Created: 01/15/2009 5:05 PM KSTP.com | Print Story By: Dave Dahl

Is the "January Thaw" fact or fiction?

The January Thaw got it's beginning in New England folklore.  It's defined as a period of abnormally warm weather usually around the middle to the end of January.  In most cases this means above the melting point, but there are places in eastern Canada who claim to have a January Thaw even though temperatures rarely exceed the 32-degree mark in January.  Even though this phenomenon is most likely in the northeastern part of the country, it is a pretty regular event in the middle of the U.S. as well. 

The thing that makes this event so welcome especially here in the Upper Midwest is the fact that we're just getting out of the coldest week of the winter on average.  This may also help explain one of the causes of the January Thaw.  Usually in the first part of January the jet stream is highly amplified, drawing frigid air from the Polar Region right down over the northern part of the country.  This type of flow, which dips and arcs across the country, is also usually a very changeable pattern.  Since the atmosphere tends toward balance and away from contrast, storms form along these boundaries, acting like big mixers to even out the temperature contrast.  The upper wind flow then changes from northwesterly to southwesterly in a matter of days, warming those areas as air from the Southwest moves northward.

This adjustment in the jet stream also helps build the Bermuda high-pressure system over the southeastern part of the country, which keeps the warm flow of air around for several days.  This makes the "Thaw" last about a week rather than just a day or two.  If you thought the January Thaw would miss us this year, think again.  Temperatures are expected to climb well into the 30s over the next 7 days, which should be just long enough to be classified as our January Thaw for this winter.   

Have a great day!

Meteorologist Dave Dahl