... High pressure will maintain dry and cooler than average weather through Monday. A warm front will bring rain chances and seasonable temperatures back to the region on Tuesday.
NEAR TERM /UNTIL 6 AM MONDAY MORNING/... No changes were needed for the eve update as dry high pressure will maintain a clear sky and good radiation conditions. Morning lows will thus be well be well below the averages with readings around the 40F mark, and into the 30s in the sheltered valleys.
SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY/... Our next rain chance arrives late Monday night into Tuesday, as a shortwave drops into the area in NW flow on the front side of the main ridge, while a warm front crosses at the surface. The resultant lift will combine with increasing moisture to create at least scattered showers. Thunderstorms can also be expected on Tuesday with increased instability. While GFS/NAM indicate 40-50 knots of 0-6km bulk shear during the afternoon, instability is more questionable given the likely morning presence of morning clouds and showers. Thus, severe risk is muted at this point. Continued warm advection will bring high temperatures up to or slightly above seasonal levels.
LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... A relative lull in precipitation is expected Tuesday night into Wednesday morning as the warm front and upper wave depart. The associated cold front will then approach from the north Wednesday afternoon and evening. With mid-level flow becoming increasingly zonal, progress of the boundary will be slowed, and models differ a bit on timing. This timing will be crucial, as the models suggest a heightened risk of severe weather. Model soundings indicate the possibility of MLCAPE in excess of 2000 J/kg, along with 0-6km bulk shear of at least 40 knots and an elevated mixed layer. The upper NW flow pattern will be favorable as well.
The front will eventually work its way at least into the southern CWA by Thursday night, with the best chance of afternoon/evening thunderstorms lying to the south of Pittsburgh. Another boundary from the north could provide continued rain chances into Saturday, with drier weather by Sunday. Temperatures will remain near or above normal for the bulk of the extended period.