Before the rains came, the month of July got off to its second-driest start on record. Now it’s been the wettest July ever and it’s nearing the mark for the second-most rainfall in any month.
And meteorologists said the tropical weather pattern that brought the precipitation is likely to return before August arrives.
With the surge of tropical moisture over the past week, Baltimore already has surpassed a 129-year-old record for July rainfall and is on pace for its wettest summer since observations began in 1870.
As of Wednesday morning, BWI Marshall Airport reported 15.04 inches of rain for the month to date.
Such extreme precipitation is more common in late summer and early fall, when tropical cyclones bring moisture levels more common in Florida or the Caribbean. But in this case, forecasters blamed an unusually potent weather pattern with low pressure over the Southeast and high pressure off the Atlantic coast, working together to pull a river of tropical moisture from the Bahamas directly toward Maryland.
“We’ve got a direct conduit to the tropics, and it’s pretty unusual,” said Eric Luebehusen, a meteorologist with the U.S. Drought Monitor who runs an email listserv focused on Baltimore-area weather. “You basically have tropical moisture converging on the mid-Atlantic. That’s an ugly setup.”
The region blew past the previous July rainfall record, 11.03 inches set in 1889, by late morning Tuesday. The milestone was all the more remarkable considering only a few hundredths of an inch of rain fell at BWI during the first two weeks of July, meteorologists said.
The deluge closed roads across the region Tuesday, and Baltimore County officials reported making 15 swift-water rescues of people trapped in floodwaters. That included two adults and a child on a county school bus in Sparks.
By Wednesday morning, July ranked as Baltimore’s second-wettest month on record, moving ahead of August 1933 and about 3 inches behind the record from August 1955. Historic tropical storms hit in both of those months.
While such tropical cyclones come and go in a matter of days, the current pattern has persisted since Saturday, and is forecast to return.
Dan Hofmann, a meteorologist with the National Weather Service’s Baltimore/Washington forecast office, said it’s not unprecedented for Baltimore to be caught in a pattern like this one.
Forecasters call the stormy system over the Southeast a “cut-off low,” meaning it’s disconnected from the jet stream’s steering forces and left to churn until something pushes it off shore. And they call the area of high pressure off shore a “blocking high,” because it literally stands in the way of any weather system looking to move out to sea.
Both systems are stronger than usual for this time of year, he said.
“It’s just more severe than it normally would be,” Hofmann said of the pattern. “We’re drawing up even more moisture from the tropics.”
Luebehusen said meteorologists have observed record moisture levels in the atmosphere for this time of year in Baltimore — levels that are commonplace in the tropics. He said it’s possible that warming temperatures near the North Pole are helping to set up the unusual pattern.
A large contrast between polar cold and tropical warmth helps keep the jet stream — a highway of air high in the atmosphere that drives weather patterns — strong. But as Arctic temperatures warm and ice melts, the jet stream can be more erratic and allow blocking systems like the one off the Atlantic coast to form.
Baltimore’s wet weather pattern is forecast to persist Wednesday. A flash flood watch remains in effect across Central Maryland through Wednesday afternoon.
But then spells of dry weather are forecast as a pair of weak cold fronts pass Thursday and Saturday.
“Hopefully coming out of Wednesday we get a reprieve and we can begin to further assess,” said David McMillan, Baltimore’s emergency management director.
The city was dealing with flooding along the Jones Falls valley Tuesday, forcing the evacuation of the Meadow Mill complex.
Rain clouds could return Saturday. And then, meteorologists say, the conveyor belt of tropical rains could reappear by early next week.
That could keep Baltimore on pace for its wettest summer on record. There has been about 20 inches of rain in June and July, almost three and a half inches more than the second-wettest June-July period on record, in 2015.
And since May 1, more than 28 inches of rain has fallen, 6 inches more than the previous record for that period, from 1989. That is about two-thirds of Baltimore’s typical annual rainfall.
Baltimore Sun reporters Sarah Meehan and Catherine Rentz contributed to this article.