Seven-day rainfall forecast from National Weather Service. Localized amounts could be significantly higher (or lower.)

After the stretch of three glorious days ends on Friday, Washington’s weather will take a serious turn for the worse this weekend and especially next week. An oppressively humid weather pattern, with tropical pedigree, sets up  — one that could support very heavy rainfall and flooding at times.

The soggy pattern may last for a week, at least. It will not rain continuously, and some areas will be wetter than others. But the atmosphere will be so loaded with water that heavy, even extreme, rainfall will be possible.

If you have outdoor plans next week, dry intervals will be possible at times. But you’ll need to stay weather-aware, given frequent chances for showers and storms, especially in the afternoon and evening hours.


European model forecasts extremely high levels of atmospheric moisture sourced from the Bahamas streaming up the East Coast Monday night. (WeatherBell.com)

While the rain is likely to be intermittent and scattered, the humidity will be inescapable. Dew points, an indicator of humidity, will rise to near 70 by Sunday and remain near or above that level for all of next week. We could see dew points in the mid-70s or even higher at times. Whenever the dew point exceeds 70, we classify the corresponding humidity level as “gross.”

The relentless humidity will be an annoyance, but the rainfall and flood potential could turn serious.

When we examine weather patterns from the past that resemble the one forecast for next week, the closest match or “analog” is June 26, 2006, one of Washington’s most extreme rainfall events ever recorded. On June 25 and 26, 2006, Washington received 5.19 and 4.22 inches of rainfall, its second and third wettest June days on record. Only Hurricane Agnes in 1972 produced a greater single-day amount in June.


Top 5 heaviest June rainfall events in Washington.

The June 25-26, 2006, two-day total of 9.41 inches was Washington’s most on record dating back to 1871. The storms caused “massive flooding” in the Washington area, The Washington Post wrote at the time. “The cause is a ‘tropical connection’ that has been funneling an extremely wet air mass from the Bahamas up the East Coast, said Jim Lee, meteorologist in charge for the National Weather Service in Sterling,” The Post reported. “The moist air is going up against a stationary front that has hung over the mid-Atlantic for the past week.”

A similar weather pattern may evolve next week. Weather models show an area of high pressure stuck in place in the western Atlantic. It will act like a massive pump, circulating extremely moist air from the tropics over the East Coast day after day.


European model shows high pressure over the western Atlantic Ocean more or less stationary for the next week. It will act like pump, pushing moisture from the Bahamas up the East Coast. (WeatherBell.com)

At the same time, a front may become stalled to our west, which would help focus moisture over the Mid-Atlantic.

“One of the key elements in the 2006 event was a stalled front that set up with an unusual north-south orientation,” Jeff Halverson, Capital Weather Gang’s severe-weather expert, said in an email. “Today’s National Weather Service forecast charts for Monday through Wednesday show an identical frontal configuration. One could envision deep moisture running parallel to this boundary … literally for days … setting up waves of storms.”


National Weather Service forecast shows stalled front over the Mid-Atlantic region Monday through Wednesday next week.

Both the European and American weather models show the potential for several inches of rainfall. The official National Weather Service forecast predicts two to four inches of rain through next Thursday, noting increased potential for “areas of heavy/excessive rainfall” in both the Mid-Atlantic and Northeast starting Sunday.

Additional heavy rainfall could occur beyond Thursday if model forecasts, which show the pattern persisting through next weekend, are correct.

It’s important to note that just because the next week’s pattern may try to mimic that of June 2006, it does not mean the rainfall will be as extreme or even in the same ballpark. How much rain ultimately falls will depend on the specifics of small-scale weather features, which cannot be predicted this far in advance.

Localized rainfall amounts may vary considerably, and some areas could receive substantially more or less than current forecasts of two to four inches. It is not possible to determine where the heaviest rainfall may occur, but simply that the potential exists for high amounts.