Amazing U.S. snow cover retreat in six days

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...

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.