Wednesday, December 10, 2014

December 22-26/Christmas Potential Winter Storm

I'm watching the potential for a winter storm in the December 22-26 timeframe.

Tropical Tidbits
The image above shows the forecasted 500mb vorticity values on the morning of December 16th, valid over the West Pacific. In this graphic, we see a negatively-tilted upper level low pushing east into Japan, as the elevated vorticity values indicate. We can observe the negative tilt by the height contours seeming to "dig" in a southeast-ward direction. When a trough/storm system is at its mature stage, usually its strongest stage in a storm's life, the storm is said to be negatively tilted.

Using the Typhoon Rule, which states weather phenomena occurring over Japan is reciprocated in the United States 6-10 days later, we might expect this storm to impact the United States in a December 22-26 timeframe... possibly right around Christmas.

But there's more to this story.

Tropical Tidbits
Take a look at this image, forecasting 500mb geopotential height values in colored shadings, as well as mean sea level pressure (MSLP) values in the contoured lines. In this image, valid just six hours prior to the time period in the first image we analyzed, we see a strong storm system in the Sea of Japan, correlated to the strong upper level low.

But wait... there's something else there.

Check out that sagging of contour lines just south of Japan, right where the arrow is pointing. It almost looks like another low pressure system! Other model guidance confirms this idea of a second body of low pressure forming south of Japan and skirting the nation to the east before phasing with the very strong storm in the Sea of Japan. The implications here could be huge.

If that really is another low pressure system, then we have a very interesting scenario on our hands for Christmas. We can't tell for sure just yet, but the storm shooting northward to the east of Japan could mean a few possibilities.
Let's first hypothesize that the item outlined above is indeed a storm system. The track to the east of Japan could either mean we would be looking at:

- 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 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.

Long range ensemble model guidance for the evening of December 23rd shows a relatively favorable pattern for a storm. We see strong ridging high pressure in the West Coast of North America, but another swath of high pressure also looks to exist in southern Canada. This could boost temperatures, or worse, suppress and kill the storm system altogether. Even though this is quite far out and should not be taken at face value, those in the Midwest looking for a winter storm should feel rather good from what model guidance is projecting:

Tropical Tidbits
This image from the GFS ensembles shows what I'm talking about. In this image, also valid for the evening of December 23rd, we see a few factors at work here. A strong positive PNA is currently agreed upon by these ensembles, which permits a strong ridge to form in the Western US. This ridge then allows a colder pattern to develop in the Central US, as the jet stream drops southward to accommodate the +PNA ridge. Sweetening the pot is evidence of a very slight ridge in the Southeast, something that could allow any potential storms to move more northward. That final piece to the puzzle will be a bit more difficult to come by, as that strong upper level low over Greenland will try to keep the flow over North America very zonal (west-to-east, jet stream not wavy). There's still, of course, plenty of time for this to change.

To summarize:

- A winter storm may 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.
- Relatively low confidence still exists due to the long-range nature of this threat.