March 1-2 Potential Winter Storm

Model guidance has been indicating a winter storm will track across the middle of the country, bringing accumulating snow to those further south of where much of the wintry weather has occurred this winter.

Tropical Tidbits

The image above shows the GFS forecast for precipitation type and intensity, valid for Sunday night. We see light to moderate, possibly even heavy snow falling across the Midwest, Great Lakes, and Ohio Valley. A classic signal of winter finally on its way out, the rain/snow line is displaced pretty far north in comparison to where it has been in recent days and weeks, with rain being a concern in southern Illinois into southern Ohio. A bit of mixed precipitation could impact those in the central Plains.

Instant Weather Maps

Snowfall projections have been waving back and forth with the axis of heaviest accumulations, but the latest GFS pegs a swath of 6-10" accumulations across the central Plains into the Midwest and central Great Lakes, with Kansas, northern Missouri, central Illinois and northern Indiana receiving the most snow.

To summarize:

- Model guidance is anticipating a winter storm to affect the country over the next few days.
- Snowfall on the order of 6-10" may be expected.
- While the track of this storm is still not nailed down yet, the central Plains and Midwest ought to see the most intense snow accumulations.

Andrew 

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 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

January 17-21 Potential Winter Storm

There is potential for a disturbance in the January 17-21 timeframe.

Tropical Tidbits

The above image shows 500mb vorticity values across the Western Pacific from the (soon to be retired) GFS model, valid the morning of January 11th. Here, we see elevated vorticity values pushing east from the mainland of Asia. Of note is the shortwave evident just west of Japan, as the depression in the 500mb contour lines shows. This is the storm system we are watching.

A look at mean sea level pressure (MSLP) values for this same timeframe indicates the storm would be crossing almost squarely over the middle of Japan. Using the Typhoon Rule for timeframe, extrapolating the date of this graphic out 6-10 days, we come up with a possible storm in the US during a January 17-21 timeframe.

ESRL

The above image shows teleconnection forecasts from the ESRL agency shows the Pacific North American index dropping to near-neutral levels when this storm is projected to strike. Add in the moderately-positive NAO forecast, and chances are this storm will mainly take a west-to-east track, and is unlikely to "cut" north or south in any drastic manner.

To summarize:

- A storm system may impact the US during the January 17-21 timeframe.
- Indications are this storm could affect primarily the mid-section of the country.

Andrew

January 5-6 Potentially Significant Clipper Snowstorm

Model guidance is starting to realize the potential for a rather significant clipper storm in the January 5-6 period.

Instant Weather Maps

The above image shows the 0z GFS snowfall projection for this clipper storm over a 24-hour period, ending on the afternoon of January 6th. We see a very strong clipper system dropping snowfall in the range of 6-10" across eastern Iowa, southern Wisconsin, and northern Ohio into Michigan. Northern Indiana and Northern Illinois are hit especially hard, with a stripe of 9-10" accumulations. The clipper then goes on the drop amounts over 6" in northwest Pennsylvania, with additional shovel-able snow spreading across New York and other portions of the Northeast.

Instant Weather Maps

Let's fast-forward to the most recent GFS forecast, which is the 12z forecast. In this 24-hour snowfall accumulation chart, valid for the early-evening of January 6th, we see high snowfall amounts over the eastern Dakotas into Minnesota and Iowa, extending far into Wisconsin, Illinois, Indiana, Ohio, and the Northeast. Here, the hardest-hit regions look to be eastern Iowa into northwest Illinois, where amounts are maximized to the 8-10" range.

I'm unable to share certain graphics of other models on this website due to copyright concerns, but it appears the Canadian GGEM model does not have this clipper system. However, the ECMWF model does have this clipper potential, with a decent amount of GFS ensemble members also backing the solution. Regardless, there's quite a bit of uncertainty here.

To summarize:

- Model guidance is indicating a strong clipper system may strike the Plains and Midwest to start the next workweek.
- Amounts off of one model point to maximum 6-10" accumulations.
- High uncertainty still exists with regards to amounts, storm track, and even the existence of the storm.

Andrew

January 3-5 Potential Winter Storm & Ice Storm Threat

The tables have turned (yet again) on the forecast models, as guidance is favoring a northward track for this storm system. Additionally, some guidance is seeing a major ice storm threat.

