“Any thunderstorms will be capable of producing very heavy rainfall, with localized totals of two to four inches possible,” the Weather Service wrote. “Much of the rain may fall within a one to three hour period, making rapid rises in creeks and streams possible, as well as flash flooding in urban areas.”
A Flood Watch has been issued for the entire area (excluding Cecil County), in effect through this evening. Localized heavy rainfall in these areas may cause rapid rises in creeks and streams, as well as urban flash flooding. #DCwx #MDwx #VAwx #WVwx pic.twitter.com/M8CtXPr7wS
— NWS Baltimore-Washington (@NWS_BaltWash) August 5, 2022
A few storms could also contain damaging gusts, although the wind threat is somewhat lower than Thursday.
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Looking back on Thursday’s storms
Thursday’s storms were more numerous and longer-lived than expected. We had called for hit-or-miss storms ending around sunset; instead, widespread storms developed during the evening and lasted until after midnight.
The Weather Service received about 140 reports of severe weather over the greater region; downed trees and gusts over 39 mph were the sources of most of those reports. However, there were a handful of reports of flash flooding in both the District and Baltimore. Rainfall totals were mostly between 0.5 and 1.5 inches, but there were some areas that posted between 1.5 and 3.5 inches, mostly east of Interstate 95.
The wind damage was concentrated in small pockets and was associated with downward bursts of wind high in the sky that slammed into the ground and fanned out; these are known as microbursts. One particularly intense microburst caused tree damage around Alexandria and Landmark. This same storm unleashed a gust of 58 mph at Reagan National Airport. Trees were also toppled inside the District, around Winchester, Gaithersburg, Columbia and Baltimore.
Why it was stormier than forecast
Looking at the weather maps, the overall environment seemed benign: There were no nearby fronts, and the jet stream was displaced far to the north over Canada. With the jet stream so far north and a massive “ridge” of high pressure dominating the Mid-Atlantic, wind shear was weak, and not thought to have contributed to the storm severity. There was a weak high-altitude disturbance evident that might have helped kick off storms over the region, independent of the activity that was mainly confined to the mountains.
However, given the exceptionally high surface temperatures and humidity — highs reached the mid- to upper 90s — the atmosphere became very unstable. The unstable air fueled intense but short-lived storm updrafts that quickly paved the way for torrential downpours, concentrated cloud-to-ground lightning and the microbursts.
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Additionally, the cool downdrafts of these cells spreading along the surface quickly congealed and helped lift the air over a broader region, causing loosely organized clusters of storms to evolve as the hours ticked on. In a sense, the storms “bootstrapped” themselves into expanded coverage and a higher degree of organization. The prediction models did a poor job handling this consolidation. The greatest degree of consolidation was near and east of Interstate 95, where the axis of greatest instability lay. Additionally, cooler winds off the Bay increased temperature contrasts, intensifying nearby storms.
The storms were picturesque
While hazardous, the storms produced some stunning scenes across the skies. A number of readers captured views of lightning strikes and rainbows simultaneously and, in the storm’s wake, beautiful mammatus clouds. These clouds feature pouch-like appendages that sometimes hang beneath the anvil of intense thunderstorms.
Below are some examples …