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

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

Thanksgiving Potentially Significant Storm System

Model guidance is now hinting at this Thanksgiving storm system to impact the Central and East US.

PSU

Beginning with the ECMWF model, with this forecast going out to 10 days, we see a deep trough pushing into the Southern Plains, neutrally-tilted, as the isobars pushing due south show. Note how pressure tendencies have rotated to the southeast corner of the vort max, likely telling us that this trough will start its maturing phase into a negatively-tilted trough in the next few days, if this model forecast were to go out further. A better description of tilted troughs is below from theweatherprediction.com .

The Weather Prediction

Given the freezing line / 540 thickness line in the top-right image being pulled all the way to the Southern Plains on that ECMWF graphic, as well as the expected negative tilt to this trough, the going ECMWF forecast would likey deliver severe weather to the Southeast, as well as some snow to those in the Great Lakes and southern Midwest, with exact areas to be determined.

Tropical Tidbits

Moving ahead to the GFS-Parallel forecast for the same November 24th timeframe as that ECMWF graphic, we see a very similar forecast. Once again, a strong trough is located in the Southern Plains, as shown by the deep negative height anomalies centered over Texas. If you guessed this trough was neutrally-tilted, you are correct! The height contours seem to be 'pushing' due south, which means it's neither positive or negatively-tilted. Unfortunately, I don't have access to the jet stream forecast for this timeframe from the GFS-Parallel model, which means we cannot tell if this trough is preparing to tilt negatively or not.

We don't have much to work with right now, since the storm's still about 10 days away, so here's a few graphics of precipitation forecasts from the GFS and GFS-Parallel model with this storm.

Tropical Tidbits

Tropical Tidbits

Andrew 

November 25-29 Potentially Significant Winter Storm

The storm system around the November 25-29/Thanksgiving timeframe continues to look like a significant storm system.

OPC

On the morning of November 8th, we saw the remnants of Typhoon Nuri reach peak strength via Ocean Prediction Center observation. As the chart shows here, the massive storm bottomed out at 924 millibars. This means the storm, located at about 170E and 55N, didn't break the record for strongest extratropical cyclone on record, but it certainly came close.

NWS

The graphic above, produced by the National Weather Service in Alaska, shows the observed mean sea level pressure of the storm at that 924mb reading, while the NWS office measured it at 930mb on the chart on the left, where strongest extratropical cyclones in the North Pacific are depicted. For multiple reasons, including the possibility that past storms may have been stronger than that 924mb reading, this storm was not declared the strongest on record in the North Pacific. Despite this, it's quite clear this storm was a historically-strong one, relative to storms in the last 60 or so years.

NOAA

Purely for comparison purposes, the team at KOPN Weather identified a strong storm in the Bering Sea on April 7th, 2011, bottoming out at 936mb, that was about 10 degrees W of where this very strong storm was observed yesterday in the Bering Sea. If you recall what happened about 2-3 weeks after the date of April 7th, we saw a certain tornado outbreak strike the South US, devastating thousands across the country, and causing millions on millions of dollars of damage. If we look at where the resultant storm ended up in mid-late April, utilizing the Bering Sea Rule, we find the storm situated in the Ohio Valley.

WPC

Now, keeping in mind that this sort of correlation is a tough one to use at best, not to mention all the caveats associated with long range forecasting, we could theoretically juxtapose the remnants of Typhoon Nuri and this storm in the Bering Sea on April 7th to get an idea of where the consequential storm in the US may end up. Recalling that the storm in 2011 in the Bering Sea was at about 180 degrees longitude, and almost the exact same latitude as the one observed in the last day or two, we find the remnants of Nuri placed about 10 degrees west of that 2011 storm. If we take the location of that storm system in late April (pictured above) and move it west, like the remnants of Nuri were west of that 2011 Bering Sea storm, we end up with a map like this:

WPC

Continuing this correlation, just to see what would happen, we notice that the remnants of Typhoon Nuri are moving eastward (a bit northeast in the process) in the Bering Sea right now, slowly at that. If this storm somehow does end up in that potential location outlined above, and if enough cold air is available (this will be discussed later), a significant winter weather event may strike the Central Plains, Midwest, Great Lakes, and Ohio Valley. Similarly, if the correlation works out, a severe weather event may strike the South US. Again, many caveats are associated with this method, and this should not be taken as "gospel", or at face value.

