February 15-19 Potential Snowstorm and Ice Storm

The February 15 through 19 period is increasing in likelihood that we will see a winter storm, potentially with an ice storm component.

Tropical Tidbits
Click images to enlarge

The image above shows the GFS forecast for precipitation type over the United States for the morning of February 16th. Here, we see precipitation breaking out over the Southern Plains and along the Gulf Coast as a low pressure system advects northeast-ward. In this forecast, we see a snow shield placed from southern Illinois into Missouri, Arkansas, and the Tennessee/Kentucky area, with rain blossoming in Texas, Louisiana, and southern Mississippi. We also see a rather broad swath of sleet, potentially freezing rain in southeast Oklahoma, central Arkansas, and northern Mississippi. This freezing rain/sleet delineation could prove to be a serious issue for those in the South, especially with memories of the ice storm from last year still fresh.

Tropical Tidbits

By the evening of February 16th, the low pressure system responsible for this precipitation is trekking along the Gulf Coast, located along the Mississippi/Alabama border in this graphic. A rain shield encompasses Louisiana, Mississippi, and a good chunk of both Alabama and Georgia. Snow is falling in eastern Tennessee, the western Carolinas, and extreme western Virginia, with a small band of freezing rain / sleet in northern Mississippi and Georgia. It's encouraging to see the freezing rain shield shrink from our last image, but as freezing rain is so tricky to predict, I wouldn't take that part of this forecast verbatim.

Tropical Tidbits

By early morning on February 18th, the storm has transferred offshore and is beginning to strengthen over warm waters along the coast of the East US. Snow continues to fall in much of North and South Carolina, with rain prevailing in southern Georgia and much of Florida. Heavier bands of snow are already impacting coastline locations in the Mid-Atlantic and Northeast, with New Jersey on the northern fringe of this heavy snow shield. From there, the storm continues north and east.

Snow accumulation charts are unreliable in this case, as some methods for snowfall will accidentally count freezing rain and sleet as snow, unrealistically amplifying snow totals. Thus, it would be unwise to show a snow total chart for those in Arkansas, where that unrealistic amplification of totals is likely to occur.

WxCaster

I want to now look at the forecasted freezing rain accumulation chart from the short-range NAM model over the Eastern US. In this chart, we can see where freezing rain is most likely to occur. Again, because freezing rain is so hard to predict in advance, take this with a relative grain of salt. Regardless, let's see who may be affected. The highest freezing rain totals appear in western South Carolina, where accumulations of 0.50" to 0.75" could be found. Significant accumulations of 0.25" to 0.50" extend through the rest of the Carolinas, and isolated spots of similar totals stretch back through northern Mississippi, Georgia and Alabama, all the way to southern Arkansas. While you shouldn't expect to see this chart verify exactly as-is, it gives you a good idea as to who may be affected by freezing rain from this storm.

To summarize:

- A storm system in the Southern US looks to bring wintry precipitation to states such as Arkansas, Mississippi, Alabama, Georgia, Tennessee, Kentucky, and the Carolinas.
- Accumulating snowfall is possible, particularly in Tennessee and Kentucky.
- Accumulating freezing rain is possible, particularly in southern Arkansas, northern Mississippi, Alabama, Georgia, and the Carolinas.

Andrew

February 10-15 Potential Winter Storm

I'm watching for the potential for wintry weather in the February 10-15 period.

Tropical Tidbits

The image above shows 500mb geopotential height values for the morning of February 5th, with mean sea level pressure (MSLP) contours superimposed. In this graphic, we see two areas of low pressure over the Western Pacific. The more dominant system is located south and east of Japan, with another piece of energy just west of this island nation. In the forecast hours after this graphic, it appears the energy west of Japan transfers to the storm east of Japan. From there, the dominant low moves generally northeast, away from Japan. When we use the Typhoon Rule, which states weather phenomena occurring over Japan is reciprocated in the US 6-10 days later, a storm in the US might be expected in a February 10-15th period, adding a day for some uncertainty I have on timing. Additionally, the transfer of energy over Japan tells me we could be looking at a transfer scenario from the Midwest/Great Lakes/Ohio Valley into the Northeast.

ESRL

We can take a look at teleconnections to get a glimpse at what this storm might do. For this post, we'll pay attention to the top-right North Atlantic Oscillation (NAO) outlook. Notice how the index rises from negative to neutral territory right in the timeframe of this potential winter storm. The NAO switching phases is known to be a red flag for snowstorms in the Northeast, as the risk of snow events in that area tend to rise when this switching of phases occurs. This could support the idea of a transfer storm to the Northeast, like the GFS showed (in a likely overdone projection) in its 12z forecast:

Tropical Tidbits

To summarize:

- Model guidance is indicating a winter storm may impact the country in a February 10-15 period.
- As of now, a transfer low from the North-Central US to the East Coast may be a likely solution.
- As usual, high uncertainty still exists.

Andrew

January 28 - February 1 Potential Winter Storm

We're watching for a potential winter storm in a January 28th through February 1st timeframe.

Tropical Tidbits
Click Images to enlarge

The above graphic shows mean sea level pressure (MSLP) contours superimposed on 500mb geopotential height values (colored shadings), valid from last Thursday morning. Last Thursday, we saw a positively-tilted trough forcing a rather strong low pressure system to form just east of Japan, dipping below the 1000 millibar mark on this image. When we apply the Typhoon Rule, which states weather phenomenon occurring in Japan is reciprocated in the US 6-10 days later, we come out with a potential winter storm in the January 28 - February 1 timeframe.

