Wednesday, December 11, 2013

December 19-23 Potentially Significant Winter Storm

I am rather confident that we will see a winter storm in time for the December 19-23 period, and there is a chance it could be significant.

A pretty strong storm system is expected to move across Japan in 48 hours, after dumping multiple storm systems over that area in the past few days. This storm system is looking healthy, as defined by the GFS ensemble mid-level height anomaly forecast above over Japan. As I have mentioned on this blog, there is a 6-10 day correlation between weather that occurs in East Asia, and weather that occurs in the United States. Thus, with this forecast image, we arrive at the December 13th date. Go ahead 6-10 days, and we find the potential storm timeframe of December 19th to 23rd. The reason I believe it may be significant is due to the strength of this system on the image above. Unlike the upcoming weekend storm, which saw only a moderate system in East Asia 6-10 days prior, the current East Asian projection is for a strong storm to hit Japan, and long range model guidance senses that this system will also be a strong one for the US.

The ECMWF model delivers a very strong system to the Midwest, Great Lakes and Ohio Valley before shifting northeast to hammer Canada. As is denoted above, there could be some heavy snows north of that dark blue 32 degree temperature line. This would put the states of Illinois, Indiana, Michigan and Ohio in line for such snows, though we remain roughly 10 days away from this potential storm. I'll discuss a little more on the atmosphere surrounding this system later on in this post, but for now, you snow fans can take a look at the total snowfall projection for this storm below.

The ECMWF isn't the only model showing a potential winter storm for this timeframe. It's global model rival, the GFS, is also in on it.

GFS precipitation forecast for December 20th, 12 hours earlier than the ECMWF image above.
The GFS image shown above details 12 hour precipitation totals for this storm, and you can clearly see the wide swath of potential heavy snowfall across Illinois, Wisconsin, Michigan and Indiana. The GFS model has the low pressure system in Kentucky 12 hours after this image, whereas the ECMWF has the system in central Indiana. The GFS being on a more southern track does mean the snowfall would theoretically be suppressed south, but the wide precipitation shield on the image directly above gives away the GFS' projected snowfall, which is shown below.

GFS 48 hour snowfall totals for this snowstorm.
My thoughts on this storm system are actually of rather high confidence right now. This strong Japan storm system is more likely than not to translate to a storm system (if not a strong storm system) for the United States. Once it enters the United States, it is most likely going to drop down into the eastern Rockies (think Colorado).

This relative measure of predictability map, from the NCEP, gives us a glimpse at what atmospheric conditions are likely to be unfolding by an assortment of colors, defined in likelihood by the legend at the bottom. In this case, we see that blues have a single or low double-digit percent chance of verifying, while oranges define close to a 50% chance of verification. This map is valid on the evening of December 19th, roughly two days before this storm is projected to strike.

If we look around the map, we first notice a wide swath of oranges across the Gulf of Alaska. A glance at height contours tells us there is ridging forecasted in that area for the evening of December 19th, and the presence of widespread oranges tells us that the ensembles are at least decently confident in that atmospheric factor verifying. The effect that Gulf of Alaska ridge has on the potential winter storm is that the system should follow the contour lines and drop south into the Rockies. It is unable to drop too far south into the Southwest, as a positive North Atlantic Oscillation (NAO) will keep the pattern more progressive, and thus more west-to-east (hence the relatively flat contour lines over the United States). The second item we observe is a stripe of orange extended from southwest Canada into the central and western Rockies. This looks to be the potential storm system, as it dives southward towards the Rockies. The oranges tell us there is rather high confidence, which is a good sign for the storm prospects. The third (and possibly most important) item is the swath of oranges over the Southeast. It may not look too impressive, but there is likely to be high pressure stationed over the Southeast as a result of storminess over the West.

The image above is what is referred to as a negative Pacific-North American index phase (or -PNA for short). The negative PNA results from that ridge we discussed in the Gulf of Alaska that then produces storminess/troughiness in the Western US. In response to that West US storminess, we then see the development of high pressure in the Southeast. The jet stream in this situation takes storms from the West and runs them north and east, typically delivering snow to the same areas that were outlined by the GFS and ECMWF earlier in this post. Considering the relative measure of predictability chart has high confidence (for being nearly 10 days out) in the Gulf of Alaska ridge, the storm itself entering the Rockies, and the ridge in the Southeast, I do think that the GFS and ECMWF solutions are two very possible solutions.

Let me explain why I think we could see the GFS or ECMWF work out in terms of storm track and snowfall amounts.

1. The East Asian storm system is strong. Typically, we see weak or moderate storms in Japan translate to weak or moderate storms in the US. Now that a strong system is headed for the area, it is quite possible that this winter storm is strong as well.

2. Models appear rather confident with the environment surrounding this storm system. This negative PNA environment is looking like its gaining confidence in the GFS ensemble set, and a look at the ECMWF ensemble prediction system seems to agree on both the set up and the storm system actually occurring.

3. All put together, the environment looks ripe. This one is mainly thanks to the East Asian component, but when you look at the overall picture of the mid-levels, a storm track favoring the areas depicted by both modeling systems does appear to be a viable solution on either guidance system.

It's a bit too far out for me to give a graphical outlook, but I think you get the gist here- there is some mighty fine potential with this system.