November 24-30 Potentially Significant Winter Storm

It's becoming evident that we may be dealing with a potentially very strong Thanksgiving winter storm.

Tropical Tidbits

The image above shows the GFS model's combined forecast of 500mb geopotential height values (colored shadings) and mean sea level pressure (MSLP) contours over the West Pacific and north-central Pacific basins. High and low pressure demarcations are also seen. This forecast is valid on the evening of November 7th, and may have a significant effect on our weather here.

The potential for a significant winter storm evolves out of the Bering Sea for this forecast. The method, referred to as the Bering Sea Rule, takes the occurrence of high and low pressure events in the Bering Sea, and expects a similar weather phenomenon to occur in the US about 17-21 days later. This is almost exactly like the Typhoon Rule I commonly discuss when analyzing winter storm potentials, except now the area to watch is the Bering Sea, and the timeframe from occurrence in the Bering Sea to reciprocation in the US is now 17-21 days.

Getting back to this model forecast, we can see a very strong extratropical cyclone impacting the far western Aleutian islands, with a minimum central pressure of 930 millibars. If we utilize the Bering Sea Rule here, and extrapolate the forecasted time frame above of November 7th out seventeen to twenty-one days, we find ourselves with a potentially significant winter storm in a rough November 24-30th timeframe.

The evidence supporting this storm doesn't end here; we have a long way to go...

Tropical Tidbits

The model now shown above is the Canadian GEM forecast model, with the same 500mb geopotential height and MSLP contour parameters as the GFS model (for future reference, all model guidance we will analyze here today will use those 500mb and MSLP parameters). This forecast is valid on the afternoon of November 8th, nearly a day later than the GFS forecast above. Despite this time difference, we still see that very strong storm system from the GFS model now appearing in the GEM's forecast, with a minimum central pressure of 944mb spread across a heavy majority of the Bering Sea. Once again, this supports the prospect of a strong winter storm, if not solely a strong blast of cold air, to the US in late November.

Tropical Tidbits

Let's now examine the ECMWF model, commonly referred to as the most accurate weather model in the world at this time. The forecast panel above is valid for the evening of November 7th, and even though the storm hasn't reached the Bering Sea yet, its minimum central pressure is already down to whopping 930 millibars!

Tropical Tidbits

If we fast forward that same ECMWF model forecast up twenty-four hours to the evening of November 8th, we find that the storm system has gotten a bit stronger, and quite larger. The minimum central pressure is now down to 928 millibars, with the storm's influence spread from Russia to Alaska! The ECMWF model joins the GFS and GEM in supporting a strong winter storm for late November, possibly in time for Thanksgiving.

I could keep adding more and more model guidance projections to prove my point, but the general premise is already pretty clear by now. The three aforementioned models, in conjunction with the GFS-Parallel, ECMWF ensembles, and GFS Ensembles are favoring a very strong storm system in the Bering Sea for around November 7th or 8th.

The next question becomes, who could it target?

Well, if these model forecasts do verify as they appear now (which is nowhere from certain), each model projection would hit a difference part of the nation. The forecast members with the storm further west in the Bering Sea would find the Thanksgiving storm system in the Central US, while the model guidance members further east into the middle Aleutian Islands (or even further east) would likely find the consequential Thanksgiving timeframe storm in the East US, possibly along the coast.

Because this potential storm is still over 7 days away from appearing in the Bering Sea, and thus guidance can be expected to change with uncertainty, it's not worth putting down an accurate estimate as to where the storm may hit. However, just glancing over trends from these three shown models, as well as others available, would only slightly support more of a Central US/inland East US storm system.

To summarize:

- Model guidance is favoring a very strong storm to hit the Bering Sea in the next few days.
- If this storm ends up as strong as projected, it may result in a very strong winter storm around the Thanksgiving timeframe.
- A lot of uncertainty exists with this storm at this time.

Andrew

March 25-27 Potentially Significant Blizzard

I'm watching the potential for a potentially significant snowstorm that model guidance believes will become a blizzard.

The image above shows 500mb geopotential height anomalies from the latest run of the GFS model over the Western Pacific, valid for March 20th. As this image shows, we see strong negative height anomalies present over Japan on March 20th, indicative of a rather strong storm over the area. 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. So, if we take this March 20th storm and move it ahead 6-10 days, we find a potential storm system around the March 26th through 30th period.

