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

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

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:

-

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 9-14 Potentially Significant Snowstorm

It's looking like we'll be seeing another snowstorm between the February 9-14 period, and some indications are that this could be a rather significant snowstorm.

Above, we see the 500mb height anomaly forecast for the evening of February 3rd. We see a strong trough digging southeast towards Japan. The arrow I drew gives you an idea of where the storm system goes in the couple of days after this forecast image. The storm then continues to dig before it is a fully negative-tilted storm, indicating the system has reached peak maturity and strength. Then, the system cuts north, as the arrow shows you. Using Joe Renken's idea that a storm system over Japan correlates to a storm system in the United States 6-10 days later, we can project that a storm system will hit the United States around a February 9-13 time period. This fits almost completely inside the timeframe I put in this post, with the one day extension to February 14th brought in to account for any delays model guidances may encounter, since model guidance can have a bias that results in forecasts being too fast with storm systems.

The curve in the East Asian storm system suggests that the eventual storm system in the US may also curve north. This includes a variety of possibilities, including a Colorado Low storm, a Panhandle Hook storm, an Apps Runner storm, or even a Nor'easter.

Also supporting this potential storm system is the Lezak Recurring Cycle. 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 9th mark. 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. If you read my February 4-6 posts, you may recognize this storm, as I talked about how it would correlate with that storm system. It seems I miscounted a bit, and the 57 day period from December 14th goes out to February 9th, rather than 7th. Therefore, this December 14th system looks to correlate with the February 9-14 storm threat.

Now, I named off a few possibilities for this system's track, which included a Colorado Low track, a Panhandle Hook track, an Apps Runner track or a Nor'easter track. If the LRC is to be believed, we could narrow down the track possibilities to a Colorado Low track or Panhandle Hook track. It's too far out to nail down exact probabilities of these tracks, but the two aforementioned tracks may be more likely than the other two tracks.

The 12z GFS actually showed a solution not unlike the one being printed out by the LRC and East Asian correlation.

TropicalTidbits

In the 12z GFS model, we saw a storm system dig southeast into the southern Plains before going negative tilt, and eventually closing off as it began to move north. This forecast scenario looks rather similar to the one we're seeing with the February 4-6 system, though this forecast is certainly much more ominous than the February 4-6 forecast.

Just for a little eye candy, here's the snowfall forecast from that 12z GFS model run. Please note this is not my forecast, nor do I endorse it- just showing you what this beast could become if it really is as strong as projected (though the probability of such an occurence working out is rather low).

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

February 4-6 Potentially Significant Snowstorm

It's looking like we're going to see a storm around the February 4-6 period, which could very well be significant.

(The following is copied from the January 25 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. (End copied portion)

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.

We'll begin by taking a look at the predicted Pacific-North American (PNA) index. We see on the top panel that ensemble members from the Climate Prediction Center want the PNA to plummet as we head into February. By the time this potential storm system comes around, the Pacific-North American index looks to be well into its negative territory.

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.

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.

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.

Lastly, here are a few snowfall forecasts from different models to show what the snowfall track (NOT necessarily the snowfall amounts) may be like if this northern track works out.

January 28 12z GFS Snowfall Forecast

Andrew

Polar Vortex to Unleash Historic Cold Across Plains, Midwest

There are increasing signals that we will see the polar vortex unleash potentially historic levels of cold air upon the nation, mainly targeting the Plains and Midwest.

Kyle MacRitchie

The Madden-Julian Oscillation (MJO) is projected to move into Phase 6 as we progress into January. For those who don't know, the Madden Julian Oscillation involves the placement of enhanced tropical convection over the Equatorial Pacific, extending from the Indian Ocean to roughly the middle of the Pacific Ocean nearly due east of northwest South America. The MJO entering Phase 6 means that the enhanced tropical convection will be northeast of Australia. After that Phase 6 movement, it looks like we will head on in to Phase 7 of the MJO, and that's the one we need to keep an eye on. This forecast, made by four separate CFS ensemble members, shows us entering Phase 7 around January 15th.

MJO Phase 7 500mb Height Anomaly Composite

If we take a look over RaleighWx's Phase 7 500mb anomaly chart, we see that Phase 7 of the MJO in January sees major ridging forming over the Western US and well into Canada, even over the Bering Sea. This is a classic example of a positive Pacific North American (PNA) index, which alters the jet stream so that the jet stream spikes north in the West due to that ridging, and then plummets southward in the Central and East US. This leads to a pattern highly conducive for cold weather in those areas. Adding to that conducive pattern is the highly negative North Atlantic Oscillation (NAO), as is exemplified by the heavy ridging over Greenland. All of this ridging across the upper latitudes then allows a negative Arctic Oscillation (AO) to form, which only adds to the intensity of the cold air in the US. All in all, the Phase 7 really creates the best environment that we can look forward to in the medium/long range for some cold weather.

