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Posted at 1:15 PM ET, 02/15/2011

Amazing U.S. snow cover retreat in six days

By Jason Samenow

snow-cover-shrink.jpg
Snow cover extent on February 10 (top) and February 15 (bottom) across the lower 48 U.S. states. Source: NOAA, National Snow Analyses

Just last Thursday snow was on the ground in every state except Florida. In a blink of an eye, snow cover extent has diminished from nearly two thirds of the lower 48 states to not even a third.

The cause?

Let's look at the flow in the atmosphere last Thursday at 35,000 feet compared to today's...

jet-stream-flip.jpg
Upper level winds at roughly 35,000 feet (250 mb) on February 10 (top) and February 15 (bottom). Source 12z GFS initializations from those dates courtesy StormVistaWxModels.com

In a stunning reversal, the jet stream has flipped from a huge dip or trough ("u" shape) over the eastern two thirds of the U.S. to a giant bump or ridge ("n" shape). Notice how the yellows, oranges, reds, pinks and purples - indicative of the strongest upper level winds - are now positioned mainly north of the Canadian border whereas just six days ago, they streamed across the Deep South.

This region of fast winds (as high as 140 mph or so), a river of air along which storms track, is the dividing line between warm tropical and subtropical air to the south and Arctic air to the north.

So with this river taking a remarkable northward excursion, characteristic of what we see during the spring transition, it's not a coincidence that unseasonably warm air is bubbling up and snow cover is abruptly evaporating.

How dramatically have temperatures changed? Nowata, Oklahoma - which reached the lowest temperature ever recorded in Oklahoma last Thursday of 31 below zero - is near 50 degrees this afternoon. That's an 80-degree swing.

But late winter and spring can be a volatile time, with the jet stream plunging south again even after a big retreat to the north. So snow could yet again cover a large portion of the U.S.

By Jason Samenow  | February 15, 2011; 1:15 PM ET
Categories:  Latest, Science, U.S. Weather  
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Comments

George Costanza wants you to know the reason for the shrinkage. It was cold!

Posted by: rwalker66 | February 15, 2011 2:00 PM | Report abuse

Makes me proud of the little pile of snow we still have in front of my house here in Stafford. :)

Posted by: SpeedLimit186000 | February 15, 2011 7:13 PM | Report abuse

Al "Gopher" Gore must have delivered a speech so full of his usual hot air that he created a personal El Nino.

But, to his credit, we all have seen pics of Al when he was a young, chubby kid. Have you seen similar pics of the Kenyan President. To paraphrase William Buckley, he of God and Man at Yale, "what ain't-can't be."

Posted by: fregameeate | February 15, 2011 7:25 PM | Report abuse

Global warming deniers typically quiet down when the cold goes away. fregameeate is quite creative in the keeping up the cold air blast.

Posted by: slydell | February 16, 2011 2:57 AM | Report abuse

The comments to this entry are closed.

 
 
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