Happy first day of Meteorological Winter! BTW, unlike astronomical winter than runs from December 21 through March 21, Meteorologists define winter as December 1 through February 28, often referred to DJF.
A look back
Back on November 4th, I started mentioning the possibility of a high impact storm that could occur around Thanksgiving. At that time (and in later posts) the location of such a storm was in doubt. The storm finally did materialize but impacted the eastern states and up into New England. If you traveled by air last week, did you experience issues? The storm dumped large amounts of snow and did impact commerce and air travel nationwide (that had connections to one of the major hubs).
Forecasting a significant storm several weeks in the future (in this case about 3 weeks) is difficult at best. But, the spatial accuracy is almost impossible, especially for the first few storms of the fall. After a pattern is established and starts to repeat the accuracy of the location of such storms will improve. But is this even usual information if the spatial accuracy is lacking? In this case just knowing that there would be travel delays by air should of helped.
Behind the storm of last week a build-up of frigid air across Canada began to race south into the states. The low temperatures for Monday morning were very cold behind the Arctic front.
This cold came on the heals of a ridiculous warm spell. Ahead of the Arctic front very warm air descended from the high terrain of the southern Rockies (called adiabatic warming) and spread across the plains. At Dodge City the high temperature of 78 and 79 on Friday and Saturday (28th & 29th) was the third warmest two day period for late November since records have been kept. This was the second time in less than a month that record warmth was followed by very cold temperatures. I would bet this will occur again sometime this winter.
Looking ahead
The jet stream across the northern hemisphere is active, but changing. There are three features that have caught my attention. One feature is the "wave train" of energy that is propagating from the Asian continent in an arcing manor into the eastern Pacific west of California. This downstream propagation of energy is referred to as a Rossby wave or a planetary wave train.
I've highlighted this occurrence on the following map showing the arcing nature.
This wave train is likely responsible for the storm that is bringing MUCH needed precipitation to drought ravaged state of California. Some of the affects of this will be moving into other areas of the U.S. during the next 5 to 7 days. The Weather Prediction Center (WPC) has the following possible outcome through December 8th. Much of the precipitation will be liquid (except for the higher elevations).
Back to the Dec 2014 500hPa map above....another feature that is important is the big red L over the Hudson Bay area and the associated circular flow around it. This feature has been rather persistent so far this fall and I fear that it may be dominant this winter. The reason for the "fear" is that is will unleash Arctic air periodically. The Arctic air will become quite intense due in part to the other big red L associated flow around it that has be parked over Siberia. If the Hudson Bay feature becomes the dominant contributor to our weather, then a cold and dry winter can be expected for much of the eastern half of the country with the high plains on the western fringes.
However, the wild card in all this will be the flow across the Pacific. There is another feature currently that is contributing to this weather pattern. This feature is referred to as a Madden Julian Oscillation (MJO). I recommend that you take a minute and read a FAQ on the MJO by clicking here. If you're bored and need something to read, take a look at detailed information on the MJO by clicking here.
Often an MJO causes coherent changes in the jet stream from the tropics into the mid or high latitudes, across BOTH hemispheres. Look a the map below. This is a satellite image and I've highlighted the flow from the tropics. Notice how the flow is mirrored across the equator into both hemispheres.
I believe the "wave train" I discussed earlier in this post and the coherent MJO signal could both be indications of an El Nino response. If this is true, then the contributions from the Pacific will become more pronounced and continue into the deep of winter. Combine this with the semi-permanant Hudson Bay circulation and suddenly we're back to the below normal temperatures and above normal precipitation that the previous winter outlook was indicating.
So, for the balance of this month, the weather pattern will be relatively "mild" through at least the 13th of December (with a few minor cold shots - but also mild periods). Any major changes will likely wait until mid-month. I'm seeing some indications of another significant shot of Arctic air, especially during the last 10 days of the month. I'll be watching closely for the signal because if it does come to pass, there could be serious cold issues across much of the central part of the country, including the high plains. Serious = lows 10 to 20 below and highs around zero to single digits. That type of cold combined with any contributions from the Pacific could could result in a major storm.
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