Saturday, December 03, 2011

Forecast for December 4, 2011

The biggest change in our extended forecast has been to account for the 12z Guidance which is suggesting a later arrival of a cold front and associated deepening low pressure area along the boundary. Trends over the past few months have been to delay the initial expectations of arrival with our area storms as we get closer to an event, so this seems reasonable. This now appears to be moving in for Wednesday Afternoon into Thursday Morning. The 12z GFS has a typical La Nina storm track with the storm cutting up through Central and Western Pennsylvania. The GFS shows the low creating a pretty tight gradient developing on the backside of the system as the low deepens to 988 MB and this could translate to a period of strong wind gusts. In addition, based on the 12z GFS track, there could be a pretty good swath of heavy rain across much of the region, perhaps with some areas approaching at least two inches of rain. Each model is handling the heaviest axis of rain a bit differently. Certainly, if the GFS comes out on top and is correct with the heaviest rain over our flood prone communities, then we will have some issues to contend with during the Wednesday into Thursday period. The GFS model currently shows the 540 line pushing through our area after the precipitation shuts off, so chances for frozen precipitation are limited at the moment in the Delaware and Lehigh Valleys. This could change though in future forecasts, such as if the low center and front are further east than expected. Someone is bound to end up with a good wet snow or ice event in Western and perhaps portions of Central Pennsylvania if all the cards fall into place and the storm track is to our west.

This later timing also means a brighter early week and milder temperatures lingering until the low pressure area and cold front passes and these changes have been accounted for. But, when this front passes and low pressure area heads off to Northern New England, cold air will puncture into the Middle Atlantic creating a cold weekend. It does appear that for the first time this season, the 540 Line remains south of the region for an extended period of time after our storm this week.

18z Update:

The 18z GFS shifts the storm further alignment with the 12z Euro...which would introduce mixed precipitation chances in our NW Suburbs especially. We will see what 00z brings along....this track is going to bounce around over the next 24 to 36 hours.