Tropical Tidbits

The above image shows temperature anomalies at the 850 millibar level (about 5,000 feet off the ground) over North America, as forecasted by the ECMWF ensembles ten days from today. In this image, we see a large swath of warmer than normal temperatures in the Bering Sea, with colder than normal conditions encompassing much of the United States and southern Canada. This looks to be a persistent pattern in coming days and weeks, as a large block of high pressure looks to set up shop directly over the Arctic, providing for a very cold period for North America. Extrapolating this to Thanksgiving, enough cold air should be in place for at least a modest threat of a significant snow event. Again, bear in mind long range caveats, but such a prognosis is favored right now.

To summarize:

- A potentially significant storm system still looks to evolve in the United States around Thanksgiving.
- Severe weather will be a possibility, namely in the South US.
- Significant snow will be a possibility, predominantly in the Central Plains, Midwest, Ohio Valley, and Great Lakes (for now).
- Thanksgiving travel may be severely hampered by this storm, if it does come to fruition as currently projected.

Andrew

November 8-12 Potentially Significant Winter Storm

Model guidance is in agreement of a strong winter storm traversing the Central US. As a result, this storm's title has been upped to a 'Potentially Significant' storm.

InstantWeatherMaps

The image above shows the most recent GFS model forecast for the next 120 hours' snowfall accumulation. In this map, we see the winter storm pounding parts of northern Iowa, southern Wisconsin and northern Illinois with over 6 inches of what would likely be some rather heavy snow, before dropping 4-6 inches in most of Michigan. The GFS model is taking the furthest south track out of major guidance, and we'll discuss that later on. The point is, however, the GFS is favoring a significant storm in the Midwest.

Tropical Tidbits

Next up, we have the GFS-Parallel model. This model is the new version of the GFS, which will be implemented in coming months. It has been rumored that this new model resolves biases in the current GFS model, with more accurate forecasts. This outlook has a swath of 12-18" of snow slamming central Wisconsin into southern Minnesota, with that heavy snow also hitting Michigan. Eyeballing it, maximum accumulation totals look to be on the order of 18-20". Clearly, this model favors a further north, even stronger storm track than the GFS. Let's see what other guidance has to say.

Tropical Tidbits

It just keeps getting worse, I suppose. This is the latest GEM model forecast of snowfall over the next five-ish days. The GEM model, from the Canadian Meteorological Centre (one of the other agencies that also spells the word as 'centre'), is notorious or exaggerating snowfall forecasts, and that is well seen in the image above. Here, we see a bullseye of around 24" of snow (two feet), pounding northern Iowa into Southern Minnesota. Wisconsin sees lesser, but still impressive amounts well above the 12" benchmark. Michigan also observes plowable snow.

We've gone through three model guidance forecasts, and have come to the idea that the GFS-Parallel and GEM support a northern track, while the GFS supports a southern track. Let's throw some ensembles into the mix and see what we can see.

NCEP

This image shows the NCEP (GFS) ensemble forecast of extratropical cyclone tracks in the next handful of days, where each line shows a different track from a different ensemble member. Ignoring the mess in the East Coast, we look around Illinois and see several lines, all very spread out. These spread-out lines, indicating spread out tracks on the upcoming storm, tell us that high uncertainty still remains with this storm. Some ensemble members go south to downstate Illinois and mid-Missouri, while others stay north into southern Wisconsin. We can't really derive anything of significance from here, other than confirming that high uncertainty remains.

For my personal view of the storm, I feel there are two possibilities.

1) Model guidance has a good handle on what's going on right now. Heavy snow will hit central Wisconsin into Minnesota, with accumulations nearing 12", if not slightly more.

2) Model biases come into play, in this case the GEM and GFS-Parallel being too slow with advancing the Arctic air southward. This would push the storm southward as well, resulting in a track similar to the GFS.

For the time being, I'll side with the North Camp of models, just because the GFS seems to be on its own with the more southern track. I'll have more updates as we move closer to the storm.

Andrew 

April 16-17 Potentially Significant Snowstorm

I'm monitoring the potential for a significant snowstorm on April 16-17.

