As much as 6 inches of rain, and more in isolated areas, could cause widespread flooding across Central Maryland through Friday — including in Ellicott City, still recovering from the July downpour that triggered a deadly flash flood.
A slow-moving low-pressure system coming from the Great Lakes is forecast to pull in tropical moisture as it spreads over the region, likely soaking Maryland. Thunderstorms and pockets of intense rainfall could inundate some areas, National Weather Service meteorologist Chris Strong said, but it's impossible to predict where.
"It's like predicting where bubbles are going to be in your boiling pot of water," he said. "There's potential for spotty areas to see exceptional rain and extreme flash flooding."
Those predictions had Baltimore officials urging residents to clear storm drains, and meteorologists reminding them of the dangers of fast-rising, swift-moving waters.
Six inches of rainfall in a two-hour span July 30 sent a river down Main Street in Ellicott City that drowned two people and swept away dozens of vehicles. Officials were preparing for the possibility of another flood.
Howard County officials have been in "constant communication" with the weather service and relaying meteorologists' warnings to business owners and residents in Ellicott City, spokesman Mark Miller said. Crews put up sandbags along a stream bank that has not been restored since the flood, he said, and building inspectors have been checking on some of the properties still repairing flooding damage.
Miller said officials aren't expecting any flooding as severe as Ellicott City saw in July, because the rain is expected to fall over a longer period of time.
"We are just making sure we're taking every preventative measure we can," he said.
The heaviest rain was forecast to begin falling Wednesday night and continue through Thursday, Strong said. The system has the potential to drop as much as a foot of rain in isolated areas, he said.
Showers could then linger through Friday and into early Saturday, he said.
Public schools in Wicomico County were delayed for two hours and morning pre-K was canceled Thursday due to inclement weather, officials said.
The weather service declared a flash-flood watch in Central Maryland from 6 p.m. Wednesday through Friday morning. Meteorologists urged residents to keep an eye on streams and drainage ditches, and be prepared to quickly seek higher ground.
They also warned of the dangers of fast-moving water. Just 6 inches of rushing water can knock over an adult, and a foot can carry away a small car.
Baltimore-Washington International Thurgood Marshall Airport, the region's point of record, got 1.71 inches of rain overnight. The Inner Harbor had a little less, 1.41 inches.
Calvert County in Southern Maryland was deluged Wednesday night, however, with more than 5 inches reported in Prince Frederick.
Rainfall this year has been about normal at BWI, with 32 inches of precipitation. September has been dry, with just 0.91 inches of rain through Tuesday.
Amid the rain, temperatures are forecast to be cool, with highs in the lower 70s through the week and the mid-70s this weekend. Sunshine is expected to return by Sunday and through the early part of next week.
Baltimore Sun reporter Colin Campbell contributed to this article.