Synopsis
As high pressure exits the state to the northeast tonight, a low pressure system will begin to approach the state Tuesday morning. Southeasterly flow ahead of the precipitation will bring warmer air into the region and result in rain showers across western Pennsylvania while precipitation begins as a wintry mix across the rest of the state. Expect it to quickly change to rain across eastern Pennsylvania Tuesday afternoon while cold air damming keeps enough cold air across central & northern Pennsylvania to remain a mix of snow, sleet, and freezing rain. Higher elevations of the Laurel Highland should also remain a mix of snow and freezing rain. Precipitation will change over to scattered snow and rain showers Wednesday morning as precipitation moves off to the northeast Wednesday afternoon. Scattered snow showers will spread across the state late Wednesday night through Friday morning, as the low pressure system slowly crosses the state. Arctic air will then make its way into the state behind the exiting system as a mean northerly flow will persist for the weekend producing scattered lake effect snow showers.
Short Term (Monday Night thru Wednesday Afternoon)
High pressure currently sitting over the region will move toward Maine tonight. Clouds will slowly make their way into the state as a strong low pressure system to the southwest approaches. A southeasterly flow will escort warmer air into the area as rain begins to fall in extreme southwestern counties by about 16Z Tuesday according to both 18Z NAM and GFS. Precipitation could start out as snow Tuesday afternoon for central and northern portions of the state, but then change to a mix of snow, sleet, and freezing rain to the north and for higher elevations, and rain mixed with some snow elsewhere. Precipitation will be falling statewide by 00Z Wednesday. The majority of the precipitation will fall ahead of the warm front. The models split the low pressure system into two separate centers of low pressure due to cold air damming. One center of low pressure appears to develop off the coast and produce heavy rain for southeastern PA late Tuesday night. The other low pressure remains stationary over western Ohio during the day Wednesday. The inland low pressure should remain dominate through Wednesday as it pushes east through northern Pennsylvania and eventually off the New England coast.
Long Term (Wednesday Night into the Weekend)
Wednesday night will bring colder air as the cold front continues to make its way east. The inland low pressure will slowly make its way across the state from west to east along with scattered snow showers Wednesday night through Friday morning. The heaviest snowfalls will occur in western portions of the state due to lake effect snow persisting through Friday night due to a mean northerly wind from the wraparound of the low pressure sitting off the coast according to the 18Z GFS. A deep trough will develop for the weekend bringing cold air and a northerly flow allowing for a chance for scattered lake effect snow showers for the weekend, especially across the western half of the state.
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