Instant Weather Maps

Initially, the ECMWF takes this system down through the Midwest and Ohio Valley, as the above image shows on the evening of January 29th. We see a minimum sea level pressure value of about 1008 millibars, if not a bit below that. This won't be a significant storm, per current forecasts, but could drop some wintry precipitation across the aforementioned regions.

Instant Weather Maps

By the evening of January 30th, the ECMWF model sees this storm transferring offshore the Mid-Atlantic, strengthening at an appreciable pace to a minimum sea level pressure value of ~997 millibars. This would likely produce accumulating snowfall for parts of the Northeast, as the GFS model is also alluding to, but again, nothing incredibly significant.

To summarize:

- A winter storm may affect the US between January 28th and February 1st.
- This storm may affect the Midwest, Ohio Valley, and Northeast the most, if at all, given the relatively-weak strength of this storm.

Andrew

January 21-25 Potentially Significant Winter Storm

It's looking as if a winter storm will impact the United States between January 21st and January 25th. As of now, this storm has potential to be a strong one.

Tropical Tidbits

The above image shows mean sea level pressure (MSLP) and precipitation values for the morning of January 15th over the West Pacific. In this graphic, we see a strong low pressure system moving up the eastern coast of Japan, delivering heavy precipitation to areas offshore of the island nation. A look at 500mb vorticity values (not shown) depicts this storm phasing with another piece of energy to strengthen and mature the energy into a substantial event.

If we recall that the Typhoon Rule states weather phenomenon occurring in East Asia is replicated here in the United States about 6-10 days later, we should expect a storm system, possibly strong, to hit the US in a January 21-25 timeframe. The orientation of this storm striking the east coast of Japan tells me it may come up from the South US and hug the East Coast here in the US.

Interestingly enough, model guidance is approving of this theory.

Tropical Tidbits

The new GFS (the old GFS model was retired with this morning's 12z / 6AM central time runs, and replaced by what was known as the GFS-Parallel model) is showing a storm system developing in the Southeastern US on January 23rd. Here, we see a large swath of heavy rain, likely containing thunderstorms draped across the Gulf Coast into the Mid-Atlantic, as well as a heavy snow swath spread across the Ohio Valley and into the Northeast. This is something like I would expect to see happen with the way our energy acts when it skirts around Japan in the earlier graphic we analyzed.

Tropical Tidbits

By the evening of January 23rd, we find our storm has moved offshore, dragging an impressive liquid precipitation shield behind it, which then drags behind it a very cold airmass to introduce the anticipated cold blast to close out January. A strong snow swath remains present, but the most intense snow appears to strike Long Island in New York, as well as other coastal spots. Lighter, but still impressive snows then impact more inland regions.

To summarize:

- A storm is expected to impact the United States between January 21 and January 25.
- This storm has the potential to be strong, per current guidance.

Andrew

Long Range Regional Outlook (Ohio Valley): September 20-October 12

This is the latest Long Range Regional Outlook forecast for the Ohio Valley area, valid for September 20th through October 12th.

CMC Ensembles

I have elected to use a combination of the CMC and ECMWF ensembles for this outlook, after finding the GFS ensembles to be in disagreement with the two aforementioned systems, which do hold a consensus.

The image above shows the CMC ensemble mean 500mb height contours on the date of September 22nd. In this forecast, depressions in the contours indicate troughing (cold and unsettled weather) while arcing motions in the contours depict ridging, which results in warm and relatively quiet weather. In the graphic shown above, we see a broad depression in the contours across the Plains and into the Midwest, though we then see slight ridging emerging over the Ohio Valley. This would tell me that the forecast period of September 20th to October 12th likely opens with some seasonable to above-normal temperatures, before colder weather moves in.

ECMWF Ensembles

Now shown above is a two-panel forecast from the ECMWF ensemble system. We see forecasted 500mb height contours and anomalies (with legend on the right) in the left-hand panel, with the ensemble 'spread' on the right image. An ensemble 'spread' indicates the degree of uncertainty among individual ensemble members on a particular area and its forecast. For example, we see deep purples on the right panel over the Great Lakes, which the left panel says will experience troughing (due to the depressed contours). This tells us that the ensembles are uncertain as to how likely this particular factor is to actually occur, and results in a wider spread of ensembles.

In the image above, valid for September 21st, we see a similar layout as that of the CMC ensembles. We see suppressed ridging over the Western US, created by some stormy weather in the Gulf of Alaska (see green shadings of below-normal height anomalies). This ridge in the West results in cooler weather for the Plains and Midwest, as the CMC ensembles depicted, but that then leads to some slight ridging over the Ohio Valley. This jives well altogether with the CMC ensembles.

Tropical Tidbits

In the image above, valid September 17th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we use this rule for the Ohio Valley, we might expect to see some cold weather, though I wouldn't call it a 'cold blast'. In this forecast, the upper level low looks displaced north enough that the core of the cold would likely stay in Canada. This fits in with the CMC and ECMWF ensemble projections of a weak trough moving east from the Plains and Midwest (as we discussed earlier), and is a reason why I disagreed with the GFS ensembles.

* The image used above is a forecast from the GFS ensembles, but the part I disagree with is for its forecast in the Northeast Pacific, not over Japan. Still, this part of the forecast must be monitored closely for the discrepancies described above.

Tropical Tidbits

Once again using this Typhoon Rule, we see the forecast on September 21st calling for continued deep troughing over Japan, now pushed deep into the country. Using the guidelines set forth for this rule, we might expect some chilly weather in the Ohio Valley around the September 27th - October 1st period.

After going over the factors listed above, the temperature and precipitation outlooks for the Ohio Valley over the September 20th - October 12th period are as follows:

Temperature Outlook:

The Weather Centre
Temperature Outlook

Temperatures for the Ohio Valley are expected to remain around average to slightly below-average, due to a warm start to the forecast period and a few weak cool shots.

