The most probable timing for any storms would be between about 6 and 10 am, arriving first along the Interstate 81 corridor and last in Southern Maryland.
Early Monday afternoon, the storm complex was just starting to organize in Wisconsin. By late afternoon, an area of severe storms was expanding near the border of Indiana and Ohio.
Computer models generally project the complex to intensify over the Ohio Valley on Monday night into early Tuesday before barreling southeast across the Appalachians toward Pennsylvania, Maryland, Northern and Central Virginia and the District.
The complex is expected to develop along the northern periphery of a heat dome, generating record high temperatures in the central United States. This zone — where hot, unstable air meets cooler air and storms tend to erupt — is sometimes called a “ring of fire.”
Record-setting heat wave expands east; over 100 million under alerts
There’s an outside chance that the storm complex, known as a mesoscale convective system, meets the criteria of a derecho — which is an extensive, fast-moving, long-lived and violent bow-shaped squall. However, if a derecho forms, it is very unlikely to be as strong as the infamous event about a decade ago on June 29, 2012.
The National Weather Service Storm Prediction Center has placed the region under a Level 2 out of 5 risk for severe storms Tuesday. Areas to the northwest are under an even higher risk — up to Level 4 out of 5 — “in anticipation of a highly organized MCS/possible derecho developing.”
The storm complex has the potential to generate a swath of widespread wind damage spanning multiple states as it approaches the Mid-Atlantic region early Tuesday.
The complex will track along a stalled front along the apex of the heat dome as it races from northwest to southeast. This front is depicted in the forecast surface weather map, valid 8 am Tuesday. The solid maroon coloring over West Virginia indicates the likely location of the severe weather threat:
Whether this complex will continue in an intense and damaging state, east of the Appalachians, is highly uncertain. Model projections indicate it’s a possibility, as shown in the high-resolution NAM model simulation below.
We define derecho as a fast-moving complex of thunderstorms that generates a damaging wind swath at least 250 miles in length, with multiple, continuous gusts exceeding 58 mph, and sometimes higher (ie into the 70-80 mph range).
But not all derechos are alike, in that they seem to exist along a spectrum of intensity and size. If this complex is deemed a derecho, its intensity may be mitigated by its morning timing, when there has not been enough sun to strongly destabilize the atmosphere.
Derecho science: The debate about what a derecho is and what it isn’t
The severe June 29, 2012, derecho arrived here during the late evening, one to two hours after sunset, after an afternoon when the temperature reached 104 degrees — Washington’s highest June temperature on record. This heat wave promoted an extremely unstable air mass—among the most unstable ever measured in the region—that was able to persist for a few hours beyond sunset.
Conversely, the derecho of June 13, 2013, one of two that impacted the Mid-Atlantic that day, managed to cross the Appalachians in the very early morning hours, but ended up in a fairly ragged state with minimal instances of damaging winds by the time it reached Washington.
In other words, the time of any storm complex’s crossing of the Appalachians and arrival on the eastern slopes does seem to matter in terms of derecho outcome. The odds may favor a less organized or weaker system when Tuesday’s storm complex arrives.
However, as some models continue to simulate an angry-appearing storm arc affecting the DC area on Tuesday morning, we have to take this threat seriously. Even if the mountains do take a bite out of the storm complex and the atmosphere is only weakly unstable, an extensive and deep “cool pool” containing high-momentum air could enable the system to sustain itself at an intense level for a time east of the mountains.
CWG will be monitoring upstream development trends, updated models and early evening weather balloon observations and will post updates on the situation at the top of this story.