Where did that snow come from, anyway?
Okay, so it wasn't much of a snowstorm. But it was beautiful as it fell. It made the place look lovely at daybreak today and, best of all, it was a wintry surprise that caused little disruption.
So where the heck did it come from? The forecasts for Thursday had called for a little bit of snow across southern Maryland - Calvert, Charles and St. Mary's counties. But Washington and Baltimore weren't supposed to see anything. Then the morning and midday forecasts began to suggest a chance of snow in the city. Then came warnings about slippery spots on the homeward commute. It was snowing in Washington by 2 p.m., and by 3 p.m. Baltimore looked like one of those snow-filled glass paperweights.
In the midst of it all, the forecasters at Sterling said it would taper off by 5 p.m. But it kept on snowing - big, fat flakes on the slow ride home, and it was still snowing in Cockeysville at 10 p.m.
So, what happened? Blame a stubborn "blocking pattern" a thousand miles out in the Atlantic, and an unexpected injection of energy into our storm from another one in the Ohio Valley, says Geoff Cornish, a meteorologist with the Penn State Weather Communications Group.
Our storm developed as a low pressure center in the Gulf Coast of Louisiana and Texas. It gained strength and moved across the Southeastern states, riding the southern branch of the jet stream.
"I imagine most forecasters thought this would pass as a relatively weak low pressure system and quietly pass of the Carolina coast," Cornish says. "I got the impression it did behave as forecasted in the Blue Ridge and the Carolinas. It was just in the northern fringe where many of the forecasts didn't work out as well as expected."
A second low had moved along the northern branch of the jet stream from north of Minnesota into the Ohio Valley. That system injected new energy into "our" storm, moving the snow zone north into the metropolitan areas and intensifying it.
At the same time, Cornish said, another area of low pressure far out over the Atlantic that had been stationary for most of the week, began to push westward. "So we had two different disturbances converging," he said. The low from the east smushed into our storm and forced the air at the collision point to rise. He compared it to toothpaste in the center of a tube that's squeezed from both ends at once.
The rising air meant mixed precipitation for southern New England and Long Island early Friday.
"It was a little bit bizarre, but certainly not unprecedented," Cornish said. "The atmosphere does these things whenever you get into a blocking pattern. Computer models don't always handle blocking patterns all that well."