A winter storm produced a protracted mix of snow, sleet and freezing rain across most of Eastern Pennsylvania. Snow and sleet accumulations averaged 1 to 3 inches (highest amounts in the Lehigh Valley and Poconos) and ice accretions ranged from around one tenth of an inch in the local Philadelphia area to around three tenths of an inch in the Lehigh Valley and the Poconos.
Precipitation started as snow throughout Eastern Pennsylvania during the evening of the 17th. As warmer air moved in aloft, the snow changed to sleet and freezing rain in southeast Pennsylvania between Midnight EST and 3 a.m. EST on the 18th and between 3 a.m. EST and 6 a.m. EST on the 18th in Berks County, the Lehigh Valley and the Poconos. The erosion of cold air near the surface took longer and the transition to plain rain occurred around the local Philadelphia area between 7 a.m. and 10 a.m. EST on the 18th, but took the rest of the morning into the early afternoon to reach the northern Philadelphia suburbs, Berks County and the Lehigh Valley and until the middle of the afternoon on the 18th just as the precipitation was ending in the Poconos.
Untreated roadways were very hazardous and the prolonged period of sleet and freezing rain impacted commuting on the 18th especially in the Lehigh Valley and Poconos. Numerous fender benders were reported. In the local Philadelphia area, Forty-two flights were cancelled at the Philadelphia International Airport.  One hundred fifteen passengers were stranded overnight on the 17th. SEPTA canceled several commuter trains on the morning of the 18th and had delays of up to 30 minutes on other rail lines. The Norristown (Montgomery County) High Speed Line was using shuttle buses between Radnor and the Norristown Transportation Center because of a downed tree. Service was also curtailed on SEPTA bus routes. Hundreds of schools opened late. PECO Energy reported scattered trees were knocked down because of the ice, but the overall number of outages was small. In the Lehigh Valley, in Northampton County, Eastbound Interstate 78 east at Pennsylvania State Route 33 was closed for more than an hour on the morning of the 18th because of a jack-knifed tractor-trailer. School districts in the Lehigh Valley were closed for the day on the 18th.

Representative snow and sleet totals included 3.8 inches in Schnecksville (Lehigh County), 3.0 inches in Pottstown (Montgomery County), 2.6 inches in Weatherly (Carbon County), 2.5 inches in Perkasie (Bucks County) and Pocono Summit (Monroe County), 2.0 inches in Lehighton (Carbon County) and Nazareth (Northampton County), 1.9 inches at the Lehigh Valley International Airport, 1.6 inches in Martins Creek (Northampton County), 1.5 inches in Hanburg (Berks County), 1.2 inches in Drexel Hill (Delaware County), 1.0 inch in Exton (Chester County) and the Philadelphia International Airport.
The wintry mix of precipitation was caused by the combination of a low pressure system that emerged from the Gulf of Mexico and an arctic high pressure system that left cold air in place, especially near the surface. The low pressure system moved from the eastern Gulf of Mexico on the 17th northeast to near Cape Hatteras, North Carolina at 7 a.m. EST on the 18th. It moved northnortheast and was just east of Atlantic City, New Jersey at 1 p.m. EST on the 18th. From there it turned more to the northeast and passed south of Montauk Point, New York by 7 p.m. EST that evening. The surface high pressure system was over New England on the evening of the 17th and at 7 a.m. EST on the 18th was centered over Nova Scotia. As it moved farther to the east, the combination of that motion and the low pressure system moving up the east coast cut off the supply of sub freezing temperatures near the coast.

Winter Storm in CARBON, PENNSYLVANIA
From 17-JAN-11 23:00:00 to 18-JAN-11 13:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Storm in MONROE, PENNSYLVANIA
From 17-JAN-11 23:00:00 to 18-JAN-11 13:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Storm in BERKS, PENNSYLVANIA
From 17-JAN-11 21:00:00 to 18-JAN-11 12:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Storm in LEHIGH, PENNSYLVANIA
From 17-JAN-11 22:00:00 to 18-JAN-11 13:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Storm in NORTHAMPTON, PENNSYLVANIA
From 17-JAN-11 22:00:00 to 18-JAN-11 13:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Storm in BUCKS, PENNSYLVANIA
From 17-JAN-11 21:00:00 to 18-JAN-11 13:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Storm in MONTGOMERY, PENNSYLVANIA
From 17-JAN-11 21:00:00 to 18-JAN-11 09:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Weather in PHILADELPHIA, PENNSYLVANIA
From 17-JAN-11 20:00:00 to 18-JAN-11 08:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Weather in DELAWARE, PENNSYLVANIA
From 17-JAN-11 20:00:00 to 18-JAN-11 08:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Weather in CHESTER, PENNSYLVANIA
From 17-JAN-11 20:00:00 to 18-JAN-11 08:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter