Wednesday Afternoon Technical Discussion

Synopsis:

Earlier today, high pressure has moved offshore. A low pressure area, partially fueled by a shortwave trough, is making its way northeastward. It will bring mostly rain to the Commonwealth on Thursday, but mixed precipitation - both at the onset and as the cold front moves through - cannot be ruled out. High pressure will build in during the wee hours Friday and control our weather for Friday and Saturday. We will watch an area of low pressure (currently located in the southwest U.S.)- move along the I-10 corridor throughout the remainder of the week. The 00Z & 12Z GFS trends toward a nor'easter for Sunday, but no other models agree. We have to wait and see.

Models Used: 12Z NAM, GFS, & Euro + 15Z SREF; Navy & CMC models consulted

Short-term (Thursday's Precipitation):

A shortwave is causing the 0°C isotherm and the 540-Dm line to bulge northward right over PA. The precipitation starts around sunrise Thursday -  as rain with some sleet and wet snow mixed in - as the warm front moves north. The cold front zips through after sunset with the chance for a snow shower as it passes. No more than one-half of an inch will fall, with an avg. precip. amount of 0.39".

Long-range (THON weekend forecast, Possible Nor'easter):

High pressure will be in control for Friday and Saturday. It will sit over PA.

A cut-off low is feeding Sunday's potential nor'easter as it sits over the SW US. For the nor'easter scenario to play out, the cut-off low needs to be absorbed back into the flow as it nears TX. While the 00Z and 12Z GFS say it will, the 06Z GFS and all other models say that the cut-off will not interact with the northern branch ! on Saturday AM as it nears TX. The precipitation, if any, will start around sunset Saturday. One thing is clear: the low is likely to interact with the left-exit region of a jet streak (an area favorable for enhanced upper-level divergence), causing the low to rapidly intensify to around 984 mb - IF the scenario plays out. There is the potential for a foot of snow in Philadelphia, but model disagreement and run-to-run inconsistency gives this scenario little chance to play out. It is worth watching closely over the next few days. Our call: a chance of snow showers for the eastern half of PA on Sunday.

-Jaron Breen