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

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 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

Long Range Outlook: Cold, Stormy Period Expected in Mid-December

It's looking like a cold and stormy pattern will overtake the US for the mid-December timeframe.

Tropical Tidbits

The image above shows 500mb height anomalies over the Western Pacific on the evening of December 1st. In this image, we see what is very likely a lobe of the polar vortex passing just north of Japan, occluding and cutting north as it does so. Minimum values here at the 500mb level in this image, off the GFS ensembles, appear to drop close to 490 dam, an indication of either an incredibly strong upper level low, or the more-likely polar vortex lobe.
This situation appears similar to the case we had last winter, where we saw a very strong upper level low crash into Japan to round out the year 2013, which then led to incredibly cold weather in the first week of the new year.

ESRL
December 27, 2013

If we recall the Typhoon Rule, which states weather phenomena occurring over East Asia is reciprocated in the United States about 6-10 days later, we could expect a bout of  substantial cold here at home in the December 7-11 timeframe. Should this verify, which is becoming increasingly likely given model consistency, the Central and parts of the East US may expect a bout of intense Arctic air to hit in the middle of December.

But it doesn't end there...

Tropical Tidbits

The image above once again shows 500mb height anomalies from the GFS ensembles, now valid on December 6th. Notice that we still see below-normal heights over Japan. This means Japan has been under a strong upper level low's influence for nearly a week, and still going. Building off of the first graphic, it's quite possible that the United States may also undergo a harsh beating from cold weather for nearly a week straight. The degree of this cold air is still under examination, but if it's anything like the upper level low expected to trek north of Japan to open December, watch out. 

JMA

The stratosphere is also working in conjunction with this strong upper level low over Japan. The image above shows the last week or so of temperature anomalies over the North Hemisphere at the 10 millibar level.

When we look at the final days of November, we find ourselves on the tail end of a stratospheric warming event, as the pink colors show in the boxes labeled November 24th through 26th. When we factor in that stratospheric warming events typically reflect in the troposphere about 2-4 weeks after the warming event, evidence for a synoptic cold weather outbreak strengthens for the early-mid stages of December, only enhanced by this Typhoon Rule component.

From here, the waters get a little murky.

CPC

The image above shows the projected phase space for the Madden-Julian Oscillation (MJO) off the GFS Ensembles. Each triangle-esque shape represents a specific phase of the MJO, with the phases number 1 through 8 on the exterior of each shape. Typically, in the winter, cold weather is favored in the Central/East US when the MJO is in Phases 7, 8, 1 and 2, while warmer weather prevails in the remaining stages. The GFS Ensembles predict the MJO to translate eastward with time, shown well by the quick movement through Phases 4, 5, 6, and 7, arriving in Phase 8 around the mid-December timeframe. Hey, isn't that when we're supposed to get a cold blast? Yep, and the MJO moving into Phase 8 only re-assures this outlook.

But this time, things aren't as clear.

CPC

The graphic above shows the same Madden-Julian Oscillation forecast as the GFS Ensemble one above, but this one now shows the forecast from the prestigious European model, abbreviated as the ECMWF ensembles. These ensembles take the MJO through Phases 4, 5, and 6 rather quickly, but then the MJO event weakens, as ensembles minimize strength into that circle in the center, which indicates the MJO event is too weak to be put into a certain phase.
Big difference from the GFS ensembles, right? If this ECMWF forecast verified, the chances of a chilly mid-December would likely be on the decline. Of course, that East Asian component would play a role, but this ECMWF forecast could very well dampen the cold in areas.

So who do we believe? For now, we see the United Kingdom's UKMET and Japan's JMA model guidance supporting a GFS ensemble-like solution, where the MJO would move into those latter phases supportive of a cold weather event. I'll take a middle ground and stick with a weaker version of the GFS ensembles, but still keeping an eye on a full-on GFS ensembles solution.

Kyle Griffin

Now this is where the fun begins...

The image above shows the GFS model's forecasted jet stream wind speeds (colored regions), mean sea level pressure (black contours), and 1000-500mb thickness values (dashed red and blue lines).