We begin by looking at the GFS model. The GFS model shows a strong storm system dropping down from Canada, taking advantage of the cold air already displaced south in the United States and producing an intense snowstorm in the Upper Midwest. The GFS indicates we may see amounts over 12 inches across northern Wisconsin and upper peninsula of Michigan, with spotty amounts nearing or exceeding 18 inches possible. Given model guidance tendencies to be too snowy/too cold in their forecasts, as well as the fact that models may be overdoing snow because they're stuck in a wintry pattern when we're really transitioning to spring (though the snow falling out my window begs to differ), I'm skeptical of these 18 inch-plus amounts. However, if the storm is as strong as projected, a big snowstorm would be expected. I'm just skeptical of temperature profiles right now.

The NAM model, a short-range, higher-resolution sibling of the GFS model, is actually showing similar amounts. I'm expressing surprise here because the NAM is notorious for over-doing snowfall guidance, in that it can (and frequently does) forecast more snow to fall than what actually does fall. However, comparing the GFS and NAM models, they are pretty similar. While this raises confidence in this outlook, it's nothing to get too excited about. I'm still very skeptical of this event, and I want to emphasize how this has a high 'bust potential' (meaning the forecast has the potential to 'bust', or fail). However, if the forecast ends up to be on target, then I am on board for a big snowstorm. With spring, model guidance starts to struggle due to the pattern changing, and that messes up model guidance. This could be a flash in the pan, or something more.

Shown below is a hand-drawn image of amounts as depicted by the GFS and NAM, for easier interpretation.

To summarize:
• There is the potential for a significant snowstorm in the Upper Midwest on April 16-17.
• Amounts may exceed 12 inches in many places.
• I am skeptical of this forecast, as model guidance may be too cold and may be forecasting too much snow to fall.

Andrew

April 14-15 Potentially Significant Snowstorm

Model guidance is suddenly converging on a potentially significant late-season snowstorm.

The image above shows the most recent forecast from the GFS model, highlighting something I discussed the other day: a potential late-season snowstorm that may drop significant amounts of snow. We had seen the ECMWF model dropping similarly high amounts of snow, as the hand-drawn graphic below shows. Remember, this was yesterday's ECMWF MODEL forecast, NOT my forecast.

Yesterday's ECMWF model snowfall map

While these late-season snowstorms are typically weaker, due to warmer soil temperatures that reduce sticking snow, as well as a genrally unfavorable environment, the Gulf of Mexico can provide a basis for intense amounts of moisture and fuel. If there's enough cold air in place, a strong snowstorm can result.

Tropical Tidbits

There is some merit to the idea that we may see a strong storm system, as the ECMWF is suggesting. Shown above was the observed 500mb height anomaly chart over the Western Pacific on April 6th, where cool colors depict negative height anomalies/stormy weather, and warm colors represent quiet, warm weather. On the morning of April 6th, we saw a trough moving through Japan, possessing a pretty decent strength. This is significant, as it directly relates to what we may see in this April 13-15 timeframe .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. Thus, we can expect a storm in the US on an April 12-16 period, and the April 13-15 ECMWF snowstorm falls right within this timeframe. Additionally, this trough in Japan brought along some pretty cold weather, which COULD contribute to additional chances for snow in this potential storm system. 

The image above shows the projected mean surface pressure by the ECMWF ensembles, as well as the spread among all ECMWF ensemble members in the shown colors for April 14th. We first observe how the ensembles are much less enthusiastic about the idea of a storm system for this timeframe, only bringing a minimum 1004-1002 millibar swath of low pressure over an area from northeast Indiana into southeast Michigan. This sort of weaker projection is to be expected, as these ensemble means take the average of 52 ensemble members, unlike the ECMWF model, which only shows one solution. The next item we observe is the swath of oranges and yellows over the Plains and Midwest. These warmer colors indicate a higher spread in the ensembles; in other words, the ensemble members are in higher disagreement with one another over this higher spread envelope, thus indicating increased uncertainty. However, looking back at the post on this storm from the other day, we note that the uncertainty has decreased, with the ECMWF ensembles showing mainly yellows (decent uncertainty), rather than a swath of oranges, which depicted even higher uncertainty. This bodes well for the idea of a snow event.

If we look at the projected ECMWF ensemble mean 850mb temperatures over North America for April 14th, we find a startling scene. The ensemble mean actually seems to support an upper Midwest snow event, with the freezing line located just west of the dashed black line (marked as the number 0 ). I don't have access to precipitation products that display each ECMWF ensemble member, but just analyzing this map alone, it does seem like the ECMWF ensembles would support at least a slight snowfall event.