Precipitation Outlook:

The Weather Centre
Precipitation Outlook

The precipitation outlook calls for predominantly below-normal precipitation over the forecast period, due to somewhat dry signals over the Japan region for the next week or two, among other factors.

The next Long Range Regional Outlook will be published Saturday, September 20th.

Andrew

Long Range Forecast for Late April, Early May

Let's examine the long range forecast for late April into the beginning days of May.

Long range analog guidance from the ESRL/PSD division, a special physics-based meteorology branch of the government weather service, indicates we will see troughing setting up in Western North America in late April as a strong upper level low drops into the Southwest, provoking high pressure out ahead of it in the Central and East US. This ridge out ahead of the upper level low will likely make for anomalously warm weather across the aforementioned sections of the country, a real treat in the face of such a nasty winter.

Beyond the last days of April, it is expected that the weather will take on a slightly cooler tone. In the wake of a Kelvin Wave currently pushing across the Pacific, enhanced tropical convection is expected to develop near the 60E Longitude demarcation, a classic Phase 1 MJO signal. When we see enhanced tropical convection in this Phase 1 signal, it typically means we can anticipate cooler than normal weather here in the United States.

I am a bit skeptical of this cold weather forecast, due to the response we're looking to see in East Asia around April 26th. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. In this image above, we see projected tropopause pressures, vector winds, and wind speeds way up in the middle-upper regions of the troposphere into the stratosphere. If we look to this forecast image, valid April 26th, and find Japan in the top left corner of the image, we can make out a bulge of orange pushing towards the center of this image. That orange bulge signifies the presence of a Rossby Wave. In simple terms, this Rossby Wave will 'break' over Japan and initiate an intensive warming spell. This may continue for some time, but if it does happen in late April, we would likely see the cooling effects of the Phase 1 MJO hurt, as this East Asian development would likely overrule it.

To summarize:
• A warm end to April is expected.
• A cool start to May is possible, but there are hints that the late April warmth may just carry over into May. More time is needed to investigate this potential.
• A severe weather event is possible in the final 7 days of April, due to the upper level low in the West US.

Andrew

April 9-15 Multiple Potentially Significant Storm Systems

I'm seeing the threat arise for not one, but two potentially significant storm systems.

Tropical Tidbits

The GFS model has been consistently bringing in a strong upper level low into Japan around April 4th, beginning to attain a negative tilt on the image above, valid for the afternoon of April 4th (for more information on negatively-tilted storms, please click on this link to see the post published yesterday on this topic). There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. If we take the April 4th day and extrapolate it out 6-10 days, we arrive at the April 10-14 timeframe for what could be a hefty storm. I say it could be strong, as the strength of these East Asian systems has been reflected in the resultant United States storm . For instance, a strong storm over Japan does usually result in a strong storm in the US 6-10 days later, and that's what we're looking to see in this April 10-15 timeframe.

But we're not just looking for one system. This time, there are indications we could see two systems.

Tropical Tidbits

About a full day after the original system moves out from Japan, we see another swath of significantly below-normal heights enter Japan. The GFS image above, now valid for April 5th, reflects this, and we can see our first storm system that was discussed above now located just west of the ridge in the Bering Sea. This second storm system is kind of a tricky one. I'm watching closely here, as it could end up being one storm with residual cold weather just hanging behind. However, this forecast says we are in for two storm systems, and since we're entering spring, these strong storm systems can create nasty severe weather. For that reason, I'll err on the side of caution and highlight two storms in this post, but do realize that this may switch back to one significant storm.

The pattern I had highlighted earlier last week, which showed how the Northeast was at the most risk, is now a bit more hazy than when we last analyzed this timeframe. Model guidance is no longer as favorable for an East Coast impact, but rather than drop that region from a potential impact zone, I'll still tentatively keep the Central and East US in line for this storm. We should know much more about what this storm(s) could do in about 4 or 5 days from today.

As you can tell, there's a lot of uncertainty. Let's sum up what we do know.

- There is the potential for at least one significant storm system around the April 9-15 period.
- Severe weather does look to be a potential factor in this timeframe.
- Cooler and unsettled weather can be anticipated for this timeframe.

Andrew

March 26-31 Multiple Potentially Significant Snowstorms

I'm examining the idea of multiple significant winter storms over the March 26-31 period.

The above image shows a recent ECMWF forecast, projecting 500mb height anomalies over the North Pacific on March 20th, where blues and purples show negative height anomalies (cool and stormy weather), and oranges and reds depict positive height anomalies (warm and quiet weather). Looking towards the continent of Asia, specifically zeroing in on Japan, I marked two points of interest using the letter X. These X's denote the location of storm systems in the area. There is a storm system moving east over Japan, tilting negatively as it does so, and a storm to the west of Japan, just beginning to drop south and racing towards the country. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. The two storm systems both retain impressive intensities, with the bright purples indicating that both systems look to be strong.

About 18 hours later, we see that the situation over Japan has changed. The first storm system that was previously over Japan has shot north, influenced by the even-stronger system shown in green shades south of far northeast Russia. The second storm system that was previously racing towards Japan is now impacting Japan, maintaining rather-strong characteristics, as shown by the tinted purples over the country. This confirms there may be not one, but two storm systems in this timeframe.

Shown above is the ECMWF Ensemble mean forecast of 500mb height anomalies, over North America. The same color rules, where blues indicate stormy weather and oranges show calm weather, still apply. This forecast is valid on March 26th, the beginning of the five day period we're watching for these two potential storms. We see storminess evolving over the West Coast, shown by the darker blues just offshore of California, Oregon and Washington state. We see this provoking slight ridging in the southern Plains, well illustrated by the erosion of blues in that region. We also see an old friend in a piece of the polar vortex still sticking around in Canada. The storminess over the West Coast should enable the high pressure over the Southern Plains to gradually strengthen and push east, as it's possible we see a negative Pacific-North American pattern evolve, which is characterized in the image below.

