A composite of the path Friday's derecho storm took over… (National Weather Service,…)
The Baltimore region was left out of the earliest severe storm watches issued in the mid-Atlantic on Friday, giving it about three hours less to prepare for the deadly winds than the Washington area and points south and west.
At the Storm Prediction Center in Norman, Okla., where severe systems around the country are tracked for the National Weather Service, the first severe thunderstorm watch in the mid-Atlantic was issued at 6:35 p.m. Friday. It included the Washington metropolitan area, but went no further east than Frederick, Montgomery and Prince George's counties.
It was three hours later, at 9:51 p.m., that the Baltimore area received its first severe thunderstorm watch, according to Steve Zubrick, science and operations officer at the National Weather Service's Sterling, Va., office, which covers the Baltimore area. A severe thunderstorm warning for the region followed at 10:17 p.m., and the worst of the storm's winds arrived around 11 p.m.
Thunderstorm watches caution residents of the potential for severe weather to develop, while warnings alert the public that severe weather is imminent. Sterling officials strive to give the public two to four hours notice with thunderstorm watches, said Jim Lee, meteorologist-in-charge at the Sterling office.
Greg Carbin, warning coordination meterologist for the storm center in Oklahoma, said he and his colleagues were tracking the storm system as early as noon, as it developed in northern Indiana. It was difficult to predict whether it would weaken once it reached the Appalachian Mountains and the sun went down, as storms often do, he said.
It was the storm center that broadcast the information for the 6:35 p.m. watch, alerting the Sterling office to pass along the storm watch to the public, Carbin said.
"Anyone that would be looking at radar or getting alerts from the broadcast media should have been well informed that destructive winds were on the way," Carbin said.
But the center did not include the Baltimore region in the watch area. The Sterling office added Baltimore City and the surrounding counties to the watch area once the storms' reach became apparent.
Because storm systems typically weaken or break up upon passing the Appalachian Mountains, Sterling forecasters were reluctant to add the Baltimore area to the thunderstorm watch zone until the force of the storms became clear, Lee said.
"We didn’t have absolute certainty plus a time angle as well that it would make it to the Chesapeake Bay," Lee said of the storm system. "These are exceedingly rare events. Seeing these things go through and being able to forecast them accurately, you’ve really got to weigh that."
The Sterling office did make efforts to express the severity of the storms, Lee said, issuing what he called a "strongly worded" special weather statement that covered all of the state east of the Chesapeake Bay at 7:47 p.m. The statement cautioned that the storm had a history of producing wind gusts up to 75 mph.
"In a perfect world," Lee said, a thunderstorm watch would have been issued for the Baltimore area at least half an hour earlier. But there is also some degree of a sort of "warning fatigue" making it less likely residents would have taken a storm watch as seriously as was warranted, Lee said.
Also, predicting that the storm would have reached Baltimore with the intensity that it did was difficult given the rarity of the "derecho" storm. Such storms hit Maryland once about every four years, but impacts as severely and widely felt as those in Friday's storm are even more rare, meteorologists said.
Derechos have also been expected to make turns toward the south, Zubrick added.
"It's not unreasonable that you would expect that thing to go more south, but with time it would become clear it was holding its own further to the north," Zubrick said. "They're difficult to predict."
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