Let's take a look over current model guidance. Shown above is the latest forecast from the Canadian GGEM model, valid on March 26th. We see an extremely strong storm system positioned just offshore of the Northeast, dumping heavy precipitation on the order of over 12" on inland areas, with multiple FEET of snow falling on coastal locations. The GGEM model has already been shown to be suffering from convective feedback issues on this run, meaning precipitation amounts are grossly exaggerated and incorrectly altered due to this specific bias. Thus, this forecast run is overdoing precipitation amounts and is not correct in its forecast at this time. What I do agree with, however, is placement of the storm. We look to be in a northwest flow regime, a pattern notorious for providing a very favorable environment for East Coast snow events, and we'll discuss this a bit later.

We now move on to the ECMWF model, which also shows a very strong storm system displaced just east of the Northeast coastline. This is of similar strength to the GGEM model and would likely be putting down some pretty darn heavy snow amounts, but because we lost access to the main provider of our weather maps, we can't give an indication as to just how severe this storm looks to be on the ECMWF. However, once again, we're seeing this strong storm in the midst of a northwest flow pattern, as we see the dashed lines dropping down to the southeast from Canada. This is crucial to the forecasts of a strong East Coast storm.

The American GFS model is far less enthusiastic about this storm, only putting down modest precipitation along the coastline and sparing inland regions from any accumulating precipitation, snow or otherwise. This is considered the outlier forecast when taking into account how the GGEM, ECMWF and ECMWF Ensembles believe we'll see a strong East Coast-hugging storm, but in the grand scheme of things, there is high model agreement on a storm at least threatening the East Coast in this timeframe, and that's what we're looking at here.

Take a look at this graphic above. This image shows a few things important. The main item we see first are the negative 500mb and positive 500mb anomalies, marked by contour lines in blue and red, respectively. Just glancing over the image, we find a sustained northwest flow scenario unfolding, with deep negative height anomalies pressing into the East US, with high pressure surging northward in the West US. The second item we need to discuss are the dates on the bottom right hand corner. They are analog dates, listed from top to bottom, and the date at the top means its pattern is most similar to the pattern forecasted in the graphic itself, which is valid around the March 26-30 period, right on top of the timeframe outlined for this potentially significant storm. Look closely at the dates- are you picking up on one particular date in that list? If not, check out the second date from the top. That pattern was deemed to be the second most similar to the pattern expected to unfold as this storm system impacts the East US. And that date - March 16, 1993 - spawned the 'Storm of the Century'.

Now, there are some who are anticipating another Storm of the Century because its date has shown up on these analog images, and they might tell you to prepare for feet of snow. All that we know now is that model guidance is hinting rather consistently at the potential for an all-out blizzard in the East US, specifically the Northeast. As of right now, there is disagreement on who could get hit by the storm, and there's even disagreement on if there'll be a storm at all. As of now, the potential for a significant storm is rather high due to the East Asian correlation, but it remains to be seen if this could be a blizzard for the East Coast, hence all the caveats throughout the post.

Andrew

Long Range Discussion: Severe Weather, Snowstorms To Kick Off March

This long range discussion will focus on the expected weather to end February and kick off March.

The image above shows the latest 500mb height anomalies across the Northern Hemisphere on the left, with cloud cover, pressure contours and high/low pressure denotations on the right. If you've been with The Weather Centre for a while, you know how we use a rule created by Joe Renken that states a storm system in East Asia then results in a storm in the United States 6-10 days later. This situation is no different- today, we saw ridging/high pressure emerge in East Asia. If we extrapolate that out 6-10 days, we arrive at a warm-up in the February 20-24 timeframe. I'll discuss this more a little later down this post.

Lately, as the image above shows, the weather pattern has been dominated by west-northwest to northwest flow, thanks to deep troughing in the Gulf of Alaska and suppressed ridging in the Southwest producing a pattern favorable for East Coast snow events. This whole pattern is expected to flip in the medium range.

The changes begin as ensemble members foresee strong ridging shifting east into far eastern Russia, which will then shift the Gulf of Alaska storminess to the east, resulting in the loss of high pressure in the Southwest US. That Southwest ridging then propagates east into the contiguous United States, producing mainly zonal flow, which keeps anomalous warm or cold outbreaks out of the picture. However, with the ridging now gone, the storminess in the Gulf of Alaska can drop south into the Western US. The result is a stormy West US, and a very warm East US.

Model guidance (with the exception of the CMC model on the far right) is in agreement on a pattern change evolving, where we see the troughing drop south into the West US, as high pressure prevails yet again in the Gulf of Alaska. The storminess in the West US isn't as well defined on the ECMWF forecast (left) or GFS forecast (center), primarily because this image shows projected 500mb anomalies over the 8-10 day forecast period. This means the forecasts may seem a little less 'intense' than they end up being, because the anomalies are averaged out over a 2 day period, rather than a 6 hour period like most models print out.

