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Hazardous Heat in the Western U.S.; Heavy Rain and Flooding Potential in the Southern Rockies

Widespread high temperatures in the 90s with heat indices exceeding 100 degrees will persist across the western U.S. this weekend into next week. Some daily high temperature records are forecast to be tied or broken. Monsoon conditions continue to linger across the Southern Rockies posing a heavy rainfall threat which may lead to additional flash flooding concerns. Read More >

Overview

A strong low pressure system at the surface and aloft slowly filled in as it tracked over Lake Michigan during the day on Wednesday. Meanwhile, its associated warm front extended SE through the OH Valley and into far southern VA during the morning of 4/3. A secondary surface low had begun to develop over SW Virginia during the morning, with a trailing cold front extending south of the low just east/southeast of the southern Appalachians. As the secondary low moved NE during the day, the warm front lifted north before stalling from the south-central VA Piedmont to the Richmond Metro to the Northern Neck by early afternoon. Elevated convection moved into the Piedmont by noon-1 PM. While a couple of those cells exhibited strong rotation on radar, they failed to produce tornadoes, likely due to being on the "cool" side of the front (and thus having no surface based instability to work with). However, one of those elevated cells became surface based as it tracked into Essex/Richmond Counties SE of Tappahannock. The cell, located just to the SE of the stalled frontal boundary, produced an EF-1 tornado (estimated max winds of 95-100 mph) as it tracked from north-central Richmond County to just SW of Lewisetta. The environment was characterized by very strong shear with a meager to moderate amount of surface based instability. With the strong deep-layer shear, several cells in the warm sector (across SE VA/NE NC) exhibited supercellular characteristics, but failed to produce tornadoes. Importantly, surface winds were more SSW deep into the warm sector across SE VA/NE NC, while they were SSE near that secondary low over the northern neck. While 0-1km SRH values were similar in those two locations, there was more low-level directional shear closer to the low in the northern neck due to the SSE winds. This may very well have been the difference that resulted in one storm producing a tornado while others did not.
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