Typical negative PNA pattern
NCSU

We also see how a lobe of the polar vortex still sits over Canada, as it has been doing for much of the winter, and should continue to do into spring. If this forecast were to verify, it wouldn't be too hard for the storm track to be suppressed, from the Midwest down into the Ohio Valley. This could favor areas that have been hit multiple times already this winter, including the lower Midwest and Ohio Valley regions.

The GFS Ensemble forecast for 500mb anomalies at about the same timeframe have a pretty similar pattern, even though they may seem different at first glance. The PNA remains negative, like the ECMWF ensembles project, as we see strong negative height anomalies over the West Coast. We also see a lobe of the polar vortex positioned in Canada, as we did in the ECMWF Ensemble image. The only 'difference' here is that the ridge we discussed in the ECMWF Ensemble image is more pronounced on this image, and located further east. This solution would likely still drive storm systems into the Midwest, but they would also be suppressed- just not as much as the ECMWF ensembles say they would be. This sort of projection would take the two potentially significant snowstorms into the Midwest and Ohio Valley, as ridging in the Southeast would deter an East Coast solution.

Caveat: The ECMWF projection that says we would get two major storm systems is a long range forecast, and has a chance of not verifying completely. There is a pretty good chance we would be seeing a potentially significant storm system in this timeframe, it's the idea of two storm systems instead of one that isn't really solidified just yet.

To summarize:
-One or possibly two potentially significant storm systems are expected in the March 26-31 timeframe.
-Both winter and severe weather modes would be quite possible with this event.

Andrew

March 11-14 Significant Snowstorm

I'm still looking at the idea of a significant snowstorm on the March 11-14 period.

An upper air analysis at the 500mb level of the atmosphere shows our storm system as the depression in the contour lines over the northeast Pacific, which tells us the system isn't onshore yet. The storm not being onshore means that model guidance will continue to change until the storm actually gets onshore. The reason being that National Weather Service offices release weather balloons twice a day, and the data those balloons gets goes into the models to enhance their forecasts. Thus, when the balloons are sent into the storm when it comes onshore, the models get a better handle on the storm, and that's why we tend to see a model consensus come about when the storm system in question comes onshore. We also see suppressed ridging over the Western US, which will impact the eventual track of this event, as we'll go over later.

Shown here is the GFS 500mb vorticity projection for the evening of March 11th. On this image, it seems our storm system is separated into two storms, located over Nevada and Kansas. In this case, the GFS model wants to take the westernmost portion of the storm and retrograde it into the ridge located along the West Coast. This piece of energy then closes off and separates from the other piece of vorticity that continues progressing eastward. This move is a bit suspect to me. It is well known that model guidance has the tendency to hold energy in the Southwest for too long, and this could very well be one of those situations where the GFS is too eager to bring a piece of the storm into the Southwest US.

By the time we get to the evening of March 12th, about 24 hours later from the image we just discussed above, we see a lot has changed. We now see that the storm has indeed separated into a closed system along southern Nevada and California, which is linked to its parent storm by an elongated lobe of positive vorticity draped across the Plains. We'll get to that parent storm in a second, but first let's go over the storm in the Southwest. The system has retrograded directly into the West Coast ridge, and has closed off in the process, leading to a Rex Block-style pattern, where we have a ridge directly north of a storm system. While the typical Rex Block produces a zonal flow (west-to-east flow) synoptic set-up across the areas downstream of the block (to the east of the block), the GFS prefers to initiate a northwest flow regime, with the West US ridging leading to deep troughing across the East US. This leads us into the parent storm, which is in its own category here. The aforementioned northwest flow has led to a phasing (merge) of the easternmost storm we saw in the second image of this post (the first GFS image we discussed) with a weak system originating from northern Canada. This comes as no surprise, with the northwest flow regime being very supportive of phasing should the opportunity arise. I'm a little skeptical on this idea, as models are notorious for phasing storm systems too eagerly. This could mean we actually see two weak, unphased systems when the March 11-14 timeframe comes around, but with guidance supporting this phasing more and more, I find that the positives for phasing outweigh the things going against it.

This is the ECMWF 500mb vorticity forecast for the evening of March 11, the same timeframe as the first GFS image we analyzed. In this model's forecast we see the two systems are still trying to split up, but are nowhere near as elongated as the GFS model portrays them to be. Rather, we see the two systems fairly well defined, with one over Utah and the other over Kansas.

The above graphic shows the ECMWF 500mb vorticity forecast for the morning of March 13th, about 6 hours after the second GFS image we analyzed. Here, we see a significantly differing view as to what happens. The ECMWF takes the westernmost system and does retrograde it into the Southwest, but it does so as an incredibly weak system, so the system does not become a closed low and the Rex Block cannot form. We see a strong lobe of positive vorticity extending across the southern Plains and Gulf Coast, before we arrive on the parent storm, which has phased with the Canadian storm. This solution also results in a snowy solution for much of the Northeast, and it is a viable idea. However, because this is a northwest flow regime, and the ECMWF model has performed poorly in northwest flow situations this winter, I'm not ready to buy in on this solution just yet. It will probably take another day or so before we can at least refine the solutions we have now to try and come across a more solid consensus.

Here's an overview of current model projections for snowfall.


Andrew

March 11-14 Potentially Significant Snowstorm

As I discussed back on February 25th, we're looking at the potential for a significant winter storm in the March 11-14 timeframe, narrowed down from the original March 12-17 timeframe.