This is all good and fun, but the really interesting stuff comes up when you take a look at the Climate Prediction Center's analog-produced upper air outlook for the medium range.

Does this chart look familiar? It should, because the anomaly placements are almost identical to the Phase 7 composite above when viewing the North American continent. We see extreme ridging over the Gulf of Alaska, which then allows for the formation of not only a deeply negative EPO (a key player in this impending brutal cold spell), but also a classic McFarland Signature look.

This image from the National Weather Service in 2010 shows what the McFarland Signature looks like. You have the massive ridge set up across the Gulf of Alaska and into the West Coast, and this leads to deep negative height anomalies developing just to the east, usually in the Central and East US. These deep negative height anomalies allow frigid air to be transported from the upper latitudes down into the United States, and I anticipate a very similar situation to evolve here again in the medium range time frame.

Taking a look back at that analog image above the McFarland picture, we see not only the well-established McFarland Signature/-EPO, but those deep negative anomalies being projected as very pronounced across the Central United States. Based on how pronounced those anomalies are, as well as analyzation of the jet stream and temperature patterns during this timeframe, it does appear that the polar vortex would be entering the United States. This wouldn't be any low pressure system; if this were to happen, we're talking record cold. Remember that the polar vortex is the low pressure system that sustains the cold air in the Arctic, so bringing a portion of that vortex down south into the US could be disastrous for farmers growing crops not only across portions of the Central and Eastern US, but also down in the South.

Model guidance is in good agreement about the polar vortex first affecting the United States around January 6-8. Beyond then, we may have to wait a little while until the MJO gets into Phase 7 before we see a resurgence of cold.

ECMWF Temperature valid Tuesday morning.

GFS Temperature valid Tuesday morning.

Andrew

December 19-23 Potential Significant Snowstorm & Ice Storm

I'm feeling pretty confident that there will be a significant winter storm, composed of heavy snow and accumulating ice, between the December 19-23 period.

The following text & images are from yesterday's post on this system. The end of this copied portion will be denoted as (End of December 15th post).

A pretty strong storm system is expected to move across Japan in 48 hours, after dumping multiple storm systems over that area in the past few days. This storm system is looking healthy, as defined by the GFS ensemble mid-level height anomaly forecast above over Japan. As I have mentioned on this blog, there is a 6-10 day correlation between weather that occurs in East Asia, and weather that occurs in the United States. Thus, with this forecast image, we arrive at the December 13th date. Go ahead 6-10 days, and we find the potential storm timeframe of December 19th to 23rd. The reason I believe it may be significant is due to the strength of this system on the image above. Unlike the upcoming weekend storm, which saw only a moderate system in East Asia 6-10 days prior, the current East Asian projection is for a strong storm to hit Japan, and long range model guidance senses that this system will also be a strong one for the US. (End December 11 post).

The teleconnection forecast for the next little while from the Earth System Research Laboratory (ESRL) above shows projections for the Pacific-North American index (PNA), the North Atlantic Oscillation (NAO), the West Pacific Oscillation (WPO), and the East Pacific Oscillation (EPO). For this discussion, we will focus on the PNA and the NAO.

The forecast for the Pacific North American index is negative over the time period for this storm system, even bottoming out to its minimum during when I anticipate this storm system to strike. So what does the atmosphere look like during a negative PNA?

The image above is what is referred to as a negative Pacific-North American index phase (or -PNA for short). The negative PNA results from ridging in the Gulf of Alaska that then produces storminess/troughiness in the Western US. In response to that West US storminess, we then see the development of high pressure in the Southeast. The jet stream in this situation takes storms from the West and runs them north and east, typically delivering snow to the same areas that have been getting snow recently (think Plains, Midwest, Great Lakes regions). An interesting point of interest is how the two arrows on the image above (from NCSU) merge in the Plains. That means that, if the timing is just right, two storm systems could combine (phase) and form a stronger, single storm. History suggests storms that encounter phasing do become anomalously stronger thereafter, and can produce significant wintry precipitation. I'm not implying this will be a phasing situation, just something interesting.

The next item to discuss is the projected positive phase of the North Atlantic Oscillation. During the positive NAO, we see stormy weather over Greenland, which shifts the jet stream so the weather pattern is not conducive to those big high pressure systems you might sometimes see over the Bering Sea or even the Western US. The positive NAO pattern is what is called 'progressive', meaning no atmospheric feature really sticks around in any single area, because the atmosphere wants to keep this moving, so to speak. That said, previous forecasts for this storm had the system going north-northeast from the southern Plains into the Dakotas and Upper Midwest. Because of that progressive pattern, I'm really doubting we see such a spike northward. Will we see northward movement? Yes, because the -PNA will supply that ridging in the Southeast. However, the progressive nature of the positive NAO should limit any NNE or even due-north movement by this storm. I would watch out for the storm to move northeast, or even ENE.

