Northern Virginia weather forecasting a tricky business


Ever wonder why its so difficult for Washington area meteorologists to accurately predict winter weather?

One reason is a formation called the coastal frontogenesis, a phenomenon involving the mountains and coastal plain. Throw in variables like the, winds, air masses and water temperatures over the Chesapeake Bay and forecasting down to the precise hour gets difficult for the local news as well as the plow drivers.

Its a balancing act, said Jim Lee, a meteorologist at the National Weather Service in Sterling. Where the coastal front sets up is the tricky part, Lee said.

So far this year, there were two storms that featured snow and ice as part of the picture, and both were completely different.

On Dec. 5, the storm came in from the northwest, and was called a clipper, in the weatherman world. Clippers typically come in from the northwest and hit the air mass that is over the area that many times is holding moisture from the south. Predicting the precipitation depends on how cold the air mass is when the clipper arrives, Lee said. For that particular storm, NWS had two computers that predicted two different outcomes for that storm one with just rain and the other with the wintery mix. In the end, a trained meteorologist had to step in adding value to the models, Lee said. The other model [with snow] had it pretty much pegged, he added.

Bob Ryan, chief meteorologist at NBC4 in Washington, noted that the storm dumped the amount of snow that he said it would, just a little earlier than predicted. Predicting within an hour or two is difficult, Ryan said.

A week later, the area was hit by another storm that came up from the south, along the coastal front that made forecasting more difficult. At first, there were predictions of a mixture of snow and sleet that Friday, but it all turned to rain by Saturday afternoon. In these cases, the ocean temperatures and the air masses play a bigger factor. That weekend, at 6,000 feet, the air mass was way too warm, Lee said. At the same time, the colder air hung in the Shenandoah Valley, and there was a lot of freezing rain in Loudoun County.

At the Virginia Department of Transportation, snow removal is a big deal and everything hinges on the National Weather Service forecasts. The snow guru at VDOTs Northern Virginia Headquarters is Branco Vlacich, who is responsible for snow removal on 16,000 lane miles of roads in Northern Virginia. He checks the forecasts twice a day and when snow and ice are forecast, Vlacich coordinates a plan based on the storm track, road temperatures and the timing. With rush hours lasting from five to 10 in the mornings and three to eight in the evenings, its a juggling act at VDOT.

Salt trucks and plows dont do well in rush hour. Throw in school systems with late openings, and the Federal government snow schedule, and putting trucks out on the roads gets complicated. It all depends on how accurate that forecast is, and how timely it is, Vlacich said.

When the temperatures hover around 32-33 degrees, the situation is even trickier, and sometimes there is a thermal line that goes right through the area with ice on one side and water on the other. The challenge for us is where that line is drawn, said Vlacich, noting that the National Weather Service has a tough time drawing that line as well. Many times, their computers come up with nine different scenarios and officials try to narrow it down from there. Who can be that accurate? Vlacich said.

Although forecasting in the Washington Metropolitan area seems to be tricky sometimes because of this coastal front, its that way with several east coast cities, even in Boston. Same thing up there, said Lee, who lived there several years ago. The geography and the coastal area create a natural boundary that traps air masses cold and warm. Although, Frederick and northern Maryland are in an area of more snow, it is not always a sure thing. Even up there can be a little tricky, Ryan added.

Salmon works for VDOT public affairs and is a free-lancer for The Times.