The ECMWF model's 500mb vorticity forecast for March 12th is shown above, and we see the two ingredients for our storm on this map. We see the main piece of energy as an elongated swath of positive vorticity stretching from Nevada to Missouri, with our second player dropping south on the lee side (east side) of the ridge stretching across the West Coast. We also see a piece of energy in the Gulf of Mexico which will do its part to interrupt the severe weather aspect of this storm, which is why we'll be focusing on the snowy side.

The ECMWF develops a 997 millibar low centered over southern Illinois on the morning of March 12th, with overpowering high pressure to the north suggesting we aren't likely to see any big northward shifts with this storm in the future. Model verification confirms this idea. As far as the projected storm, this 997mb low is just the elongated system, not accounting for the system dropping from Canada which will eventually phase with the aforementioned system in Missouri.

By the morning of March 13, the system is rapidly deepening in the Mid-Atlantic as the two pieces of energy have begun to phase. This means that they are combining into a single storm system, which only intensifies the snow potential for this event. From March 12th's jet stream forecast, I would expect we see the system try to push east due to a rather zonally-oriented jet stream across the United States, which isn't that favorable for the system to shoot up the coast. However, by the time it hits the coast, the jet stream is essentially laying out the red carpet for the system to go northeast and affect the Northeast.

The snow map for the ECMWF would dump amounts of upwards of 12" across much of the Northeast, including coastal regions, while laying down over 6" in Ohio and a portion of Indiana. This would all depend on phasing of these two systems, which the folks at the National Centers for Environmental Prediction (NCEP) indicate may not be so likely. They indicate that models tend to phase systems too often, when in reality, they don't end up phased. This could be one of those situations, but I guess we'll just have to wait and see.

The GFS model, on the other hand, is much less enthusiastic when compared to its European counterpart. We see both systems shown above, valid for the same timeframe as the 500mb vorticity map we looked over at the top of this post, but in the GFS' case, the layout is different. The GFS model keeps the system back in the Southwest, on a positively-tilted axis, meaning the strongest vorticity values are pointing in a southwest direction. I have a feeling the GFS might not be grasping this system correctly. The NCEP agency also states that models in general have a bias to keep storms in the Southwestern US for too long, when in reality they eject from the Southwest quicker than forecasted. This would work in favor of the ECMWF's solution. However, the GFS does not phase the two systems, which results in noticeably less snow and precipitation in general. We'll need to watch both models for this system in the next few days, as both appear to be hanging on to one bias while letting go of another one. In this case, the ECMWF may be phasing the systems too eagerly but not holding the energy back in the Southwest, while the GFS looks to be keeping the system in the Southwest but not phasing the storms.

Andrew

Weekend Snowstorm Targets Midwest, Ohio Valley

I'm expecting a snowstorm to impact the Midwest, Ohio Valley and East Coast this weekend.

As the graphic above shows, I anticipate the heaviest snow to extend from northern Missouri through Illinois, Indiana, Ohio and into Pennsylvania and New Jersey. West Virginia and Virginia will also be impacted by heavy snow. I anticipate we see amounts somewhere in the 6-8" range over the Midwestern states I just mentioned, while areas along the East Coast have a higher upside, possibly near 12". With this storm comes the threat for freezing rain, and I expect this threat to reside from Arkansas into Kentucky, eastward through the Virginias. Pinpointing the location of freezing rain is difficult, and with model guidance still having numerous issues handling this event, do not be surprised if my forecast chances later today. I based this map off of the 12z GFS, seeing as the system has finally made it ashore and a general forecast with this layout appears to be the consensus at this time.

A look at enhanced water vapor imagery shows the storm system making its way onshore, throwing out massive amounts of moisture ahead of it, as the warmer colors show. The circulation of the storm remains just offshore of California, a bit east of that swath of dry air depicted in dark blues. As the system makes its way onshore, we should see model guidance improve and (hopefully) settle on a solution, as the weather balloons sent up by each National Weather Service office at 6 AM and 6 PM (central time) each day will be able to ingest data from the storm, which then feeds into the models to make a more accurate forecast.

There is some talk about model guidance underdoing precipitation in this storm, and also being too far south. The jet stream is predicted to be aligned in such a way that the storm would normally push further north than it is currently projected to go, and we would typically see the heaviest snow shifted to the north as well. However, with models standing their ground on the storm staying south, and both the northern and southern jet streams now "sampled" by those weather balloons, having been onshore for some time, tells me that the storm will most likely stay on its southern track. The under-doing precipitation claim is a bit more difficult to decipher. On one hand, model guidance is notorious for over-doing precipitation when you compare observed precipitation to forecasted precipitation, but in this scenario, we will have a strong jet stream and Gulf of Mexico moisture feeding into the environment, which would usually help enhance precipitation totals. I want to wait for a bit before making a call on that issue, because it'll most likely take a close analysis of radar trends to determine how model guidance has been handling that aspect of the storm.

Andrew

Arctic Cold to Abruptly End in Mid February, Return in March

It's looking like we will see the recent Arctic cold come to a quick end in mid-February.

The image above shows the GFS Ensemble 500mb height anomaly forecast for February 17th over the western Pacific. We see strong ridging present over Japan and much of eastern Asia, with another swath of positive height anomalies displaced further east to the south of Alaska. Using the idea that a storm system in Japan can signal a storm in the US 6-10 days later, we can find that this strong ridge over Japan should also come back to haunt the US 6-10 days later. That would place warmer and quieter weather over the nation in the February 23-27 period. Considering high pressure should be affecting Japan both before and after this February 17th forecast graphic, it would be reasonable to think that this warmer/quieter weather may extend into early March, as well as closing out the last week or so of February.

