Synopsis
Energy at 500-mb has created two surface features: one is moving through PA now, and the other, located over northwest-central Ontario at 00z, will move through the Commonwealth Monday night into Tuesday. A weak high pressure area will allow for some clearing on Tuesday afternoon before a slow-moving warm front crosses the state Tuesday evening. The warm front will head into upstate NY and hover near I-90. A strong cold front will cross the state on Thursday night with rain and thunder. Temperatures will fall rapidly toward daybreak and back-end snows will return on Friday.
Models Used: 12z ECMWF, 18z GFS, 18z & 00z NAM & Hi-Res NAM
Short-Term
As the cold fronts moves through, temperatures may be slightly under-predicted by MOS. The Ontario-system is only starting to get its act together at the surface. Some rain is likely as it crosses the state, especially in the western areas. The NAM placed the warm front into New York while the GFS & Euro kept it along the PA-NY border on Wednesday into Thursday. Once the warm front crosses the PA-NY borde, MOS will be too cold for highs on Wednesday and Thursday and a bit too warm with Wednesday night's lows, so use the 850-mb method with super-adiabaticy for high temperatures and subtract a few degrees off of MOS' low for Wednesday night.
Long-Range
The cold front will arrive in western PA around 9 PM Thursday and pass through Philadelphia and Scranton around noon on Friday. Thunderstorms will be likely in the lower elevations in western PA with thundershowers in the central and eastern part of the Commonwealth overnight.
This is the same cold front that will bring severe thunderstorms to the southern states. If the front arrives in western PA earlier, then thunderstorms could occur across PA, but heavy rain and a few rumbles of thunder are likely.
As for the cool-down, the 850 mb temp at 9 PM Thursday will be above 10°C and then fall to -5°C by midnight Friday into Saturday. The heavy rain will also help cool things down. The upper-level low will cause the back-end snow (it could be just back-end rain and graupel) on Friday afternoon and evening.
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