“All 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 can fall over a period of one to three hours, making rapid rises in streams and streams possible, as well as flash flooding in urban areas.”
A flood alert has been issued for the entire area (except Cecil County), in effect until this evening. Localized heavy rainfall in these areas can 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 a little lower than Thursday.
Back to Thursday’s thunderstorms
Thursday’s thunderstorms were more numerous and longer than expected. We had called for random storms ending at 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 these reports. However, there have been a few reports of flash flooding in the district and in Baltimore. Precipitation totals were mostly between 0.5 and 1.5 inches, but some areas had between 1.5 and 3.5 inches, mostly east of Interstate 95.
Wind damage was concentrated in small pockets and was associated with downward gusts of wind high in the sky that struck the ground and fanned out; these are known as microbursts. A particularly intense microburst caused damage to trees around Alexandria and Landmark. That same storm unleashed a 58 mph gust at Reagan National Airport. Trees were also knocked down inside the district, around Winchester, Gaithersburg, Columbia and Baltimore.
Why it was stormier than expected
Looking at the weather maps, the general environment seemed benign: there were no fronts nearby and the jet stream moved far 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 severity of the storm. There was an obvious low altitude disturbance that could have helped bring storms to the region, regardless of the activity that was mostly confined to the mountains.
However, given the unusually high surface temperatures and humidity – the highs reached in the mid-90s – the atmosphere became very unstable. The unstable air fueled intense but short-lived storm updrafts that quickly gave way to torrential downpours, concentrated cloud-to-ground lightning and microbursts.
Additionally, cold downdrafts from these cells spreading along the surface quickly froze and helped lift air over a wider region, causing loosely organized clusters of thunderstorms to evolve over hours. . In a sense, the storms have “primed” into widespread coverage and a higher degree of organization. The prediction models mishandled this consolidation. The greatest degree of consolidation was near and east of Interstate 95, where the axis of greatest instability was located. Additionally, cooler winds off the bay have increased temperature contrasts, intensifying nearby storms.
The thunderstorms were picturesque
Although dangerous, the storms produced stunning scenes across the sky. A number of readers simultaneously captured thunderbolts and rainbows and, in the storm’s wake, beautiful mammatus clouds. These clouds feature pouch-like appendages that sometimes hang down under the anvil of intense thunderstorms.
Below are some examples…