This warm trend can be seen by the Earth System Research Laboratory's (ESRL) Analog Temperature 8-14 day outlook, as is shown below:

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As we move into early March, it looks as though we will be heading into new phases of the Madden Julian Oscillation (MJO). Looking at the March 8th MJO OLR anomaly forecast, in the middle panel of the left side of the images above, we see deep blues spread out across the waters to the southwest and southeast of India. These deep blues represent enhanced tropical convection, and it also means an active phase of the MJO will be evolving.

If we compare that March 8th forecast image to the MJO OLR Composite image from the Australian Bureau of Meteorology, we see that the blues on the forecast image line up the best with MJO Phase 2 and MJO Phase 3, which are seen as the second and third images down from the left-hand side of the panels above. If we look at the low amplitude Phase 2 MJO (which guidance predicts we see when the MJO does enter Phase 2 before transitioning to Phase 3) 500mb height anomaly image below from scotlandwx.co.uk , we find that deep negative height anomalies are favored over the East US, resulting in colder than normal weather. The troughing in the Gulf of Alaska remains present, with suppressed ridging over the north central Pacific Ocean. Long range ensembles favor this type of set-up for the middle-late portions of February, and it's very possible this continues into March. To clarify, while the set-ups may be similar from mid-February onward, I currently only expect warmer weather for the mid-late parts of February due to East Asian ridging. Beyond that period, into early March, indications of East Asian ridging dissipate, allowing cold weather prospects for early March to flourish.

Low amplitude Phase 2 MJO 500mb height anomaly composite

The 500mb height anomaly composite for all amplitudes of a Phase 3 MJO also favors some cooler weather in the East US, though the cold is more restricted to northern parts of the nation.

So, let's sum it all up.

•We are looking at warmer weather over the United States in the closing week of February, possibly extending into the opening days of March.

•The pattern after the early-early March warm-up looks chillier, with a potentially favorable MJO (keyword is potentially due to the typical long range forecast caveats and inaccuracies) and favorable signs from East Asia after ridging moves out.

Andrew

February 4-6 Major Snowstorm

There will be a major snowstorm over the February 4-6 time period, which has the potential to drop up to a foot of snow.

The GFS model, whose 87 hour snow projection is shown above, indicates we will see heavy snowfall in the 8-12" range across easterk Kansas, southeast Nebraska, northern Missouri and into Iowa. Amounts then taper off into the 3-6" range in Illinois, before ramping back up in the Northeast, where amounts over 12 inches are possible. The GFS model has been consistent with this storm thus far, and with the storm only a couple of days away, it looks like this is the track that is more likely to work out. The snow amounts have moved north of the far south storm tracks that we saw days earlier, and this north shift was expected, per earlier posts I put out.

The NAM model is much more enthusiastic with snowfall, laying down 6 to 10 inches of snow in Kansas, Nebraska and much of Missouri before pounding the Lower Great Lakes region with over 12 inches of snow. Northern Indiana appears to be heading for 20 inches of snow if this model is to be believed! However, the NAM model had previously been showing a far northwest solution, so additional nudges south may be anticipated. I do expect snowfall amounts will be lowered in future forecasts, as the NAM model is notorious for over-estimating snowfall. The stripe of snowfall extending from Texas to the Mid-Atlantic is not associated with this storm.

Because the storm system has not come ashore yet, slight changes in storm track and snowfall amounts can be anticipated. We should begin to see a solid model consensus tomorrow or tomorrow evening, when the system comes onshore and the balloon network can get fresh data from the storm.

Andrew

February 4-6 Potentially Significant Winter Storm - Updated January 30

There remains a good chance of a significant winter storm in the February 4-6 timeframe. There is also a good chance of the storm track shifting north, so let's dive right in.

(The following is copied from the January 28 post on this storm)

Shown above is the 500mb anomaly forecast off the ECMWF model, valid on the morning of January 26th. If you've followed this blog for a while, you know that we can use that area of negative height anomalies over Japan to forecast a storm in the United States. As Joe Renken states, we can extrapolate a storm over East Asia out 6-10 days to arrive at the timeframe for a storm here in the United States. Going that length past January 26th gives us a storm timeframe of February 1-5. That's not the only thing we can take away from this predictor, however. Looping the ECMWF forecast tells me that the storm system will be moving essentially due east as it passes through central Japan. This tells me that we could (keyword here is could) see our February 1-5 storm system also moving on a west-to-east track, not really curving north or dropping south too much. The 12z and 18z GFS models caught on to that idea of a west-to-east storm, but had the storm hitting the US on January 31st to February 1st, which is just outside the timeframe presented here. We'll discuss the models a bit later on in this post.

Adding to the evidence of a possible storm in the early February timeframe is what we saw happen in the Bering Sea on January 18th. The image above shows contoured 500mb height anomalies over the north Pacific on January 18th. We see a storm system moving east from far northeast Russia, which is the westernmost 'X' on the image above. The next day, we saw the storm move nearly due east to the second 'X' in the image, only moving just a bit north in the process. Going back to that link I posted above, there is also something called the Bering Sea Rule, developed by Joe Renken. The Bering Sea Rule states that a storm in the Bering Sea can result in a storm over the United States 17-21 days later. If we extrapolate the January 16th date out using the 17-21 day timeframe, we arrive at a potential storm in the February 2-6 timeframe. This fits well in the February 1-5 timeframe we got from the East Asian correlation, and the nearly west-to-east movement in the Bering Sea tells us the storm in the US may be west-to-east as well, like we saw in the ECMWF forecast. 

The East Asian correlation and the Bering Sea Rule aren't the only things rooting for this storm- the Lezak Recurring Cycle is also in on this storm potential.

