“Repeated rounds of showers and thunderstorms with heavy rain may result in at least isolated instances of flooding through much of the week,” the National Weather Service forecast office in Sterling, Va., said early Monday.
The National Weather Service’s rainfall forecast calls for two to three inches averaged over the area this week. But it’s a situation where amounts may vary considerably, depending on where the heaviest downpours concentrate. Some areas could see double that amount, while others end up with a much more manageable total of an inch or less.
The locations of heaviest rainfall will depend on where narrow corridors of showers and storms set up and “train,” or pass repeatedly.
There’s some indication that our western locations may have the highest rainfall potential this time because of the configuration of weather systems, while our eastern locations were hit the hardest last week. A lot of the tropical moisture last week was sourced from near the Bahamas, while this week the atmospheric river is originating more from the Gulf of Mexico.
It will not rain all the time. Like last week, we’ll have intervals, sometimes extending much of the day, with little or no rainfall. The threat for storms with heavy rain may be highest Wednesday or Thursday, when an approaching front serves as a focus for all of the moisture streaming our way. Generally, showers and storms will be most numerous in the afternoon and evening hours.
Because of all the recent rain, the ground is nearly saturated, and streams are close to full. This means torrential downpours would pose the risk of flash flooding, and flash-flood watches could be issued at times this week.
The recent rains in perspective
This week marks the third straight in which exceptional amounts of moisture have streamed up the East Coast. Rainfall totals in region have approached or surpassed records for July in the Washington and Baltimore region:
- Washington’s 9.19 inches ranks as the seventh most on record (since 1871).
- Baltimore’s 16.40 inches ranks as the most on record (since 1871).
- Dulles’s 9.75 inches ranks as the most on record (since 1960).
Dunkirk, Md., in Calvert County, logged 16.55 inches of rain in five days.
The moisture siege has pushed many other locations in the Mid-Atlantic and Northeast to their wettest Julys on record, including Harrisburg and State College in Pennsylvania and Elmira, N.Y.
Cape Hatteras, N.C., has now posted more than 20 inches this July, the most in any month on record. This rainfall tally is particularly notable at a location that’s a magnet for hurricanes but has not yet seen any tropical storm activity this month.
The abnormally wet weather is largely the result of a weather pattern that has been more or less stuck in place. A big area of high pressure has parked itself in the western Atlantic Ocean, acting like a pump and circulating very humid air over the East Coast.
At the same time, the jet stream has dipped unusually far south in the East Central United States. A front has set up along the leading edge of the jet stream, dividing cool air behind it and muggy air ahead of it, and serving as the focus for rainstorms.
This midsummer jet stream pattern is unusual and more typical of the colder months.