WxMaps

The forecast s valid for the morning of December 7th, and tells us a lot about the coming weather pattern. On the right (if I'm aligning this post correctly), you'll see the current jet stream pattern laid out across the Northern Hemisphere. Notice the ragged jet stream in the Pacific, but there's a very strong ribbon of the jet crossing Eurasia and currently pushing into Japan. The model guidance above indicates this strong jet will shove off mainland Asia in a matter of days, overpowering the Pacific basin and shunting moisture towards North America. As this happens, and we see extratropical cyclone formation on the surface (as indicated by the tight black SLP contours in the image above), pressures will lower just west of the Gulf of Alaska, even into the basin itself. In response, high pressure will form along the West Coast, which then in turn creates a trough in the East US, and suddenly you've got yourself a cold start to the second full week of December. This brings the Typhoon Rule, stratospheric, and MJO concepts full-circle to favor a cold weather pattern for the December 7-17 period.

The 'fun' part is that moisture I mentioned earlier in that paragraph. As the jet forces that moisture east, it will dive into North America on the heels of a powerful jet stream. With aforementioned high pressure in the West and low pressure in the East, conditions look to be ripe for storm formation in the Central US in this December 7-17 timeframe. Depending on how the atmosphere unfolds closer to this event, it could end up being a decent winter storm.

ESRL

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA and NAO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.
 The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

Looking at the forecast at hand, the PNA is expected to spike positive and stay positive throughout the extended range. This comes as that jet stream makes its way into the Pacific, and resultant stormy weather in the Gulf of Alaska forces high pressure to form in the West US. The NAO, however, will remain positive. This could be a bit problematic. If we see a positive NAO and positive Arctic Oscillation (AO; +AO supports warm weather, -AO supports cold weather), as forecasts currently indicate, we may also see this cold weather set-up slip away. I'm not sold on that happening yet, and in addition to all the content above, here's why.

The image to the right shows the Climate Prediction Center's verification of Arctic Oscillation forecasts. From the long-long range forecasts on the bottom panel, to observed AO values for the same time on the top panel, we can see what, if any, biases models are retaining with the AO. If you can't read this sort of chart well (I don't blame you, this one's a bit tricky to maneuver), it can be summarized by saying model guidances tends to forecast the AO to be positive too often; it ends up negative more often than not in those cases. That's not to say this time around will see the positive forecasts drop, but it must be noted that model guidance is too "trigger-happy" about forecasting a positive Arctic Oscillation event. If the same holds true for this mid-December time period, we'd likely see a handful of colder outlooks.




To Summarize:

- A warm start to December is expected (December 1-7).
- A predominantly-cold weather event is anticipated in the middle portion of December (December 7-17).
- A storm system (possibly more than one) could be seen in this December 7-17 time period.
- Following a brief warm spell in the mid-late December period, things could turn stormy again in time for Christmas.

Andrew

December 2-6 Potential Winter Storm

Another winter storm threat is on the horizon.

Meteostar

The image above shows forecasted 500mb vorticity values with superimposed 500mb height contours over Japan for November 26th, off the GFS model. In this graphic, we see a depression in the contours over Japan, as well as a swath of red positive vorticity values,  indicating the presence of a storm system. Using the Typhoon Rule, which states weather phenomena in Japan is reciprocated in the United States about 6-10 days later, we might expect a storm system to trek across the US in a December 2-6 timeframe.

PSU

The ECMWF model is already hinting at this potential storm, as the above frame shows a positively-tilted trough entering the Central Midwest on December 4th and 5th. The extent and intensity of snowfall remains in question, but for now, the possibility is certainly present.

Andrew

Thanksgiving Potential Coastal Snowstorm

It looks like the Northeast will see a pretty white Thanksgiving this year, as a coastal storm will lay down plowable snowfall.

Instant Weather Maps

The image above shows accumulated snowfall from the GFS model, valid over the next five days. We see a swath of snowfall across the Northeast region, primarily coastal areas, as the storm will be located relatively offshore, restricting snowfall. This forecast lays down a general 3-5" of snow for many on the coast, with spots of 6" and slightly above for a handful of areas. Given that this is November, warm air immediately near the coast will be acting to put a damper on any intense snow activities, something that will have to wait until later in the season.