The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a regular (non-changing) 40-60 day interval for the next ~10 months before it dissipates over the following summer.

This season's LRC cycle is on a 57 day cycle length, meaning the weather we saw in December can be expected to "come back" as we enter February. The image above shows the surface condition map on the left and 500mb map on the right from December 14, 2013. If w extrapolate that date out 57 days, we come to the February 7th mark. To make up for natural shifts that occur in the weather pattern, we can reasonably stretch the timeframe out to February 6-8 if we want to. In this case, it wouldn't be the strangest thing for this system to bend the cycle length a bit to fit in with the East Asian + Bering Sea correlations, and I feel that they will be working in sync on this particular storm threat. As the 500mb map above shows, we have high pressure over the West Coast, with only slight ridging in the Southeast and the storm system easily working its way across the Plains. Based on the pattern I'm seeing to start February, I'm thinking a storm track similar to the one this system took in mid-December could very well be in the cards.

Now that we've got the trio of long range predictors backing this storm potential, confidence is rather high on the idea of a storm system happening around February 4-6. Now, we can start to outline a possible track. 

This is the Pacific-North American (PNA) forecast for the next several days, with the forecast shown on the top panel, composed of various ensemble forecast members. We see that the PNA is forecasted to remain negative before, during and after the time period of this potentially significant winter storm. Beyond the storm, it looks like the PNA wants to move back towards neutral or positive territory, but the majority of ensemble members want to stay in a solidly negative state.

The pattern in a negative PNA state is shown above, and we can see how cold and stormy weather invades the West US as a result of persistent high pressure in the Gulf of Alaska. This stormy West then allows high pressure to form in the East, essentially cutting off any chances of a Nor'easter. What the -PNA does allow is the Great Lakes Cutter type storm, where a storm system begins in the Plains and 'cuts' north towards the Great Lakes, laying down potentially significant snowfall in its wake. It's very possible we see somewhat of a cutter storm here- the long range predictor East Asian correlation and Bering Sea Rule argue for a more west-to-east track, which does look like the preferred track at this point, but models may want to shoot the storm north when it hits the Ohio Valley, which also is in the realm of possibility. I'm supporting the more northern track as a result of the Southeast Ridge showing its strength when the system tries to dig towards the Gulf of Mexico.

Now, we have to discuss the models themselves, and where they've gone wrong. Model guidance has really pulled the storm track to the south (though recently they have been correcting north again). I'd like to explain why this south track is unlikely, and why the more northern track is probable.

There's a key model bias that has been (and will continue to) impacting model forecasts for this storm. The model bias references a negative height bias, which is shown when a storm system enters the Plains. In those cases, model guidance lowers heights in the East US too much, leading to a storm track too far to the south. In reality, high pressure systems in the East should be stronger than what they are projected to be on model guidance, leading to a more northern storm track. This system fits the bill for that bias; for the full explanation from the NWS office in Louisville, you can read below:

The model has a slight cold (heights too low) bias, especially for about the eastern third of the country, with respect to the prediction of mid/upper tropospheric geopotential heights and resultant thickness calculations. Often the MRF will dig troughs in the height field too far south across the Great Lakes and Northeast regions, most noticeably after about Day 3. During the cold season (mainly October-April), the MRF will depress the storm track too far south across the Plains states as a response to this cold bias. Forecasters across especially the Northern Plains/Upper Mississippi Valley areas need to be aware of this characteristic and be careful predicting, for example, snowstorms to "go to their south".

How have we already seen this model bias? Take a look at this side-by-side forecast from the 0z GFS model on the left, and the 12z GFS on the right, both forecasting 500mb vorticity values for the morning of February 4th, when the storm is just beginning.

If we look at the 0z GFS on the left, we see the storm system on February 4th is digging into the Plains, but the high pressure in the East US, shown as the arch-like contour lines, isn't strong enough to push the storm system north. This lets the storm system stay to the south and dump snow on the midsection of the US. However, in the 12z GFS, we see what looks like a correction of that height bias we mentioned earlier, as the heights are noticeably higher in the East. This allows the storm to push north and dump snow across the northern US, including the Plains, Midwest, Great Lakes and parts of New England. The negative PNA (whose forecast went even more negative today when compared to yesterday) raises an immediate red flag to the forecasts that want the storm to stay further to the south. The increasing strength of the -PNA projections only highlight that red flag. There are indeed items that could make this storm take the southern track, but I find it more reasonable right now to take that northern track.

We could compare maps all day, but a better idea would be to observe biases from different models and see what we can deduce. Let's take a look at a few charts below.

This image above shows 500 millibar height contours in two different colors, valid at the same time. The blue lines represent observed 500mb heights as of January 29th, while the red lines show 500mb heights from an earlier ECMWF model forecast. This ECMWF model forecast is at Hour 144, and is valid for January 29th; the same time period as the blue lines/observed 500mb heights. Why are we using this? By comparing the observed 500mb heights and ECMWF Hour 144 500mb height forecast (for future reference, I chose Hour 144 because we are about 144 hours away from this potential storm system as of this posting; it is to get the best grip on potential biases), we can find model biases. Just glancing around the image and comparing numbers on a red line to the same number on a blue line tells us that the red lines are far too south. For example, if we see the 552 dam height contour on the red line, and then find the 552 dam height contour on the blue line, we see that the red line is much further south than the blue line. This tells us that the ECMWF model has a bias in this case that forecasts 500mb heights too low. How does this affect our storm? Well, if the forecasted heights are too low, it could mean that the Southeast Ridge will actually be stronger than what is being forecasted, and that would result in a northward track.

