Thursday, March 13, 2014

March 26-31 Multiple Potentially Significant Snowstorms

I'm examining the idea of multiple significant winter storms over the March 26-31 period.

The above image shows a recent ECMWF forecast, projecting 500mb height anomalies over the North Pacific on March 20th, where blues and purples show negative height anomalies (cool and stormy weather), and oranges and reds depict positive height anomalies (warm and quiet weather). Looking towards the continent of Asia, specifically zeroing in on Japan, I marked two points of interest using the letter X. These X's denote the location of storm systems in the area. There is a storm system moving east over Japan, tilting negatively as it does so, and a storm to the west of Japan, just beginning to drop south and racing towards the country. 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. The two storm systems both retain impressive intensities, with the bright purples indicating that both systems look to be strong.

About 18 hours later, we see that the situation over Japan has changed. The first storm system that was previously over Japan has shot north, influenced by the even-stronger system shown in green shades south of far northeast Russia. The second storm system that was previously racing towards Japan is now impacting Japan, maintaining rather-strong characteristics, as shown by the tinted purples over the country. This confirms there may be not one, but two storm systems in this timeframe.

Shown above is the ECMWF Ensemble mean forecast of 500mb height anomalies, over North America. The same color rules, where blues indicate stormy weather and oranges show calm weather, still apply. This forecast is valid on March 26th, the beginning of the five day period we're watching for these two potential storms. We see storminess evolving over the West Coast, shown by the darker blues just offshore of California, Oregon and Washington state. We see this provoking slight ridging in the southern Plains, well illustrated by the erosion of blues in that region. We also see an old friend in a piece of the polar vortex still sticking around in Canada. The storminess over the West Coast should enable the high pressure over the Southern Plains to gradually strengthen and push east, as it's possible we see a negative Pacific-North American pattern evolve, which is characterized in the image below.

Typical negative PNA pattern
We also see how a lobe of the polar vortex still sits over Canada, as it has been doing for much of the winter, and should continue to do into spring. If this forecast were to verify, it wouldn't be too hard for the storm track to be suppressed, from the Midwest down into the Ohio Valley. This could favor areas that have been hit multiple times already this winter, including the lower Midwest and Ohio Valley regions.

The GFS Ensemble forecast for 500mb anomalies at about the same timeframe have a pretty similar pattern, even though they may seem different at first glance. The PNA remains negative, like the ECMWF ensembles project, as we see strong negative height anomalies over the West Coast. We also see a lobe of the polar vortex positioned in Canada, as we did in the ECMWF Ensemble image. The only 'difference' here is that the ridge we discussed in the ECMWF Ensemble image is more pronounced on this image, and located further east. This solution would likely still drive storm systems into the Midwest, but they would also be suppressed- just not as much as the ECMWF ensembles say they would be. This sort of projection would take the two potentially significant snowstorms into the Midwest and Ohio Valley, as ridging in the Southeast would deter an East Coast solution.

Caveat: The ECMWF projection that says we would get two major storm systems is a long range forecast, and has a chance of not verifying completely. There is a pretty good chance we would be seeing a potentially significant storm system in this timeframe, it's the idea of two storm systems instead of one that isn't really solidified just yet.

To summarize:
-One or possibly two potentially significant storm systems are expected in the March 26-31 timeframe.
-Both winter and severe weather modes would be quite possible with this event.


March 22-26 Potential Weak Storm System

There's the potential for a weak storm system in the March 22-26 timeframe.

It's looking like we'll be seeing a storm system cut across Japan on March 16th, with rather meager negative height anomalies, indicating a weak storm system. 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, 6-10 days after the date where we see a storm hit Japan (March 16) brings us to a potential storm system in the US around the March time period.

Looking deeper into that ECMWF forecast, we see strong high pressure over the West Coast. This is strongly indicative of a positive Pacific-North American pattern (positive PNA), which is demonstrated in the above graphic. The positive phase of the PNA sees high pressure form over the West Coast, which then leads to storminess and cold weather in the Plains. This sort of pattern can give snow to the Midwest and Ohio Valley in situations where ridging tries to push north in the Southeast, but can also send wintry weather to the East Coast if we see high pressure over Greenland.

If we go out in the long range, we can see what the weather pattern may be like on March 22nd, the first day of this window that has been designated for a storm system. We see that the pattern includes warm weather in the West, as persistent ridging continues, but also pushes east into the Plains. More importantly is the presence of a piece of the polar vortex digging well into the Northeast, likely providing the chance for another intense round of cold weather. This sort of pattern tells me we would likely see some severe weather in the South Plains, but that would be about it. This pattern isn't conducive for wintry weather in the Plains, Midwest or Ohio Valley, and the polar vortex will be suppressing the storm south to likely keep it away from the East Coast.

This looks to be a weak storm, and impacts should be minimal on the wintry side. I would monitor severe weather prospects in the near future, however.