Technical Discussion

Synopsis:

There will be a mix of sun and clouds on Monday, as weak high pressure affects the region.  However, a significant disturbance already developing in the western United States will bring moderate precipitation to the state as it approaches later in the week.  Expect light to moderate rain showers beginning on Tuesday and lingering into Wednesday.  After this low pressure system passes through Pennsylvania, cloudy skies and unseasonably cool temperatures will prevail on Thursday.   

Short Term:

Both the GFS and the WRF forecast weak high pressure briefly impacting the region on Monday, after the weak cold front passes through tonight.  They also project a brief 'dry' slot of sorts in association with this synoptic feature in both the middle and lower atmosphere.  We forecasted a mix of sun and clouds across the state for the day, before clouds begin building in Monday night.

Long Term:

The high amplitude trough currently developing over the Rockies will progress towards Pennsylvania by Tuesday.   The models are in decent agreement regarding the timing of the precipitation in association with this moderately strong low pressure system.  The WRF's 1200 UTC run has the rain showers entering the most southern regions of the state beginning late Tuesday morning.  The GFS's 1200 UTC run is virtually identical in this respect.  The ensemble mean of the SREF 900 UTC run was a bit quicker in the arrival of the precipitation, though.  We therefore forecasted rain showers beginning first around the morning in our southern zones and around mid-day in the more northern zones of the state.  The models exhibit a wide spread in projected precipitation amounts on Tuesday into Wednesday.  The 3-hour precipitation amounts, as seen in the plumes of the 1500 UTC run of the SREF ensemble, range anywhere between a tenth and four-tenths of an inch at the peak of the precipitation event.  Most of the ensemble members consistently projected this maximum to occur Tuesday evening into the early hours of Wednesday morning.  The ensemble mean was approximately around three quarters of an inch for this time period.  The SREF plumes, as well as the WRF and GFS runs, show the swatches of precipitation mainly leaving the state by late Wednesday afternoon, the northern zones last.  We therefore forecasted moderate to heavy rain showers for Tuesday night, with only lingering showers in most zones on Wednesday.  Finally, we made mention of breeziness in some zones for Wednesday. The models suggest that subsidence occurring under the left entrance region of an upper level jet to the south of Pennsylvania, in combination with cold advection, will lead to moderately strong winds across most of the state.

-Alicia Klees