This image shows observed 500mb heights in blue and the hour 144 500mb height forecast, but this time the forecast is from the GFS model, not the ECMWF model. Despite the model change, the bias remains the same- comparing the same number on different colored lines results in the GFS forecasting 500mb heights too low. This bias would once again argue for that northward track. The GFS model has been trending north with its latest forecasts, which could mean it is shedding this bias. The UKMET (not shown) also has this heights-too-low bias.

There is also some concern about high pressure coming down from Canada being too strong, which would then force the storm system south. However, model guidance has problems here, too.

This image shows observed mean sea level pressure (MSLP) values in blue, and the ECMWF 144-hour forecasted MSLP values in red. If we look in the Southern Plains, we see a small swath of 1032 millibar MSLP values, indicative of high pressure. But, if we then look for a corresponding 1032mb contour on the red lines from the ECMWF forecast, we see a big swath of 1032mb values displaced a bit to the south. If we keep comparing numbers, the result is the same: it appears the ECMWF forecasts MSLP values to be too strong than what they are when the MSLP values are actually observed. In our case, this bias could mean models are being too aggressive with the Canadian high pressure. Weakening of that Canadian high pressure would then help out chances for that northward track.

If we look at observed MSLP values in blue and 144 hour forecasted MSLP values from the GFS model in red, the bias remains the same. We can clearly see by comparing the location of any pressure contours in the US that the GFS is overdoing high pressure systems when you compare the forecast to what's actually observed. This pressures-too-high bias would really strengthen the argument for that northward track.

Still don't believe these biases (or you just don't get what's going on here)? The two two-panel images below show you computed 500 millibar height biases from the GFS' 0z forecast, which is top image, on the left, and 12z forecast, seen on the top image, on the right. The UKMET (United Kingdom forecast model) 500mb bias' for the 0z forecast (bottom image, left panel) and 12z forecast (bottom image, right panel) are shown.

GFS 500mb model bias.
0z forecast bias on left, 12z forecast bias on right.
Blues mean the model is too low with 500mb height anomalies.
Reds mean the model is too high with 500mb height anomalies.

UKMET 500mb model bias.
0z forecast bias on left, 12z forecast bias on right.
Blues mean the model is too low with 500mb height anomalies.
Reds mean the model is too high with 500mb height anomalies.

The result is quite clear: model guidance is far too low with 500mb heights in the East US. This means that if the bias transfers over to current forecasts (which it very well may do), we can expect a stronger Southeast Ridge in future forecasts, leading to a more northern solution.

As you can see, there's a lot of evidence arguing for these models being too far south in their forecasts. And while I do believe that there's a rather strong case to be made for the models being too south, both due to bias and not getting the incoming negative PNA regime right just yet, there is some evidence that the storm could go south. However, I'm more confident in this northern solution at this time.

But we aren't done yet! Let's talk about how this storm could get to be a monster, and what's prohibiting it as being so on current forecast models.

I. Gulf of Mexico is Open for Business

This is a forecast of the 700mb relative humidity forecast, with 700mb wind barbs and pressure contours superimposed. The point I'm making with this image is not when it was made, but rather what's going on here. Check out that huge moisture fetch from the Gulf of Mexico! Strong winds coming straight from the Gulf will provide this storm with copious amounts of moisture to work with. This moisture will translate into heavy snow, heavy rain, and even sleet and freezing rain. This is a big component of the storm, and thankfully, it is a component that models are not fighting over, unlike what seems like every other part of this potentially significant storm. Needless to say, whoever wins the snowy side of this storm will win big. Not 2 feet of snow, but probably just over a foot or so.

II. Jet Stream Phasing Needed

In order for this storm to really give its all, we will need to see the two branches of the jet stream phase. This means the 'polar jet stream' and 'subtropical jet stream' would need to come together when the storm comes along. Above, I have a GFS forecast showing a lack of phasing on the left, and a forecasting showing phasing occurring on the right. The left image shows the two jet stream branches completely disorganized, obviously separated from each other. That particular GFS forecast resulted in a weak winter storm. However, on the right panel, we see the branches of the jet stream are phased as one, and that particular GFS forecast resulted in a substantially more snowy solution. If we want to see this storm really get going when it hits, we should be rooting for a phased jet stream out west. More recent forecasts aren't as gung-ho on the phasing shown in the right panel above, but they are certainly more open to the idea of combining than the left panel.

Lastly, let's discuss the MJO. 

Another item rooting for this northern storm track is the Madden-Julian Oscillation. The Madden-Julian Oscillation, also known as the MJO, involves the placement of enhanced or suppressed tropical convection year-long. The image on the left shows precipitation anomalies for Phases 1, 2, 3, 5, 6 and 7 for January, February and March. Phases 4 and 8 were left off, as they are not needed in this post. The forecast on the right shows 200mb velocity potential anomalies forecasted by the GFS. We see blues (dashed lines) highlighting active tropical convection, thus an active MJO phase, while oranges and solid lines indicate the lack of tropical convection. We see that there is a swath of blues covered by a dashed line oval at the bottom of the image, near the 180 longitude line. The MJO is predicted to be just west of that line, and if you are to look at the outgoing longwave radiation (OLR) composite for December-January-February MJO phases, you would find that Phases 6 and 7 are defined by enhanced tropical convection (blue colors on the right panel) being placed on or just west of the 180 degree longitude line. Taking a look at Phases 6 and 7 on the precipitation composite on the left tells us the MJO favors a storm track through the Midwest, Great Lakes and Ohio Valley. This would enhance the chances of that northern storm track working out instead of the southern storm track for this storm system.

Here's a glimpse at a recent forecast showing the northern track at this point. To be perfectly honest, it does look rather realistic, with maybe a slight bump up or down in totals in the near future.

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Andrew