It is no coincidence the south has been under the gun for severe weather lately. After all, fall is considered the second severe weather season in the U.S. (see the November 10th "Veteran's Day" tornado outbreak of 2002), spring being the first.
Why?
The location of the 300mb jet stream is key. As the transition occurs from summer to winter, the sub-tropical jet (associated with warmer air, as the name implies) retreats south. This "void" is filled by much colder air that has been bottled-up over the poles. Among other factors, the location of this air mass is dependent on the polar jet.
The polar jet has remained mainly north, however. Evidence of this lies in the historical stretch of cold temperatures that have been plaguing Alaska. To-date, Fairbanks has experienced 5 consecutive days of temperatures at or below -35 degrees Fahrenheit.
Once again, however a weak La Nina will cause a ridge of high pressure to amplify north across the Pacific. This, combined with a negative North Atlantic Oscillation (NAO) and a blocking ridge over Greenland will force much colder air south across the eastern third the Contiguous United States.
What does this all mean?
For starters, the extended stretch of mild air Erie has been enjoying for much of November is about to take a break, probably until mid-January. That means an active December shaping-up, with heavy lake effect snow for the snow belt. It also means frequent northern-tier storms and perhaps Nor'easters for the New England States.
Stay-tuned...
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