Inauguration Weather Forecast Update: Weekend Snow Threat Still Evolving

As noted here this morning, one of the major U.S. weather models (known as GFS) has been suggesting the development of a storm system over the Southeast which could bring some snow to the Washington area late Sunday and into Monday. As of the afternoon runs based on 1 pm data, this is not being confirmed by the shorter-range, higher-resolution model (NAM) or the earlier European medium-range model (ECMWF). The afternoon GFS model output, which is still being generated, is generally consistent with that model's earlier results, however.

This afternoon's forecast discussion from the local National Weather Service office notes:

WHAT HAPPENS BEYOND SATURDAY AFTERNOON IS MUCH MORE INTERESTING AND CHALLENGING. 12Z GFS DEVELOPS LOW PRESSURE ACROSS THE SOUTHEAST STATES ON SUNDAY AND TREKS IT NORTHEAST ALONG THE MID ATLANTIC COAST THROUGH MONDAY MORNING AND UP THE NORTHEAST COAST MONDAY AFTERNOON. THIS SOLUTION WOULD BRING INCREASED CHANCES OF SNOW ACROSS THE CWA ESPECIALLY AREAS EAST OF THE BLUE RIDGE SUNDAY NIGHT INTO MONDAY MORNING. THERE/S SOME SUPPORT FROM THE 09Z SREF AND GFS ENSEMBLES IN THIS SOLUTION. HOWEVER...THE 00Z AND 12Z ECMWF DOES NOT SUPPORT THIS SOLUTION AND HAS LITTLE OR NO PRECIPITATION CHANCES SUNDAY NIGHT INTO MONDAY MORNING EXCEPT FOR UPSLOPE AREAS OF THE HIGHLANDS. THE 12Z NAM IS IN THE SAME CAMP AS THE ECMWF. THE DIVERGING SOLUTIONS ADD TO THE UNCERTAINTY OF THIS FORECAST. AFTER MUCH CONSIDERATION AND COLLABORATION...WE HAVE DECIDED TO INCREASE CHANCES OF SNOW ACROSS THE CWA IN LIGHT OF THE GFS/GFS ENSEMBLE/SREF FORECASTS. IF THEY ARE CORRECT...SOME SNOW ACCUMULATION IS LIKELY. ON THE FLIP SIDE...IF THE ECMWF/NAM SOLUTIONS WIN OUT...THEN LITTLE OR NO SNOW MAY OCCUR. WE WILL NEED TO KEEP A CLOSE WATCH ON THIS SYSTEM AND ADJUST DETAILS AS MODEL GUIDANCE BECOMES IN BETTER AGREEMENT.

Meanwhile, bitterly cold air is entrenched in the area. After a morning low of 11° at 8:23 am, the high has made it up to only 18°. This makes it the coldest daily high since it was 17° on Feb. 4, 1996. Tonight promises to be the coldest of the current spell, with lows around 8°.

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Image (click to enlarge): GFS model sea level map of pressure and precipitation, 7 am, Monday, January 19, 2009, from NCEP/NWS/NOAA