TwisterData

The image above shows observed 500mb vorticity values and height contours as of this morning. We can see our primary piece of energy located in the West, digging to the south and slightly to the west as it begins to close off in the Southwest. Interestingly enough, this will be the same piece of energy that will bring the rare snowfall to the area that news agencies have been reporting on. Eventually, this energy will eject into the Plains, and that's where the real action begins.

TwisterData

By 102 hours out, we see our primary piece of energy has moving into the Midwest, and we also see another piece of energy diving south from the North Plains to interact with the primary energy. This interaction is critical to the evolution of this storm. If the system coming south from Canada is delayed, or speeds up, this storm will take on an entirely different approach, which is why model guidance still retains some problems with sorting this storm out. Lately, guidance has agreed on this storm being a "cutter" event, where the wintry precipitation will impact the Great Lakes, Plains and Midwest (those of you who read my previous post know this as the track I did not side with), but beyond that consensus, there isn't a whole lot of agreement.

We'll analyze three different model forecasts in today's discussion.

Tropical Tidbits

The above image shows the latest GFS model's forecast of accumulated snowfall out to next Monday. We see the track this storm has taken in this projection, with the bullseye of the snow hitting northeastern Missouri. A heavy swath of snowfall extends northeast from that corner of Missouri, where an additional 6"+ of snow is forecasted through the Quad Cities, Chicago, Milwaukee, and into North Michigan. This solution has been one that has shown up rather consistently, particularly in recent runs of all guidance, as the trend has been to go north. However, as was stated above, model disagreements are still evident.

Tropical Tidbits

The above image shows the snowfall forecast off the GFS-Parallel model, the incoming upgrade to the current GFS model. This forecast has one band of snow dropping about 6" or 7" in much of northern Illinois, while what may be a second band delivers similar amounts in central Wisconsin. The intriguing part about this model is its poor verification, especially as of late. It remains to be seen how this model does with this storm, but so far, recent history suggests one ought to be wary of this model's forecasts.

Lastly, but certainly not least, we need to discuss this ice storm threat.

CMC/EC

The above image shows forecasted precipitation types from the Canadian GGEM model, valid for the morning of January 3rd. We initially see a swath of moderate to heavy rain across the southern Midwest into the Ohio Valley, but areas north (particularly the western Great Lakes into Indiana and Ohio) are under the gun for a significant ice storm. Earlier projections, made not even 24 hours ago, favored freezing rain accumulation of 0.8" in Chicago, IL, though this model has since lowered those totals to a "better" 0.4" outlook of freezing rain. The GGEM model isn't the best at verification, either, so I wouldn't get hung up on this forecast. I would, however, keep an eye on forecasts for the potential for an ice event, should these ominous outlooks continue to surface.

To summarize:

- Model guidance is coming into agreement on a winter storm impacting the Plains, Midwest and Great Lakes.
- Some forecasts are predicting a major winter storm to hit the aforementioned regions, while other forecasts favor rain. Still others favor a miss to the south.
- There is some indication of an ice storm threat, but due to model uncertainty I wouldn't begin worrying yet.

Andrew

January 3-5 Potential Major Winter Storm

** This post is dedicated to the lives potentially lost on the QZ8501 flight, reported missing last night. **

The potential for a winter storm in the January 3-5 period appears to be growing.

TwisterData

The above image shows 500mb vorticity values across the United States from the GFS model, valid for the afternoon of January 2nd. Here, we see a rather potent piece of energy pushing eastward across the southern Plains at a neutral tilt. This energy is the piece we will be watching to impact a significant portion of the Central and East US. As of now, there are two tracks: one takes the system north and lays down plowable snowfall in the Great Lakes (Madison, WI or Chicago, IL), while the other track goes into the Ohio Valley, putting down snow in that area, even into the Northeast.

I'm going to go through the different mechanisms for this storm, and why ingredients are here for both a north and south track.

TwisterData

Shown here is the mid-level wind speed chart from the GFS model, valid at the same time as the first image we looked at. Notice the streak of enhanced wind speeds rounding the base of the trough/storm system in the South Plains. This streak of strong air is a textbook signal for a strengthening storm system, as the storm will then attempt to attain a 'negative tilt', where those vorticity values in that top image will try and "push" in a southeast direction. This negative tilt indicates the storm has reached peak intensity, and is now a mature storm. Additionally, it allows the storm to curve northward, and qualify the northward track.