Regardless, the pattern will involve high pressure setting up along the East Coast in response to troughing out West, and this sort of pattern just screams for not only a snow event, but a severe weather event. We will see anomalously warm and humid conditions present across the Central and Eastern US as the jet stream bends north to accommodate for the ridging out East. This ridge also tells me we should keep an eye out for a storm system cutting north through the western Great Lakes, something model guidance is already picking up on. I have a feeling such a storm cutting north would result in not only a northern Plains snowstorm, but also a severe weather event for the Central US, something I discussed in yesterday's post.

As we head into the last days of February, ensembles give the Central and East US another shot of cold weather, as shown by the GFS Ensembles' portrayal of the trough at the 500mb level on the left panel. Again, the pattern we discussed above would strongly support this solution. It wouldn't surprise me to see additional chances for storms, both snowy and severe, before the month of February ends, apart from that other storm system we discussed yesterday in the link above.

For the opening days of March, I wouldn't be surprised to see us start on a chilly note before rebounding to rather seasonal temperatures. Depending on how the sudden stratospheric warming event continues to evolve, we could be looking at a cool March, though the chance of that would be maximized in the north-central US (primarily the Plains), and minimized along the East Coast and West Coast, where ridging and warm weather would be expected.

Andrew

February 12-14 Potentially Historic Winter Storm

Things appear to be coming together for a potentially historic winter storm, though model biases mean intense model forecasts are likely overdone.

This image shows the ECMWF model's snowfall forecast for the next 144 hours, and it is not that hard to see the big storm system this model believes will strike a large portion of the Eastern US. We see that the ECMWF model begins by dropping heavy snow in excess of 12" on the areas of northern Georgia and the Carolinas into western Tennessee and Kentucky, before amounts max out near 2 feet in the Virginias. Heavy snow continues into the Northeast, with widespread 12-18" totals across the map for that region.


The ECMWF model has remained consistent on this idea of a major winter storm for the East Coast, including the Southeast. These big amounts are relatively unchanged from previous forecasts, except for the 24" jackpot zone in the Virginias. The environment looks highly favorable for an East Coast storm synoptically, with a west-northwest flow solution prevailing over the contiguous United States. This is shown well by storminess in Alaska leading to strong yet suppressed ridging over the Southwest US. Considering northwest flow scenarios typically can lead to Northeast snow events, it is of little surprise that a strong storm system in the Southeast is able to shift northward and ride the coastline to produce heavy snowfall across the far Eastern US.

The CMC model (Canadian model) above shows heavy snow extending from far northern Georgia to eastern Canada, in a similar fashion as the ECMWF (though not identical). The heaviest snow hits Maine, where amounts flirt with the 16" benchmark, and the remainder of the East Coast stays within the realm of 6-12" or a little higher, as the model shows some higher totals in New Jersey and a few other coastal places.

Surprisingly, the CMC model may have shed its too-snowy bias in this forecast and is actually showing a more reasonable solution compared to the ECMWF. It does look like the crucial point will come when we determine how far south the storm system goes. In the ECMWF's forecast, the storm is a bit to the south of the CMC, hence the heavier snow totals located deeper into Georgia. In future forecasts, I wouldn't be surprised to see totals bumped up a bit along the coast, but other than that, this forecast isn't that far fetched in my eyes.

Lastly, we see the NAM model's snowfall forecast above. The NAM drops heavy snow amounts of over 12" in a large swath of Georgia in what would be a truly paralyzing snowstorm for those who went through the agonizing snow event in Atlanta just a couple of weeks ago. If we look at the NAM forecast further north, we see the heavy amounts extend into the Carolinas, and a strip of 24" to 31" snow amounts in eastern Virginia and the coastal portion of the Mid-Atlantic. The heavy snow then continues into the coastal New England area, but the full storm is not in the NAM model's forecast timeframe yet, so we cannot get total snow amounts in the Northeast yet. Regardless, it's clear that this event would be historic for those down south.

The NAM model is notorious for making forecasts laden with a noticeable too-snowy bias. I expect that this forecast is well within that bias, and it would be wise to cut totals in half or so to be safe until we get closer to the event. Amounts in the Southeast also are unlikely to end up as high as they are shown- I would expect totals maxing out at 6" at this time.

Andrew