Friday, March 15, 2013

March 15, 2013: Spring Ain't Here - More Snow Coming?

Well, it sure feels mild out there in the strong mid-March Sun, but the large-scale weather patterns have another idea in mind, to close out this month.

One of the big indicators of short-term (7-10 day) climate variation, called the North Atlantic Oscillation (NAO) Index, has swung highly negative - in fact the most negative it's been all winter long.  A negative phase of the NAO means a jet stream pattern that favors persistent, abnormally cold air over the Eastern U.S. 

You can see it here:

Courtesy NOAA
The red curves at the very end of the top panel indicate that the NAO index is forecast to remain very negative through the rest of March. 

Unseasonably cold air masses will visit our region over the next week.  Tomorrow through Monday, two weak waves will move through, each producing light precipitation - mainly rain.  However, the disturbance Sunday night has the potential to work with some really chilly air, stuck against the east slopes of the Appalachians.   This is called Cold Air Damming.  The sub-freezing air layer may spell a period of light, wintry mix Sunday night into Monday morning:  Cold rain, sleet, snow.  The NWS thinks that any accumulation (again, light) will occur along the Mason Dixon Line and in the higher western elevations.   But we're in for a 2-3 day ride of chilly, gray, damp, miserable weather - much more of a mid-January type of weather pattern.

One of the medium-range forecast models, the GFS, is hinting at a more significant winter storm for our region next Saturday-Sunday, as we get into the backside of a coastal low with an air mass cold enough for all snow.  Well, we'll see.  Such a storm is consistent with the strongly negative NAO.   But climatologically, a big snow event is not very likely after the vernal equinox (which occurs late this coming week).  And I am very reticent to make any type of forecast call based on one run of a single model 9 days in the future.   It's worth noting, for the record, but so very much can change in the models over the next 8-9 days.  I would not bet money on this scenario. 

No comments:

Post a Comment