This relative measure of predictability map, from the NCEP, gives us a glimpse at what atmospheric conditions are likely to be unfolding by an assortment of colors, defined in likelihood by the legend at the bottom. In this case, we see that blues have a single or low double-digit percent chance of verifying, while oranges define close to a 65% or better chance of verification. This map is valid on the evening of December 20th, roughly a day or two before this storm is projected to strike. There are three things we have to look at with this graphic.

The first item we'll discuss in this image is the ridging over the southern portion of the Gulf of Alaska. This ridging is encased in solid oranges right over the heart of the big arcing in the waters west of California and the coast, signaling high confidence that this ridge will be present. If that ridge is present, then is essentially guarantees the set-up I described above. Why? Well, that ridge in the waters off the West Coast then creates West US troughing, which provokes high pressure in the Southeast, and suddenly we have a classic negative PNA pattern.

Second, we see a swath of oranges extending from southern Canada into the Rockies. The troughing in the West from that negative PNA pattern tells us that the ensembles are sensing the trough to be our storm system in the December 19-23 timeframe. Like the Gulf of Alaska ridge, the oranges signify over a 60% confidence among the ensemble system with this storm system. Rising confidence among the ensembles for not only the storm itself, but for the NE Pacific ridge tells me that this storm is becoming even more likely.

Third, there is a wide body of oranges in the Southeast US. Going back to that negative PNA pattern, high pressure/ridging in the Southeast is expected, so it really isn't surprising that the ensemble set is going with a >60% chance of that ridge occurring. However, with the aforementioned progressive pattern thanks to the +NAO, it is unlikely that we see this ridge take over the East entirely. (End December 15 post). 

Now, I was running through model solutions and found something that makes me believe this will be a storm for the Midwest and Great Lakes, possibly not as far south as recent solutions have been indicating.

This screenshot shows mid-level vorticity forecasts from the GFS on the left and from the GEM on the right. Both are valid for Sunday, December 22 in the morning. Right off the bat, we notice significant differences among each model. The GFS on the left has closed off our impending storm system in the far Southwest, extending into Mexico, while the GEM has refrained from closing off the low and has instead propelled it eastward into the Plains and Midwest. As a result of the GFS' closed off solution, the storm system is cut off from the jet stream and meanders eastward into Mexico before finally being pushed northward again. It is this closed off low idea that creates the southern storm track (where snow is given to southern IL, IN, OH and into the Northeast), while the GEM's more progressive solution forms the northern track (snow falls across northern IL, IN, MI, OH into southern Ontario).

To be quite honest, I feel that the GEM has the right idea here, at least in not closing it off in the Southwest. In a recorded list of model bias', the NCEP agency notes that medium range models have a tendency to close off low pressure systems too readily. I feel that this is expressed in the GFS system on the left panel above, and in the latest ECMWF forecast. The GEM also closes off the low pressure system in the Southwest, but releases it from that closed position earlier, thus transferring the storm eastward quicker and going with the northern track. The difference between the GFS/ECMWF and the GEM is that the GFS/ECMWF hold the storm in the Southwest for a longer period of time than the GEM, and that leads me to the second model bias. I have mentioned on this blog that there is a bias that indicates models will hold storms in the Southwest for longer than they actually end up there. The GFS and ECMWF are perfect examples of this phenomenon (seen well in the GFS/GEM panel above), whereas the GEM does not keep the energy in the Southwest for such a long period. Based on those two bias', I do feel like the GEM has an accurate handle on this system.

And the interesting part is, it's not alone.

This dual-panel image shows mean sea level pressure (MSLP) values for Hour 168 from the GEM on the left, and the ECMWF Ensemble Prediction System (EPS) mean on the right. Thus, both images are valid for next Sunday morning. Notice the similar tracks in the MSLP values across the GEM and ECMWF EPS. Timing is a bit off, with the ECMWF EPS faster by a good 12 hours with this system moving into the Ohio Valley. However, the main point is evident in that the GEM and ECMWF EPS agree on this faster ejection of the system out of the Southwest. I feel that the GFS and ECMWF operational models are holding the energy in the Southwest for too long due to that bias, and while the GEM is also subjected to the same bias, it would appear that the model is circumventing it and is going with a quicker solution.

Here's the GEM snowfall and freezing rain projections, respectfully (Use Caution):

Measured in millimeters

I do think that one part of the model tendency to hold that energy in the Southwest might be due to the aforementioned negative PNA during this timeframe, but with the PNA gradually rising to positive territory around Christmas time, as well as the previously-discussed model bias', I doubt the storm would be able to stick around in the Southwest for as long as the ECMWF/GFS believe it will.

It's still a little early for any projected outlook due to model confusion, as well as a couple things I want to monitor over coming days to see if a southward shift shown in the GFS/ECMWF may be a valid possibility (though as I said, I'm not sold on it yet).

Andrew