Wednesday, December 5, 2012

Colder / Snowier Outlook Still on Track...

In a post last Friday, I remarked that winter weather lovers should take heart as a major change in the weather pattern would get underway later this month, bringing an end to unseasonably warm weather across much of the nation.  

I've continued to monitor the situation since the post was initially made, and I'm happy to report that the forecast is still on track... which is great news for those of us who are sick and tired of 70 and 80 degree weather in late November and early December.

To set the stage, brutally cold weather continues in Alaska and Canada, where temperatures are in the 30s, 40s and even 50s below zero (degrees F) have been observed for the last several days:

The -36 degree weather in Fairbanks on Monday wasn't cold enough to keep some college students from throwing a beach party:

...which gives new meaning to the word frostbite (please don't try this at home, folks).  

The atmosphere likes to balance itself out.  When it's really cold in one area, it generally tends to be really warm in another.  The "other" place recently has been much of the lower 48 U.S., where temperatures continue to average above normal.  

The image below shows the departure from normal temperatures for the period today through the 10th, showing the bullseye of "above normal" conditions shifting into the southeastern U.S. during the period:

All of that cold air up in Canada and Alaska is very heavy and dense, and will begin to force its way Southeastward into the lower 48 U.S. from mid through late December, as shown on the same departure from normal computer model forecasts below, valid for days 5-10 (December 10th-15th):

...and continuing through days 10-15 (December 15th-20th):

By the day 10-15 period (December 15th-20th), you've no doubt taken note of the brutally cold air (as noted by the purple departure from normal values) shifting South from Canada into the lower 48 U.S.  That is a trend that I believe will continue through late December and into January.

Increasing jet stream energy in the middle and upper levels of the atmosphere will also come into play during the month, which will develop and increase a large snow pack across much of the central and northern two-thirds of the nation.

One such disturbance will produce a band of locally heavy snow from southwest Kansas across southeast Iowa and the upper Midwest late this weekend and early next week:

This trend is forecast to continue into the middle and latter part of the month, as indicated by the longer range computer model depictions below.  I wouldn't necessarily pay attention to the amount of snow shown on these longer range images, but they should give you a good idea as to the general areas that are most likely to be impacted.

The first set covers the period December 15-20:

...and the second set leads us from December 20 through Christmas Day:

Each panel within the image shows the "opinion" of the 4 different computer model simulations that were generated for the given period.

The snow pack will not only serve to deliver that "winter feel" to a large part of the nation later this month, but will also tend to preserve cold air intrusions from the North.  As the cold air moves South, it tends to moderate as it encounters warmer ground and air temperatures.  With a heavy snow pack in place, the ground and air will be cooler, thereby reducing the amount of warming that takes place as the airmass moves Southward.  This should generally lead to gradually colder airmasses remaining "intact" as they move further south across the nation.

So, there you have it.  I am still confident that the middle and latter half of December, and likely into January, will be much more representative of winter than what we've seen so far.  Keep in mind, winter doesn't even "officially" begin until December 21, so we still have a long way to go here!

On the other side of the coin, folks in drought stricken portions of the South and into the Mississippi and Ohio Valley region are ready-set-go for any type of moisture that will come there way, regardless of its form (rain, sleet, ice or snow).  While cold air intrusions by themselves don't guarantee an increase in moisture, there are signs that a more active jet stream pattern will gradually increase the chance of measurable precipitation across the region.

The following images show cumulative precipitation totals from the GFS and the European computer forecast models, respectively.  They are valid for the period today through Thursday, December 13th:

While the forecast model's don't provide a specific precipitation accumulation forecast beyond the time ranges above, in general, they continue to suggest that an increasingly unsettled pattern will develop across portions of the southwest and southcentral U.S., which would generally equate to greater chances of precipitation across portions of Texas and Oklahoma.  

The general position of the jet stream is noted by the green arrow on the image below, and I've circled in yellow an area of increasingly unstable conditions that should translate into the southcentral U.S. during the week leading up to Christmas.  This would generally lead to an increased chance of rain and/or snow across this region.  The image itself is valid at 6am CST on December 21st:

For those who are interested, the brighter red shaded area extending across portions of the Pacific to Alaska indicates a "ridge" of high pressure pushing into the region, which should cause the jet stream to "buckle" and send additional shots of cold air Southeastward toward the continental U.S.  I've attempted to illustrate this on the identical image below:

The bottom line on all of the above is that a much more active weather pattern is instore for the lower 48 U.S. from next week, into the foreseeable future.  Stay tuned for details as this situation unfolds...

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