Several poor souls caught in that wild storm on Interstate 95 at about 4:30 p.m. Saturday described the ice that pelted their windshields as "sleet." The National Weather Service called it "pea-sized hail." But meteorologist Jeff Warner, of Penn State Weather Communications, said it was neither. He called it "graupel." Whatever it was, it combined with rain, sun glare and steam rising off the pavement to turn the highway into a demolition derby - at least 17 multi-car collisions, involving more than 90 vehicles. Several people were injured, but thankfully, there were no fatalities.
Warner said hail forms in thunderstorms when ice crystals high in the atmosphere contact super-cooled water. The water turns to ice and the hail stone grows. It grows until it's too large and heavy to stay aloft in the storm's powerful updrafts. The storm on Saturday, Warner says, wasn't a thunderstorm.
Sleet forms in winter when a warm air mass runs up and over a cold air mass. As its moisture rises, it cools, forming snowflakes, which fall toward the surface. When they hit the warm air layer, however, the flakes melt. Then, as they hit the cold air near the surface, they refreeze and form hard ice pellets. None of that applied on Saturday, Warner said.
Graupel is sometimes called "soft hail." It forms in strong cold fronts, like the one on Saturday. The air above 5,000 feet was near freezing, he said, and snow formed. When it reached warmer air below about 5,000 feet, the flakes partially melted and stuck together. Caught in updrafts, they merged with water droplets and formed a sort of "soft" or "squishy" ice pellet.
Warner said State College, Pa. also saw an intense graupel shower at about 3 p.m. Saturday. It's fairly common in the lee of the Great Lakes when intense cold fronts sweep off the lakes but it's not yet cold enough to cause "lake effect" snows. It's much more unusual this far south and east.
He called Saturday's event a "typical autumn cold front with a sharp temperature gradient." Winds at Martin State Airport at 3:45 p.m. were out of the southwest at a lazy 7 mph, with temps at 63. Two hours later the winds were from the west at 15 mph, with gusts to 26, and temperatures had fallen 13 degrees, to 50.
At BWI, temperatures fell 11 degrees - from 61 to 50 during the same period, with a peak wind gust of 31 mph.