Why this D.C. summer is so miserably humid and stormy

Washington Post

3 days ago

It's been a broken record. Every morning when Washingtonians venture outside, they are immersed in suffocating humidity, like stepping into a sauna. Then, in the afternoon and evenings, thunderstorms erupt, fueled by the saturated air.
The repeated downpours have triggered flooding — and unleashed swarms of mosquitoes. D.C. area residents are growing weary.
To blame is a tropical weather pattern — drawing near-record amounts of moisture from the Atlantic Ocean, Pacific Ocean and Gulf of Mexico — that has settled over the Mid-Atlantic, the same one responsible for deadly flooding from Texas to New Jersey. National Weather Service reports indicate July has featured twice the normal number of floods across the country, and the agency has issued flood watches in record numbers for the D.C. area.
The onslaught that began in June may not fade until early next week when humidity levels may ease slightly. But, even then, it's not clear if the change will be sustained.
'This has been the worst summer, I am so tired of the storms and the flooding and the MOSQUITOES that are EVERYWHERE because of it,' Dax Murray wrote on the social media platform Blue Sky.
'This has to be some sort of sick joke,' Capital Weather Gang reader John Lindsey added on X Wednesday after the National Weather Service issued its 15th flood watch since May.
Summers in Washington are known for their humidity, but this one ranks among the muggiest in recent decades. Humidity is often measured by the dew point, the temperature to which air would need to cool to reach saturation. Dew points at or above 70 degrees are considered sticky and uncomfortable.
From June 1 through July 16, dew points in D.C. reached 70 or higher for 573 hours, second-most on record since 1945, only trailing 1994 (603 hours).
The 15 flood watches issued by the Weather Service since May are by far the most in a three-month period since 2007 when such data became available. The seven watches in June were the most issued in a single month. Already, six have come out in July, and more than a third of the month remains.
D.C. has received 7.79 inches of rain since June 1, which is just slightly above normal. However, many parts of the region have received substantially more. Thunderstorms tend to be hit or miss, and summer rainfall totals often vary widely over a small geographic zone.
Since May, which was also quite wet, much of the region has received 2.5 to 5 inches more rain than normal.
The benefit of the frequent deluges has been the end of the drought. As recently as early May, moderate to severe drought covered much of the region. Now, it affects only portions of far northeast Maryland.
A large dome of high pressure over the Atlantic Ocean, often referred to as the 'Bermuda High,' has been the primary driver of the muggy, wet weather pattern. It has remained stuck in place while persistently pumping steamy air westward and northward over the eastern and central United States while halting the eastward progression of cooling fronts. Ocean temperatures beneath the high-pressure zone are abnormally warm, intensifying the moisture transport.
Meanwhile, the jet stream — or high-altitude air current that sometimes dips southward and draws in cooler air from Canada — has remained too far north to send cold fronts southward. But, lurking over the Great Lakes and Northeast, it's been close enough to supply energy for thunderstorms.
This uncomfortable, tempestuous weather pattern is poised to last until at least early next week. After that, the pattern may start to change, though not necessarily in a welcome way.
The Bermuda High may fade and another zone of high pressure — or a heat dome — may build over the Southeast United States and expand north into the Mid-Atlantic in seven to 10 days. That would lower storm chances and perhaps take a slight edge off the humidity.
The high humidity and considerable cloud cover generated by the current weather pattern have held temperatures somewhat in check. The highest temperature so far this month is just 95, whereas upper 90s or even low 100s aren't uncommon at this time of year.
But, should the upcoming pattern change materialize, we may trade oppressive humidity for punishing heat.