Like commuters caught in a Beltway tie-up, Marylanders find themselves coping with the consequences of a collision that was none of their doing: A plume of warm, moist air from the tropical Atlantic has plowed into a cold front stalled in an atmospheric traffic jam.
The result has been a seemingly endless succession of showers and thunderstorms, with heavy - sometimes torrential - rains that have dumped more than 10 inches on parts of the state.
And there's more in store. Forecasters say the Baltimore region and southeastern Pennsylvania could see as much as 3 more inches before the deluge begins to ease Thursday.
"We know it's not going to get any better here in the next 48 hours," said meteorologist Barry Lambert of the National Weather Service's forecast office in State College, Pa. Its coverage area includes the rising tributaries that feed the lower Susquehanna River.
"It is a very serious situation, and in some cases life-threatening if people don't take precautions," he said.
The cold front has stalled along the spine of the Appalachian Mountains because it's stuck behind a stubborn high-pressure system over the North Atlantic far to its east.
"It's a northern extension of [a familiar summertime Bermuda high], but much farther north, centered east of Nova Scotia," said Geoff C. Cornish of the Penn State Weather Communications Group.
The jam extends clear across the country, he said. In the West, a high-pressure system has settled in, producing temperatures of 90 to 100 degrees in normally cooler places such as Portland, Ore., and Seattle.
At the same time, upper-level winds off the tropical Atlantic, from the south, are pumping moist air into this region.
When it runs up against the cold front, the air is forced to rise up and over the cooler, denser air. It cools and condenses, Cornish said, "and we have cloud formation. ... We also have plenty of rainfall - very, very heavy storms in some cases because the air is so humid and also relatively warm."
The unstable air rises in "pockets," growing into very localized thunderstorms, "maybe the size of half a county, or one or two counties," he said.
Their effect has been amplified by the fact that these storms, stoked by the continuous supply of warm, moist air, keep forming. Pushed along the front by the southerly winds, they track along the same paths, pounding the same locations with repeated downpours.
"It almost looks like a train running due north across parts of Virginia right now," Cornish said. "Some areas are picking up a great deal of rain."
Sunday's rainfall at Baltimore-Washington International Thurgood Marshall Airport totaled 2.75 inches, drowning the 1.95-inch record for the date that had stood since 1872 - a year after they began keeping records for Baltimore.
Ronald Reagan Washington National Airport reported 7.09 inches in the 24 hours ending at 8 a.m. yesterday. That is the second-greatest 24-hour rainfall on record for the capital, just behind the 7.19 inches recorded during Tropical Storm Agnes in June 1972.
Twenty-four-hour rainfall amounts varied widely across the region yesterday. Stations in Calvert County received barely a half-inch. Portions of the Eastern Shore found themselves beneath rain "hotspots," pounded by repeated thunderstorms. Storm totals have topped 10 inches in several parts of the Shore.
Greg DeVoir, a weather service meteorologist in State College, described the situation as "perhaps the most potent ... setup for flooding I've ever seen over central Pennsylvania that has not been associated with a [decaying] tropical cyclone. We will have our hands full."
Streams across much of Maryland yesterday were running at record high volumes for this time of year. Lambert said some Pennsylvania counties that have seen 4 to 6 inches of rain over 24 to 36 hours will see more of the same in the next two days.
"For numerous counties to receive 5 to 10 inches of rain in the early summer is pretty unforeseen," Lambert said.
Only small streams and tributaries have been flooding so far. "But we are going to get some large streams starting to reach bankful here in the next 24 to 36 hours as the rain trains over moist ground."
The main stem of the Susquehanna was not expected to flood, although it could fill its banks.
"People living along streams and creeks that easily flood in response to short-duration, heavy rains ... need to take some serious precautions," Lambert said.
There is some relief in sight.
"Drier air is going to work its way east, and the high pressure in the Atlantic is going to shift east finally and make room for this front to shift east," Cornish said. "There could be a storm or two on Thursday, and even possibly on Friday, but ... nothing torrential like we've seen in the past few days."
Even with 3 more inches of rain from this system, Baltimore does not appear to be headed for a June record.
Nearly 5.5 inches had fallen at the airport by last evening. But meteorologists have recorded three months when June rainfall has topped 9 inches, in 1948 (9.36 inches); 1963 (9.16 inches), and in 1972 (9.95 inches), which included Agnes.
Normal June rainfall at BWI is 3.43 inches.