Instant Weather Maps

The NAM model, a shorter-range forecast, puts down a similar 3-5" swath of snow, but enhances the coastal areas with 6-8" of snow, as the pink area shows. Far southeastern parts of Maine even look to see a shot at near 10" of snowfall, if not a bit more.

IWM

Peak storm time looks to be on November 26th, intensifying in New England that evening. Regardless of how this storm ends up, millions in the East US will have to contend with a very messy Thanksgiving commute.

Andrew

Thanksgiving Potentially Strong Storm System

I'm still watching for the threat of a Thanksgiving winter storm, this time with renewed caution concerning its evolution. Please read here for the discussion on how this potential came about in the first place.

PSU

We'll begin with the GFS model forecast for November 27th. The top left panel shows 500mb vorticity values in shaded colors, as well as 500mb height contours superimposed. The top-left image depicts 1000-500mb thickness values (people north of the red dashed '540' line generally can expect snow) as well as sea level pressure. The bottom-left panel gives an indication of low-level relative humidity, basically a measure of moisture in the air, while forecasted precipitation is on the bottom right in conjunction with 850mb temperature values. Glancing over this image, we see a significant storm evolving in the East US, with the combined energy of a trough in the southern jet stream, as well as a potent clipper system dropping down from Canada. These energies combine into one strong trough, traveling northeast and dropping intense snowfall along the East Coast. I'll provide analysis of this scenario after the GEM model, what I'm doing now is explaining what it shows.

PSU

The GEM model is different. Using the same definitions for each panel in this graphic as the GFS model, we see the storm is separated into two pieces of energy. We see our primary southern-stream storm in the Southeast US with the expansive precipitation shield, but that clipper is now located to the northwest, not conjoined with the Southeast-US storm. As a result, we see accumulating snow strike the Midwest and Plains, putting down precipitation amounts as follows.

Meteocentre

This image shows snowfall accumulation in its liquid-equivalent from right now to 10 days from today, but the clipper's snowfall is shown by the swath of greens and light yellows stretching from the Dakota into Indiana and Ohio. Doing a quick conversion tells us 10 millimeters is equal to 0.39 inches, 15mm is 0.59 inches, and so forth. At face value, the GEM would be kicking out a good 4-9" snowstorm from this clipper, with the highest amounts in those yellow shadings. With higher ratios taken into account, we would probably be facing a 5-10"+ snowstorm. Since the GEM is notorious for exaggerating snowfall amounts, this isn't something to hang your hat on. However, it does give breadth to the idea of two solutions to this storm.

For our model analysis, I want to focus on the problems with the GFS model. I've mentioned a handful of times that the GFS model (and most models in general) are prone to a progressive bias. This means that forecasts will move storms along quicker than they should, which might be leading to that merging of the clipper and southern-stream storm on the GFS. Dropping/correcting this bias might lead to a forecast not unlike the GEM model.

For now, purely due to uncertainty, I'm not willing to side with one model over the other. However, keeping in mind this bias which does appear to be showing itself in the GFS model (for now), a solution similar to the GEM forecast might be expected, something that could easily change in coming updates.

Andrew

November 23-25 Significant Storm System

A strong storm system looks to impact the US in the November 23-25 period.

Tropical Tidbits

The image above shows the temperature forecast for 5,000 feet off the ground, as forecasted by the ECMWF model, valid on November 24th. In this image, we see a storm system of 973 millibar strength shooting northward into Wisconsin, surrounded by above-freezing air temperatures on all sides. Pure observation of this chart tells us there won't be that much of a snowy side, but given the potential for model guidance to cool down as the forecast grows colder, or more likely yet, the precipitation shield does extend into the cold sector, snowfall may still occur. This snow would not be significant, at least according to this forecast, but the storm itself would be.

Tropical Tidbits

The GFS model gives a very similar story to the ECMWF. We see a 973 millibar low pressure system over the Upper Peninsula of Michigan on November 24th, but now with significantly more cold air on the western fringe of the storm. This appears to have happened as the storm wrapped itself up and occluded, pulling all that Arctic air to the south (this will have significant implications for the Thanksgiving storm, which we will discuss tomorrow (Thursday) afternoon).