Tropical Tidbits

Here's the snowfall projection off of the most recent GFS model forecast, giving you a good idea for where snow would fall if this northern track were to verify. Northeast Iowa into much of southern and central Wisconsin would see amounts near or in excess of 6", with similar amounts nearing 12"+ in Michigan. The GFS has continued to trend north as time has gone on, and is now reaching 'outlier' status, as other model guidance comes in south.

Tropical Tidbits

Here's the Canadian GEM model forecasted 500mb geopotential height values (color) and MSLP values (contoured lines) for the afternoon of January 3rd. We see our storm system displaced well south of the GFS track, with this forecast leading the low pressure system over western Tennessee, eventually into the Mid-Atlantic, producing accumulating snow as it does so. The GFS-Parallel model, the successor to the current GFS model, is also siding with this GEM projection of a further south track. This southern track would deliver snows to portions of Indiana, Ohio, and into the Northeast, finally adding a bit of winter for those regions.

Now, it's time for a bit of analysis.

TwisterData

Let's use the same image now as we did to kick off this post. As we see the energy push into the Southern Plains, notice how the longwave trough over Canada seems to almost converge with that ridge in the Southeast, right over the Midwest. This tightening up/coming-together of contour lines is a process called 'confluence'. Confluence works by piling up air, as those contour lines press together. At the surface, this results in high pressure. My concern is that model guidance (the GFS model in particular) is not accurately accounting for this high pressure. The formation of high pressure right where the storm is supposed to track would usually see the storm forced southward, similar to what the GEM and GFS-Parallel are showing. 
Adding to this concern is that all model guidance has a known bias to be too slow with the progression of Arctic air southward. Lo and behold, we have a strong Arctic high pressure system on the heels of this storm system, and if models are retaining this bias, it's quite possible the storm ends up shunted to the south, giving snow to the Ohio Valley and Northeast.

Tropical Tidbits

The image above shows the projection for 500mb geopotential height values (colored regions) and MSLP values (contour lines) for December 29th over the West Pacific. We see a low pressure system has formed on the southeastern coast of Japan, and is moving east-northeastward along the coast. As time progresses, this low will eventually shift out to sea and away from Japan.

Our Christmas Eve storm was expressed well by this Typhoon Rule application, and this storm looks rather similar to the track the storm took in Japan preceding the Christmas Eve event. The only difference here is this time around, the low pressure system drifts out to sea instead of cutting up the eastern coast of Japan. This is a huge red flag, and tells us that this storm may very well go further south and east than current model guidance is suggesting. Given the success this predictor has had in the past few years I've used it, there is reason to believe this system could go further south and east. 

For now, I'll set my sights on this storm tracking to the south, more in line with what the GEM/GFS-Parallel are showing. The GFS model, in addition to being too far north, may be too quick with the storm, so other guidance appears to be good to use at this time. The combination of potential model error, as well as expected high pressure issues, in addition to the Typhoon Rule, is too much for me to put faith in the northern solution. The GEM snowfall prediction is shown below (this event's snowfall would be through the upper Ohio Valley into the Northeast).

Tropical Tidbits

To summarize:

- Model guidance is now confirming the possibility of a storm in the January 3-5 timeframe.
- A southward track is expected at this time, with snow hitting the upper Ohio Valley and Northeast.
- Substantial uncertainty still exists.

Andrew

January 3-5 Potential Winter Storm

It's looking like the next threat of a winter storm will arrive in time for the first few days of the New Year.

TwisterData

On December 31st, a strong trough is expected to drop into the West US and close off, as the circular 500mb geopotential height contours show above. This cyclone closing off in the West slowly drifts eastward, and that's the first thing we must watch for. A common model bias involves guidance closing off these troughs too quickly. For those located in the Plains and Midwest hoping for a snowstorm, you want this trough to close off quickly. A later close-off of the trough would result in a further south track, which might be in the cards of this is an instance of that model bias. For now, however, the trend has been to close off the energy earlier and earlier.