The image above shows each individual ensemble member from the GFS Ensemble set, valid for this timeframe. In many of the ensemble members, we see substantial precipitation located to the west of that yellow-ish/brown-ish line, which defines the line between freezing and non-freezing temperatures about 5,000 feet above the ground. It's important to note that the ensembles are still pretty spread out, despite decent agreement on the precipitation location, and it is because of this spread, among other things, that I'm very hesitant to support a snowfall event for this time period.

Here's the caveats I'm currently concerned with about this storm.

• This event would be happening in mid-April, rather than mid-January. Thus, the environment is much more hostile to snowfall.
• Model guidance, including ensembles, remain inconsistent with this storm on differing scales.
• Model guidance is notorious for over-projecting cold air coverage and intensity.
• The pattern has been very "wintry" this year, meaning the models may still want to hold on to this "wintry" feel, whereas the actual pattern may be much warmer.

Let's summarize this.
• A potentially major storm system is expected in the April 12-16 timeframe.
• Colder than normal weather is expected to arrive with this storm system.
• Model guidance is hinting at a potentially significant snowfall event during the April 14-15 timeframe, in conjunction with this potentially major storm system.
• There is high uncertainty with this potential event. Caution must be used.

Andrew

Verification of the 2013-2014 Winter Forecast

Hello everyone, this is a look back at my 2013-2014 winter forecast, and an analysis of how well it performed against what we observed from December 1, 2013 to March 1, 2014.

Temperature Verification

In my Final 2013-2014 Winter Forecast, I projected that we would see the core of the cold centered in the Northern Plains, Upper Midwest, Great Lakes, Midwest, Ohio Valley and into the Central Plains. The forecast also called for warmer than normal temperatures in the Southeast, while average conditions were to prevail over the southern Plains and the Northeast. My projection had the West US in for back and forth temperatures, ending up around normal.

Actual temperature anomalies for December - January - February 2013-2014

Looking back at what actually happened this past winter, we saw the core of the cold in the Upper Midwest into the Midwest itself, with still well-below normal temperatures in the Northern Plains, Ohio Valley, Great Lakes, and Southern Plains. This part of the forecast verified quite well, with the only trouble spot being the Southern Plains, which ended up a bit cooler than forecasted. The Northeast observed overall normal conditions, verifying that part of the forecast. The Southeast did end up a bit warmer than normal, but not as much as I thought it would be. The main trouble spot was in the West, where we saw warmer than normal temperatures instead of the average conditions I predicted. This can be attributed at least partially to the drought, as well as the persistent ridge in the Gulf of Alaska.

On a self-grading scale, I might put this forecast at an A-minus, maybe down to a B-plus.

Precipitation Verification

For the precipitation forecast, I anticipated above normal precipitation from the Central Plains into the Midwest and Great Lakes, believing that was where we would see the primary storm track. I also believed a drier than normal winter was in store for the Southeast, with neutral conditions in the East and West.

This part of the forecast actually busted quite badly. The Central Plains saw below normal precipitation rather than the above normal precipitation that was forecasted, though we did see verification in the Great Lakes. Overall, the East Coast didn't stray too far from what I thought would happen, ending up only slightly above normal overall compared to my projected average conditions. The West US definitely busted hard, with a serious drought taking over instead of my forecasted average conditions. The Southern Plains also ended up drier than normal, while the Southeast did not.

On a self-grading scale, this portion of the forecast likely deserves a D-plus.

Snowfall Verification

The snowfall forecast called for snowier than normal conditions over the Plains, Upper Midwest, Midwest and Great Lakes. The rest of the country was pegged for average conditions, with the East Coast possibly in line for some big storms.

I do not have a verification image, but I can affirm that the Midwest and Great Lakes regions verified accurately in this portion of the forecast. The East Coast ended up a bit snowier than I thought it would, and the West definitely got much less snow than what is considered average.

I would put the accuracy of this forecast at a B even.

Overall Forecast Verification

The overall forecast called for some big snowstorm potentials in the Northeast, with a warm and dry winter in the Southeast. The risk for a warm and dry winter existed in the Southern Plains, with a cold and snowy winter outlined for the North Central US. Slightly below average temperatures were anticipated for the Rockies, while a warmer than normal winter was projected for the Pacific Northwest. I feel verification of this graphic went better than I actually thought it would, with most areas seeing at least partial verification of this winter forecast.

