As of midday Friday, forecasters at the National Weather Service in Sterling, Va., were uncertain that the Baltimore area would see any thunderstorms at all later in the evening.
Without a cold front or a low-pressure system in place, they lacked the markers that would guarantee violent weather. If a disturbance did occur, however, all the heat and humidity in the lower atmosphere would serve as fuel.
"We knew that any thunderstorm that did develop could become severe," said Stephen Konarik, a meteorologist with the weather service.
That knowledge did not prepare them for what actually unfolded, a line of storms from the Midwest to the Mid-Atlantic that hurled winds at hurricane force and left millions of people without power. It took only a small, upper-level disturbance near Chicago to interact with all that heat and produce a rare, straight-line windmaker known as a "derecho."
"The widespread nature of the image we saw is a very rare occurrence," Konarik said. "We would not have been planning on that magnitude of event."
Certainly, Maryland residents, utility crews and emergency workers were caught off guard. "Last night was a historic event," Gov. Martin O'Malley said, describing a "nasty little line of storms that we had been tracking, hoping it would go north of us, but it cut right across us."
Residents, who had seen nothing more than vague warnings of a possible thunderstorm, were shocked as winds bent over trees and lightning flashed nonstop.
"We got no warning from the weather forecast — none," said John Leonhart of Homeland. "I guess it was a real surprise to everyone, apparently even them."
BGE had put extra crews on the street as it would in anticipation of a typical summer thunderstorm, said spokesman Rob Gould. But even when those storms turn out to be bad, they usually leave outages in the tens of thousands, not the hundreds of thousands.
Gould said BGE officials developed an inkling of the storm's potential just before 10 p.m., but even then, they expected it to lose potency as it crossed east over the mountains. Within an hour, the storm lashed the area full-force.
"This one came with a vengeance," he said. "This is hurricane-like, no question."
The difference is that when a tropical system approaches from the south, the utility has days to import out-of-state crews and to distribute supplies such as utility poles and transformers around the area. BGE spent a week preparing for Hurricane Irene last year, Gould said, bringing in more than 1,000 out-of-state utility workers. The restoration still took a week.
For Friday's storm, which did three-fourths as much damage as Irene, BGE had no outside help in place. Worse still, the storm came from the Midwest, which often supplies crews to help with bad weather in Maryland. Those crews are instead coping with their own damage, Gould said.
Help is on the way in the form of 600 workers from Texas, Mississippi, Florida, Tennessee and Michigan, but Gould said many of them will not join the effort until Monday. Because BGE was caught by surprise, he said, customers should expect the restoration to last "deep into next week."
Gould said he has never seen a storm of this magnitude strike with such surprising suddenness. "As advanced as we are technologically," he said, "Mother Nature can still do what she wants to do, when she wants to do it."
Though Konarik didn't put it quite that way, he agreed that the storm was the kind of freak occurrence that might always defy prediction. "It's just a rare event," he said.
Baltimore Sun reporter Yvonne Wenger contributed to this article.Copyright © 2015, The Baltimore Sun