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

Widespread high temperatures in the 90s with heat indices exceeding 100 degrees will persist across the western U.S. this weekend into the week. There are fire weather concerns in the Pacific Northwest and the Great Basin. Monsoon conditions continue to linger across the Southern Rockies and Southwest posing a heavy rainfall threat which may lead to additional flash flooding concerns. Read More >


On the morning of September 17th, the remnant low of Florence was tracking across the Appalachians. The Wakefield CWA was on the east side of the low and a large area of stratiform rain was moving through much of central Virginia on the morning of the 17th with a few thunderstorms over far south-central VA/north-central NC. Winds were out of the SE at the surface at around 10 mph across much of central VA. However, as is the case with many decaying tropical cyclones, winds just above the surface were slower to diminish. The 10 AM 925hPa analysis showed winds out of the south at 30-40 mph. Thus, a large amount of low-level speed and directional shear was in place over parts of the area. This allowed for 0-1 km Storm Relative Helicity values to exceed 250 m2/s2 over our southwestern counties during the mid-morning hours. The morning thunderstorms (including the one that produced the Boydton tornado) formed in a high shear/low CAPE environment before maximum daytime heating. However, the worst of this tornado event was still to come.

Just a slight increase in temperatures (coupled with dew points in the mid 70s across central VA) during the late morning/early afternoon allowed for a noticeable increase in instability. By 2-3 PM on the afternoon of the 17th, daytime heating allowed for SBCAPE to increase to 2500-3000 J/kg just south and east of the Richmond metro (with in excess of 2000 J/kg of MLCAPE). See Environment section for more details. With most of the realized CAPE in the lower part of the atmosphere, 3 PM SPC Mesoanalysis indicated that there were localized values of 0-1km SRH values of 250 m2/s2 over central VA (likely due to local backing of surface winds). Coupled with low LCLs, this created a near-ideal conditions for low-topped supercells to produce tornadoes in the Richmond metro area. As a result, nine additional tornadoes (EF-0 to EF-2) touched down in the Richmond metro between 1:30 and 4:30 PM. This turned out to be the most active tornado day in the AKQ CWA in over a year.

***Please note that some of this data is preliminary and is subject to change***



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