Tropical Tidbits

That GFS forecast does lay down some hefty snows in the Upper Midwest, where amounts of 6-12" may be seen. The heaviest snow appears in northern Minnesota into Canada, where amounts closer to the 2' mark may be anticipated. However, it remains to be seen if this solution will win out against the ECMWF, or vice versa.

To summarize:

- Model guidance favors a very strong storm system moving into the Upper Midwest by the start of next workweek.
- Some model guidance favors heavy snow in the far northern US, while other guidance keeps this a rain/ possible severe storm event.

Andrew

November 22-25 Potentially Significant Winter Storm

A powerful storm looks to present itself to the US around the November 22-25 timeframe.

Tropical Tidbits

Since this post will primarily be a model analysis, we'll begin with the ECMWF-Ensembles. This image shows the forecasted 500mb geopotential height values in the color shadings, with sea level pressure contours and high/low pressure demarcations superimposed. In this graphic, valid for the evening of November 24th, we see a storm system placed on the border of northeast Illinois into northwest Indiana. The minimum pressure is about 1003 millibars, which is a rather weak storm system. Despite its weakness, the mere presence of a storm on this graphic tells us there is some confidence in a storm occurring within this timeframe. Confidence is low to begin with, but it is there.

Tropical Tidbits

We now move on to the ECMWF model, also showing 500mb geopotential height values and SLP contours. For future reference, those two parameters will show up on all model graphics we analyze here today. The ECMWF model favors a very strong 987 millibar storm striking central Illinois on the morning of November 24th, with windy conditions overtaking the Ohio Valley and East US within the storm's warm sector. The sub-540 geopotential height values in the Midwest tell us that a widespread accumulating snow event would likely occur, particularly in the central and northern Plains into the Upper Midwest.

Tropical Tidbits

Pushing ahead, we now analyze the GFS ensembles forecast, valid here for the evening of November 24th. We find a weak low pressure system over central Lake Michigan, with a minimum central pressure reading of about 1005 millibars. Ensemble systems typically tend to be weaker than their operational counterparts (as observed with the ECMWF-Ensemble and ECMWF graphics above), since the ensembles take into account double-digit forecasts and average them all out. Regardless, the mere presence of a storm system is re-assuring to confidence.

Tropical Tidbits

Continuing on, we arrive at the GEM model forecast, valid on the evening of November 24th. The GEM, made by the Canadian meteorological service, shows a deep low pressure system of about 995 millibars right over Chicago, Illinois. This would support some stormy activity from the Southern Plains to the East Coast, but due to the retracted 540 line into the far Northern Plains, I'm not confident that this model is supporting a snowstorm for the Upper Midwest and Central/Northern Plains.

Tropical Tidbits

We now arrive at the GFS Ensembles, which paints an interesting picture for this storm. We see a storm system of minimum central pressure 1001 millibars placed just south of Chicago. This graphic, valid on the evening of November 24th, also draws the 540 line south into the Midwest and Central Plains. Again, since this is an ensemble forecast, the storm is not as strong as individual model forecasts. However, since it is showing up in the first place, confidence continues to rise in a substantial storm in this timeframe.

Tropical Tidbits

We've saved the best for last: The above graphic depicts the GFS-Parallel model forecast for this storm. We see a 979 millibar storm striking the northeast Illinois-southeast Wisconsin border, while 500mb geopotential height color shadings tell this storm to be a closed low. With very windy and cold conditions extending across the Midwest and Great Lakes, an accumulating snow event would likely be expected in the Upper Midwest and Northern Plains.


To summarize:

- Model guidance is confirming the possibility of a substantial storm in the November 22-25 timeframe.
- Some guidance supports a powerful storm system, possibly bringing both accumulating snow to the Upper Midwest, and severe weather to the East/Southeast.
- Confidence in this solution remains low due to the extended timeframe of this storm.

Additional updates will be posted in the coming week as more information becomes available.

Andrew