Tropical Tidbits

This image shows the projected 500mb vorticity values for January 3rd, the same type of chart as above. Off of the latest GFS model projection, we can see the energy now located in Indiana, taking on a negative tilt, as the contours seem to be "pushing" in a southeast-ward direction. When the system initially exits the Southwest, it will have a positive tilt. In order for those north of Kentucky to have a shot at a good snowstorm, this storm needs to attain a negative tilt rather quickly, as the GFS has done above.

Tropical Tidbits

The 500mb geopotential height anomaly chart off the ECMWF, valid for January 3rd, shows a similar situation as the GFS is predicting. We see our energy in southern Minnesota and northern Arkansas attaining a negative tilt. Although I don't have access to the smaller-interval charts from the ECMWF, it's not a stretch to assume this storm takes a similar path as the GFS model, as well as the Canadian GGEM model (not shown).

There are a few, rather significant items standing in the way of this being a 'likely' winter storm for the Midwest.

Tropical Tidbits

The image above shows the projection for 500mb geopotential height values (colored regions) and MSLP values (contour lines) for December 29th over the West Pacific. We see a low pressure system has formed on the southeastern coast of Japan, and is moving east-northeastward along the coast. As time progresses, this low will eventually shift out to sea and away from Japan.

Our Christmas Eve storm was expressed well by this Typhoon Rule application, and this storm looks rather similar to the track the storm took in Japan preceding the Christmas Eve event. The only difference here is this time around, the low pressure system drifts out to sea instead of cutting up the eastern coast of Japan. This is a huge red flag, and tells us that this storm may very well go further south and east than current model guidance is suggesting. Given the success this predictor has had in the past few years I've used it, there is reason to believe this system could go further south and east.

Tropical Tidbits

Also of concern is that massive Arctic high pressure system to the north in Canada, pressing southward into the United States. It is well known that model guidance has a bias to be too slow with the progression of Arctic high pressure systems southward. Applying this to our storm here, this means that model guidance might be forecasting this low pressure system to be too far north. Consequentially, this would result in the storm going further south and east.

When you shed all the more 'uncertain' parts to this forecast, you end up with a model consensus favoring a northward track, and the Typhoon Rule in favor of a southward track. Usually, I would go in favor of the Typhoon Rule and predict a further south track, but we have a strong streak of mid-level winds rounding the base of the trough on the latest GFS, which is a big signal for a storm to go negative-tilt, and in this case, go north. Add up all the model biases and predicted phasing of the subtropical & polar jet streams for this event, as well as a suspicious timeframe for the system in Japan, and there's too much uncertainty to decide on one particular track at this time.

To summarize:

- There is the potential for a winter storm to impact the Central and Eastern United States between January 3-5 (good confidence).
- There is too much uncertainty to tell whether this storm will hit the Midwest and Great Lakes, or stay south in the southern Ohio Valley.
- With good Arctic air already in place prior to this storm, accumulating snowfall is a good bet at this time.

Andrew

Special Forecast Discussion - High-Impact Christmas Snowstorm

This is a special forecast discussion concerning the potential for a high-impact Christmas snowstorm.

Discussion prepared Monday (12/22/14), 22z.
Purpose of discussion: To evaluate the increasing threat of heavy snow immediately prior to Christmas.

Prognosis

OWL

The attached image shows 500mb vorticity values over the United States from the American GFS model, valid on Christmas Eve morning. Model projections see a strong trough anchored over the Central US beginning to tilt more neutrally, eventually negatively as the storm pushes east. As the storm strengthens and the trough continues to attempt a negative tilt, the low will shoot northward, bringing the potential for heavy snow across a portion of the western Great Lakes and Midwest. Recent model guidance has favored a slight speeding up in the storm's progression into the Central US, as well as a strengthening of the trough as a whole. This, in turn, leads to a more westward track.

Meteocentre

Global model guidance is coming into agreement on this westward track, as ensemble projections finally line up with operational model projections (something that had been listed as a red flag in this morning's special discussion). The thin red lines show individual Canadian model ensemble members, while the blue lines depict individual ensemble members from the American model. The dark blue line signifies the ensemble control forecast off the American suite, while the dark red line does the same for the Canadian suite. All in all, it is clear that model guidance has converged on what may be our final solution. This solution is split into two potential tracks.

