A challenging time for meteorologists

February 13, 2007|By Frank Roylance | Frank Roylance,sun reporter

Today's storm is the sort of puzzle that has meteorologists tearing their hair out as they try to figure out what sort of precipitation will fall on which parts of Maryland.

"The whole East Coast in general is a very challenging place for forecasts when you get a storm like this," said Jeff Warner, a meteorologist at Penn State Weather Communications Group.

The bay, the ocean, arctic air masses to the north, warm, wet storms from the south all come into play in very complex ways, he said. "As a forecaster, it's kinda where you earn your money. But I enjoy it. It's interesting. It's fun."

The National Weather Service issued winter storm watches and warnings for all of Maryland west of the Chesapeake Bay as forecasts pointed toward deteriorating travel conditions today. Snow, sleet, freezing rain and rain all threatened to make the day miserable for millions of residents.

The primary storm center was expected to move north and east into the Ohio River valley today, bringing precipitation to the Baltimore area - most likely starting as snow in the early morning and then changing to sleet and freezing rain.

Sleet consists of ice pellets that form as snow falls from higher altitudes through a layer of warmer air about 5,000 feet up. It melts, then refreezes as it falls though a layer of sub-freezing air near the surface. Today's forecast high temperature at BWI is about 27 degrees.

If that warm layer aloft is thick enough, and the cold surface layer thin enough, the precipitation will stay liquid but freeze on contact. Such freezing rain begins to accumulate on cars, roads, sidewalks, tree limbs and power lines.

For Baltimore, Warner said, this could continue into tonight. As much as a half-inch of ice "is certainly not out of the question. Baltimore is at high risk for more than a quarter inch of ice," he said.

The precipitation might turn to all rain tonight as temperatures rise, he said. But tomorrow, a secondary low pressure system will develop off the Carolina coast. It will begin to spin counterclockwise, drawing in the arctic air lingering just to our north.

"Then it will turn colder and blustery, and probably drier tomorrow," Warner said. The high of 32 degrees will plunge to about 14 degrees overnight. Thursday and Friday promise to be cold and windy.

The rush of cold air tomorrow could also turn any continuing precipitation back over to snow, leaving a couple of inches behind before the storm departs, he said.

frank.roylance@baltsun.com

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