Technical Discussion

Synopsis…

The ridge of high pressure is beginning to break down as a cutoff low develops and brings a somewhat significant rain event to the Mid-Atlantic region.

Short-term (Sat. – Sun.):

The cloud cover on the back side of the system seems like it is having more difficulty pushing south than the models indicate, but we expect clouds to increase in the afternoon as far south as Williamsport and State College.  The clouds then clear out tonight and skies stay generally clear with some high clouds during most of the day Sunday with the Canadian high pretty well in control.  Meanwhile on Sunday, the low to our west cuts-off and heads south.  There is a high degree of disagreement between the models, as the GFS brings the upper-level low farther south and to the west than the WRF, and forms the surface low farther west as well.  Then the GFS is slower than the WRF, which is expected.  We agree more with the WRF since the GFS is typically slow this time of year.  The CMC brings a pretty robust short wave down the cut-off low, which is something I've never seen before and something I don't think we'll see in the near future.  According to the WRF timing and precip, scattered showers will blow in off of the ocean into eastern PA as soon as Sunday night.

Long-term (Mon. – Wed.):

The 12Z WRF shows a first round of moderate to heavy precip moving NNE between Philadelphia and Johnstown late on Monday until early overnight Tues., and a period of light/moderate rain or numerous showers with the occluded front moving north and  affecting areas farther to the west.  Then a second round of moderate to heavy precip with a line of thunderstorm indicated moves through overnight and during the day Tuesday, affecting the same area but with a more eastward movement.  This looks to be the basic anatomy of this system.  SREF probability precip amounts maps show a similar story, but it's obviously more washed-out.  These maps and SREF plumes indicate a conservative half inch of rain on average across the state, but most likely many areas could get close to an inch with locally higher amounts.  This weather system is a significant change in the pattern of the last month.

Scott Sieron