The first track is favored by most of the Canadian ensemble members and a slight majority of the American members, and brings the storm through western Indiana and western Michigan. This, in turn, creates an environment favorable for accumulating snow in north-central to north-east Illinois, as well as a bit in southeast Wisconsin. Accumulations on the order of 6" may be anywhere from isolated to widespread, depending on how the warm layer projected to be present a few thousand feet off the ground reacts.
The second track, agreed upon by the remainder of American ensemble members and high-resolution NAM & regional GEM model guidance, takes the low through eastern Illinois. This drops significant snows (on the order of 12"+) on western Illinois to southern Wisconsin, in addition to eastern Iowa. This solution is observed in the latest high-resolution NAM model run below:

PSU

I don't have an image to illustrate the first track I described, but you can cut totals above in half and shift the entire band southeast to end just north of Chicago to get a good idea of what that solution entails.

Caveats to Forecast
Model disagreement on track remains a primary concern, though this concern has been somewhat alleviated in recent model runs. Placement of the low to a westward extent is rapidly becoming an important caveat, as the two solutions described above show.

Prediction
Current prediction from my end is we will see Solution 1/the first track verify, probably nudged west a bit. I feel high resolution guidance and other projections on the western edge of the envelope are too far displaced, especially considering this high resolution guidance is notorious for over-strengthening storms. Also can find reason to lower snow totals from high-res guidance, so that maximum amounts throughout the event may just touch the 10" or 12" benchmark; anything beyond that seems a bit too far-fetched for my liking at this time. NCEP control forecast on the large ensemble image above appears the most reasonable at this time.

Favored Track
My favored track at this time is represented by the pink NCEP ensemble control forecast below.

Meteocentre

Next Update
Another update to this special forecast discussion is not currently required. If one is needed in the short term, the next update can be expected prior to 1:30 AM central time (0630z).

Andrew

Christmas Potential High-Impact Winter Storm

Model guidance is still wreaking havoc on forecaster confidence, meaning this post will be laced with numerous caveats.

Instant Weather Maps

The image above shows the 12z GFS model forecast for snowfall totals from this Christmas storm. That thin swath of snow in northwest Indiana into western Michigan is the accumulation from this storm. That's a far cry from the projections of 6"+ earlier this week, even twenty-four hours ago.

This big change came about late last night, when the energy associated with this system finally crashed into North America, and the weather balloon network could take measurements of the system. This process, called 'sampling', as the weather balloons sample what the system is doing and how strong it is, gives weather models more confidence in a solution, as there are no weather balloon launches out in the Pacific.
At least, that's usually how things go. Late last night, when this first sampling took place, model guidance took a significant jump eastward and greatly weakened the overall storm, bringing about minimal snowfall accumulations. The theory among forecasters was that this would be the final solution.

But the drama appears far from over.

Instant Weather Maps

The above image shows the 18z GFS forecast for snow from this storm, the most recent forecast we have to work with. In this projection, the storm jumped west again, like model guidance was favoring about twenty-four hours ago. In this forecast, we see amounts generally of 3-6" draped across east Missouri into west and northern Illinois, with a sliver of southeast Wisconsin getting into the action.

Now's the time when I would give my personal analysis of what will happen here. But to be frank, I have no clue. This is one of the worst performances of model consistency I've seen in my five years of forecasting, if not the worst performance. I'm preparing to go about this storm on a now-casting basis, where the forecast is made as the storm actually happens.

For now, most of the population living in St. Louis on eastward will likely have to contend with travel difficulties, as this storm system still looks to be a strong one. Snow forecasts are useless at this juncture, as it's possible people could get a foot, or could get nothing. On top of that, location of this snow is nearly-impossible to predict at this point in time.

Andrew

Christmas Eve Potentially Significant Snowstorm

Model guidance continues to struggle on the concept of a winter storm around Christmas Eve.

Tropical Tidbits
Click images to enlarge

The image above shows the ECMWF model's projection of 500mb geopotential height values (shaded colors) and mean sea level pressure values (contours) for the morning of Christmas Eve. On this image, we see a deepening low pressure system pushing northward into western Ohio, as a strong trough (seen by the blue colors) begins to negatively tilt (seen in this image as those blue colors "pointing" southeastward) and drag the storm northward.

