Thursday, December 11, 2014

Christmas Potentially Significant Winter Storm

I'm now watching the risk of a strong winter storm rise as we move towards Christmas.

Tropical Tidbits
The image above shows 500mb geopotential height values (colored shadings), with mean sea level pressure (MSLP) values superimposed. In this graphic, valid on December 16th, we see a pair of very strong storm systems, one on each side of Japan. The prognosis is that a strong system will push into the Sea of Japan (located west of the country), while a second storm system will develop in the southern part of the country and move northeast-ward, skirting the eastern fringe of Japan as it does so. It is this second storm that we need to keep a close eye on, and is the one we will be discussing here today.

Using the Typhoon Rule, which states that weather phenomena occurring in East Asia is recipricated in the US about 6-10 days later, we can extrapolate this December 16th date out and predict a storm in the United States in a December 22-26th period... right in the Christmas rush.

But we can expand on this quite a bit more. The storm will be shooting north along the eastern coast of Japan. This does have an impact on the expected storm in the US. As you might expect, it raises the chance of this consequential storm also moving northeast-ward rapidly, and from there, we come out with two prevalent/possible storm tracks:

- A Panhandle Hook storm, where the system shoots north from the Southern Plains. These storms are climatologically favored to bring heavy snow to cities in the east-central Plains and Lower Great Lakes. This scenario is a possibility, as that strong storm in the Sea of Japan would likely correlate to a strong North Plains cyclone. This would keep that body of low pressure east of Japan in an area close-by, as the storms would eventually phase (not to mention low pressure areas are attracted to other low pressure areas).

- An East Coast storm. Because this body of low pressure is forecasted to merely skirt the eastern side of Japan, this could be a plausible scenario. We won't know if either of these are correct until we have more model runs to access.

The graphic above only shows the GFS model view... let's head on over to the European model projection.

Tropical Tidbits
Wow! Can you see the change?

This graphic, showing the same parameters as the GFS image, and for the same timeframe, portrays that strong storm in the Sea of Japan, but now the second storm skirting eastern Japan is more inland. It hasn't shifted much, but it has shifted nonetheless.

What does this mean? It means it's time for East Coasters to throw in the towel.

Not really, but a more inland storm does favor an inland track when the storm comes around in the US. The interesting thing is, this more inland track is an idea. Here's why.

Recall that, whether you learned it or just know it through logic, low pressure areas will try to move towards areas with the least resistance, in this case the least high pressure. My theory here is that the storm in the Sea of Japan, the stronger of the two (shown on the GFS as 993mb, 980mb on the ECMWF), is trying to pull the storm skirting east Japan towards itself. Down the road, model guidance shows the second storm absorbing the stronger Sea of Japan storm, rather than vice versa, and that's also a possibility.
My point here is, there is the possibility of a phased storm.

For those who aren't as knowledgeable with weather lingo, a 'phased storm' is a storm system which is made up of previously-two or more pieces of energy. Typically, phased storms end up stronger than either of the first two pieces of energy were. I'm not holding my breath on this Christmas storm phasing, but it probably isn't a bad idea to keep it in the back of your mind.

Regardless of if this storm phases, remember that the storm on the east coast of Japan is projected to be below 1000 millibars, so it's likely to be a nice little storm in itself.

Tropical Tidbits
We've now confirmed that not only are looking at a storm in the Christmas time period, but model guidance has amped up that threat since yesterday. Now, we have to diagnose the weather pattern here at home in that December 22nd - 26th timeframe, to see if we can pull any hints out.

I've posted the image above from the GFS ensembles, showing 500mb geopotential heights on Christmas morning. Warm colors depict ridging/high pressure, usually indicative of warm and quiet weather. Similarly, blues indicate troughing/low pressure, accompanied by colder and stormier weather. We have more than a few things to talk about with the above graphic.

First and foremost, we're looking at the Pacific driving our pattern to round out December. Tropical activity in the Equatorial Pacific will be dying off in the next few days (more knowledgeable weather folks know this as the MJO weakening), which will shift the weather pattern 'responsibilities' to the North Pacific.

We look to have a positive Pacific-North American (PNA) index pattern in place for this event. We can observe this positive PNA as a ridge forming in the West US, which allows the jet stream to buckle in the Central US. Such a pattern is climatologically favorable for a Central US storm track. In addition, a positive East Pacific Oscillation (EPO) signal will begin showing up, as ridging overtakes Canada. This will be in part due to that positive PNA, but the further east you go, the more the EPO influence takes over. A positive EPO doesn't affect the storm track so much as it does temperatures (above normal in the North US). When we factor into account slight ridging along the Eastern Seaboard, we start to see that signal for a storm system in the Central US, favoring development in the Central US.
I'm a bit skeptical, however, Many Northeast weather buffs may know that winter storms are favored in the East when the North Atlantic Oscillation (NAO) changes phases from positive to negative, or vice versa. Guess what the forecast for the NAO is around this storm's timeframe:

We've got a dilemma on our hands, with some variables favoring a Central US storm, and others favoring an East US storm. So is the world of forecasting...

To summarize:

- A winter storm appears to be in the cards for December 22-26th, likely impacting Christmas travel plans.
- A second storm system may need to be watched for the Northern Plains.
- The primary threat here may become a storm favorable for heavy snow, either in the Central/East US (ideally the Ohio Valley/Midwest) or along the Eastern Seaboard.
- Rather high confidence in the threat of a storm in this timeframe, but low confidence in who will be most affected.