January 31 – February 2, 2011 North American blizzard fell as freezing rain in the Philadelphia suburbs, a mixture of sleet and freezing rain in Berks County and the Lehigh Valley and mainly a snow and sleet mixture in the Poconos. Overnight colder air moved in aloft in over the Poconos and precipitation changed back to all snow for a few hours. Toward sunrise on the 2nd, this process started to reverse at both the surface and aloft. Warmer air was moving north again and the freezing rain changed to plain rain across the Philadelphia suburbs and Berks County around 8 a.m. EST and the Lehigh Valley around 9
Hail . GR is derived from the French word grêle. Smaller-sized hail, as well as snow pellets, use the coding of GS, which is short for the French word grésil. Terminal velocity of hail, or the speed at which hail is falling when it strikes the ground, varies. It is estimated that a hailstone of in diameter falls at a rate of , while stones the size of in diameter fall at a rate of . Hailstone velocity is dependent on the size of the stone, friction with air it is falling through, the motion of wind it is falling through,