This forecast solution is one of (quite literally) tens of ideas for this storm that have come out of recent model runs, so individual model projections are being weighted unusually low, due to unusually low confidence. Though I don't have the maps to confirm, this projection from the ECMWF would likely deliver a hefty snowstorm, with strong winds, to portions of the Upper Midwest and Great Lakes, particularly Michigan.

Tropical Tidbits

We now move on to the Canadian GEM model's projection, once again showing 500mb geopotential height values and MSLP contours, and once again valid for the morning of Christmas Eve. In this forecast, the storm system in question becomes negatively tilted quicker than the ECMWF model, which then leads to this more westward solution. Here, the GEM has the storm in central / west-central Indiana, instead of the ECMWF's west-Ohio idea.

Tropical Tidbits

As a consequence of this change, the GEM places a swath of accumulating snow in a north-south orientation, from eastern Wisconsin down through Illinois, barely scraping far western Kentucky. Additional snowfall hits Ohio and Indiana, but the amounts in Illinois are the most significant, where totals surpass the 6" benchmark. Recent model guidance has been looking more into this westward shift, but there's no consensus at this time.

Tropical Tidbits

The last model projection we can analyze, owing to an ongoing massive NOAA data outage, is the Parallel GFS model. Once again, the forecast shows 500mb geopotential height and MSLP values, and this projection is once again valid on the morning of Christmas Eve. This forecast is similar to that of the GEM model, in that we see quicker strengthening of the storm system, leading to a more westward solution.

It should be noted that ensemble guidance is a bit further east of these western tracks.

All in all, we have a very difficult forecast still evolving, which might not be resolved for another few days. For now, those across the entire Central and Eastern US should be prepared for potentially significant weather, especially if you are traveling immediately prior to, or after Christmas.

To summarize:

- Model guidance continues to suggest a powerful storm moving through the Central/Eastern US around Christmas Eve.
- Guidance remains very inconsistent, with little to no consensus currently built around a certain track.
- Regardless of where this storm tracks, significant and adverse weather remains possible, if not likely.
- Travel plans may need to be re-evaluated.

Andrew

Christmas Eve Potentially Significant Winter Storm

I'm watching the increasing potential for a significant winter storm in the Central US, created by what could be one of the strongest low pressure systems in the last few years, if not longer.

Instant Weather Maps

The above image shows 500mb geopotential height contours over the United States from the prestigious ECMWF model, valid for December 23rd. In this image, we see two pieces of energy in the Central US. One is a closed low, placed in the Northern Plains, while the other is a deepening trough in the Southern Plains, shown by the depression in contour lines. In a situation similar to that I described in my December 10th post, we see this trough begin to lift northward into the Central US, but not before the "bomb" goes off...

Instant Weather Maps

Just 24 hours later, on the morning of Christmas Eve, we find that the two pieces of energy have combined, and the trough as a whole has now attained a negative tilt, indicating it has reached its mature phase. As a result, the storm undergoes rapid strengthening, very near the criteria of bombogenesis. Bombogenesis is a meteorological term used to describe the phenomenon when extratropical cyclones rapidly strengthen, and their minimum central pressure values decrease by 24 millibars in a 24 hour period. Switching between December 23rd and 24th, we find that pressure values from the ECMWF lower by about 19.3 millibars, only a little ways off from being a true 'bomb'. Even though this storm doesn't fit the criteria, the strengthening is nothing to shake a stick at.

This solution is a tricky one. I don't have access to pay-to-view weather model graphics (yet), so I cannot see the precipitation pattern from this system. However, from what others are discussing, it appears that not only will the nearly-due-northward movement of this system foul up the precipitation shield, but temperature profiles are above freezing in many spots that would otherwise see snow. This could be placed on model error, or it may be a legitimate forecast. For now, it's just too far out to tell one way or another.

Weather Online

What is interesting, however, is the difference between the ECMWF model and the ECMWF ensembles. The above image shows mean sea level pressure values, valid on Christmas Eve (the same time as the second image we discussed). By simple comparison, note how the ECMWF model places the center of this storm somewhere in southern Ohio, while the ECMWF ensembles put northeastern Indiana in the center of this cyclone.

A further east track of this storm system could result in more of a snow impact to the Great Lakes, instead of primarily wind-driven snow in the Midwest and Ohio Valley from current runs of the ECMWF. But as I said earlier, we are still quite a while away from nailing down these details.

To summarize:

- Model guidance is beginning to sniff out a very strong storm system impacting most of the Central and East US on Christmas Eve into Christmas Day.
- This would severely impact travel.
- Snow would be confined primarily to the north-central Great Lakes into Canada.
- Very high uncertainty still exists.

Andrew

December 22-26/Christmas Potential Winter Storm

I'm watching the potential for a winter storm in the December 22-26 timeframe.

Tropical Tidbits

The image above shows the forecasted 500mb vorticity values on the morning of December 16th, valid over the West Pacific. In this graphic, we see a negatively-tilted upper level low pushing east into Japan, as the elevated vorticity values indicate. We can observe the negative tilt by the height contours seeming to "dig" in a southeast-ward direction. When a trough/storm system is at its mature stage, usually its strongest stage in a storm's life, the storm is said to be negatively tilted.

Using the Typhoon Rule, which states weather phenomena occurring over Japan is reciprocated in the United States 6-10 days later, we might expect this storm to impact the United States in a December 22-26 timeframe... possibly right around Christmas.

But there's more to this story.

Tropical Tidbits

Take a look at this image, forecasting 500mb geopotential height values in colored shadings, as well as mean sea level pressure (MSLP) values in the contoured lines. In this image, valid just six hours prior to the time period in the first image we analyzed, we see a strong storm system in the Sea of Japan, correlated to the strong upper level low.

But wait... there's something else there.

Check out that sagging of contour lines just south of Japan, right where the arrow is pointing. It almost looks like another low pressure system! Other model guidance confirms this idea of a second body of low pressure forming south of Japan and skirting the nation to the east before phasing with the very strong storm in the Sea of Japan. The implications here could be huge.

If that really is another low pressure system, then we have a very interesting scenario on our hands for Christmas. We can't tell for sure just yet, but the storm shooting northward to the east of Japan could mean a few possibilities.
Let's first hypothesize that the item outlined above is indeed a storm system. The track to the east of Japan could either mean we would be looking at:

- A Panhandle Hook storm, where the system shoots north from the Southern Plains. These storms are climatologically favored to bring heavy snow to cities in the east-central Plains and Lower Great Lakes. This scenario is a possibility, as that strong storm in the Sea of Japan would likely correlate to a strong North Plains cyclone. This would keep that body of low pressure east of Japan in an area close-by, as the storms would eventually phase (not to mention low pressure areas are attracted to other low pressure areas).

- An East Coast storm. Because this body of low pressure is forecasted to skirt the eastern side of Japan, this could be a plausible scenario. We won't know if either of these are correct until we have more model runs to access.

ESRL

Long range ensemble model guidance for the evening of December 23rd shows a relatively favorable pattern for a storm. We see strong ridging high pressure in the West Coast of North America, but another swath of high pressure also looks to exist in southern Canada. This could boost temperatures, or worse, suppress and kill the storm system altogether. Even though this is quite far out and should not be taken at face value, those in the Midwest looking for a winter storm should feel rather good from what model guidance is projecting:

Tropical Tidbits

This image from the GFS ensembles shows what I'm talking about. In this image, also valid for the evening of December 23rd, we see a few factors at work here. A strong positive PNA is currently agreed upon by these ensembles, which permits a strong ridge to form in the Western US. This ridge then allows a colder pattern to develop in the Central US, as the jet stream drops southward to accommodate the +PNA ridge. Sweetening the pot is evidence of a very slight ridge in the Southeast, something that could allow any potential storms to move more northward. That final piece to the puzzle will be a bit more difficult to come by, as that strong upper level low over Greenland will try to keep the flow over North America very zonal (west-to-east, jet stream not wavy). There's still, of course, plenty of time for this to change.

To summarize:

- A winter storm may be in the cards for December 22-26th, likely impacting Christmas travel plans.
- A second storm system may need to be watched for the Northern Plains.
- The primary threat here may become a storm favorable for heavy snow, either in the Central/East US (ideally the Ohio Valley/Midwest) or along the Eastern Seaboard.
- Relatively low confidence still exists due to the long-range nature of this threat.

Andrew