For the overall forecast, I believe a grade of B-plus would be appropriate here.

What did you think of the accuracy of this forecast? Comment below with your thoughts!

Andrew

November 27-29 Potential Winter Storm

There is potential arising for a major storm system to hit the Midwest, Plains, Ohio Valley and Northeast. Let's look at how this could all go down.

This is hour 228 of the 12z GFS, valid on the 27th of November. There is precipitation falling across the nation, with rain and thunderstorms hitting the Ohio Valley, Northeast and Midwest, while a snowstorm rages in the Plains. This comes from a 989mb storm system- quite strong for a system on land. At this point, the storm has not yet pulled out of the mountains and has not reached its full potential. Despite this, it is clear that this system will cause troubles as it is producing major precipitation across a wide swath of the nation.

Moving ahead 12 hours to the morning of November 28th, we now see this system has emerged from the mountains and is now located in the Upper Midwest. The storm's pressure rests at 988mb, ever so slightly weaker than what it was in the mountains. The GFS model now indicates the storm system has developed a strong cold front, as the frigid air wrapping around to the western side of the system would indicate. Thanks to this front, heavy precipitation has emerged ahead of the frontal boundary, with heavy rains down both sides of Lake Michigan into Arkansas, and as far east as the Michigan/Canada border. Because of the cold air now wrapping around the system, a larger portion of the Plains looks to be under threat from a snowstorm. Blizzard conditions appear to be in the cards for Minnesota and the Dakotas, the three states closest to the low pressure system yet still north of the blue rain/snow line.

Fast forward another 12 hours and we find this system has intensified quite a bit. The system is now down to 978mb, a whole 10mb lower than the morning of November 28th (the second image of this post). This strong storm system is now into south central Canada, where it is battering the region with blizzard conditions. Due to the intense cold air wrap-around from the cold front, the blue rain/snow line has now caught up with the heavy precipitation, meaning that states like Iowa, Wisconsin, Illinois and Missouri would be seeing snowflakes fly- possibly at an accumulating rate. To the east of the rain/snow line, what could be a linear thunderstorm formation is now progressing east, and hitting Indiana, Ohio, Kentucky and Tennessee with its wrath. As is the situation in the aforementioned Midwest states, these heavy storms would likely switch to snow, more likely in the northern states. Still under snowstorm conditions is Minnesota, which, should this forecast verify, would emerge with several inches of snow under its belt.

Another 12 hours later and we are on the morning of November 29th. The strong storm system has now moved off to the northeast, well off into Canada. Blizzard conditions have now ceased in the Upper Midwest and lower Midwest states, but now the Ohio Valley is north of the blue rain/snow line. Judging by the amount of precipitation accumulated in this forecast, some accumulation could be found, especially in Ohio and western Pennsylvania. Cold air would follow this strong storm system, which has now strengthened further to 965mb- another 10mb dropped in a 12 hour period. Heavy rain could be ongoing in New England before the cold front races through.

I know what you're thinking: "Why should I get excited over a forecast over 10 days away? It'll never happen- the model is too far out!" But there is substantial evidence that this isn't just another 'fantasy forecast' (a forecast that looks great but doesn't happen).

This was the observed surface map for November 13th, just 5 days ago. This surface map is for the northern Pacific and Bering Strait. As you can see, there is a very strong storm system swirling around in the waters just off the end of the Aleutian Islands on this date.

Why is this significant? There has been a connection found between strong Gulf of Alaska (GOA) storm systems, and a strong storm system hitting the Lower 48 around 17-21 days later. If we take November 13 and apply that 2.5-3 week gap, we find that this strong storm system would hit...

...between November 30 and December 4. It just so happens that the storm system forecasted above falls in this timeframe.

Is this a coincidence? I doubt it. While the GFS may be over-exaggerating some aspects of the storm (i.e. strength, precipitation, air temperature), I have a feeling that the GFS is catching on to this Bering Sea Rule, as it's called.

I can't say I am willing to pinpoint the exact track and effects of this storm system because the models are all over the place. One cannot figure all those factors out just by seeing a strong GOA system, either- all you can determine is that a strong storm system will be around the Lower 48 roughly 17-21 days later.

Andrew