Saturday, December 17, 2011

12/17 - 11:50pm - A Not So White Christmas

Positive NAO -
Christmas is just a week away and snow chances for many in the eastern half of the nation look pretty bleak. The next major system to pull through on Tuesday and Wednesday will produce snow in the Plains, but to the east it's expected to be all rain. A second low moving through the Deep South on Friday will bring more rain, but most of that will stay south of Kentucky and the heaviest south of Tennessee. After that point it looks pretty dry right through Christmas. Why so rainy and relatively warm lately? It has to do with the North Atlantic Oscillation, or NAO for short. When the NAO is negative, storms tend to ride up the East Coast or just west of the Appalachian Mountains. This is a recipe for snow for many across the East since these storms have cold air streaming in just to the north of their track. When the NAO is positive, storms begin deflecting northward to Canada before they reach the Mississippi River. This means that the cold air and snow with these storms stays out in the Plains. The NAO is in this positive phase right now and isn't forecast to go neutral or even negative until around the new year (see forecast and history chart to the right). Until we can get the NAO negative, I wouldn't bet on a winter storm for Kentucky, Tennessee, or much of anywhere else in the East.

23% of the United States is covered by snow at the moment, which is quite a bit lower than the 47.3% covered by snow at this time last year. Aside from Lake Effect snowcover in Pennsylvania and New York and snowcover in northern Maine, the East is snow-less.

A big winter storm is forecast to hit New Mexico, the Texas/Oklahoma Panhandles, and Kansas on Monday and Tuesday as an area of low pressure cuts northeastward (because of the positive NAO as explained above) through the Plains. A Blizzard Warning (green color on right image) has been issued for Northeast New Mexico while the other areas are under a Winter Storm Watch. Areas in the core of the snowfall north of Amarillo, TX up through Dodge City, KS could get well over a foot of snow.

1 comment:

Harry Hayfield said...

And judging by that map it is not looking too promising for us Brits either!