Regions of Pennsylvania

Philadelphia is the warmest and the most paved ground in Pennsylvania, and both show up in the forecast: the heat island runs the city several degrees above Chester County on a July night, and the Schuylkill and the Delaware take the runoff from everything upstream. Ida's remnants put the Vine Street Expressway under water in 2021, and the rain–snow line spends the winter somewhere between the airport and the Lehigh Valley.

Blue Mountain runs the length of this region and it is a weather boundary as much as a ridge: storms that arrive as rain in Allentown arrive as ice in Tamaqua, and the anthracite towns behind it run several degrees colder all winter. The Lehigh and the Schuylkill both drain off that ridge fast — Diane in 1955 remains the deadliest flood in the valley's history.

The Poconos are the snowiest ground in eastern Pennsylvania — the plateau stands high enough that a coastal storm giving Philadelphia rain gives Mount Pocono a foot. Below it the Wyoming Valley has the opposite problem: the Susquehanna makes a bend at Wilkes-Barre and Agnes came over the dikes there in 1972, which is still the flood of record for the whole river.

South central Pennsylvania is the state's warm corner and its snow-line argument. The Cumberland and Lebanon valleys sit low enough and far enough south to take rain when the ridges either side are white, which makes the forecast here a matter of a few hundred feet. The Susquehanna is the other story: it is a mile wide and a few feet deep at Harrisburg, and it has taken the city twice in living memory.

This is the emptiest and one of the coldest parts of the state — Potter County has darker skies and colder nights than anywhere else in Pennsylvania, and the West Branch valley below it pools that cold air until late morning. The same valley floods hard: Agnes in 1972 and the 2011 remnants of Lee both put Williamsport and Lock Haven behind their levees, and Renovo has nowhere to put the water at all.

The Allegheny Front is the sharpest weather line in Pennsylvania. West of it the ridges catch upslope snow off the Great Lakes and hold it — Somerset and Ebensburg take more than 100 inches in a good winter while Altoona, twenty miles east and 1,000 feet lower, takes a third of that. Johnstown sits where three streams meet in a gorge, which is why 1889 and 1977 both happened there.

Erie is one of the snowiest cities in the United States and all of it comes off the lake — 100 inches in an average winter and 34 inches in a single day on Christmas 2017, a state record. The snowbelt reaches inland to Corry and Kane and then stops. Behind it the Wilds are the emptiest country in the state, and Punxsutawney, on the southern edge of it, is where the whole nation asks about the forecast once a year.

Pittsburgh sits in a bowl where three rivers meet, and the terrain does most of the work: the valleys hold fog and cold air, the hilltop neighborhoods run several degrees warmer on a winter night, and the city is among the cloudiest in the country. East of it the Laurel Highlands rise 2,000 feet and take real snow — the ridge at Seven Springs gets more than three times what downtown does.