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Dangerous Heat in the Western U.S.; Flash Flooding Possible Across Portions of the South

High temperatures in the 90s to 100s and warm overnight temperatures will continue across parts of the Interior Northwest, central California, and the Great Basin. Thunderstorms and heavy rain may produce scattered flash flooding across much of the Southern Rockies into the Southwest, particularly over sensitive burn scars in New Mexico, and across the Southeast into the Carolinas. Read More >


General Summary: The day before making landfall in the CONUS (Sunday), TC Isaias was moving northward just east of the Florida coast. Meanwhile, an anomalously deep upper trough was centered over the MS River Valley with Bermuda high pressure (at the surface and aloft) well offshore of the VA/NC coast. Isaias continued to generally move northward just offshore of FL/GA/SC from Sunday night through the day on Monday as the aforementioned upper trough over the MS River Valley slowly moved eastward. Isaias made landfall Monday evening near Ocean Isle Beach in southeast North Carolina and would get picked up by the upper trough shortly thereafter. Rain (and a few thunderstorms) had already begun by Monday afternoon across the Wakefield forecast area as deep-layer moisture increased ahead of Isaias. Additionally, we were in the right entrance region of a 100-140 knot southwesterly upper (200-250 mb) jet on the east side of the trough. This placed our forecast area in a favorable region for upper divergence/lift which played a part in allowing for rain/t-storms to form well ahead of Isaias. Isaias would proceed to accelerate and track north-northeast through eastern NC and VA from Monday night through Tuesday morning as a strong tropical storm before quickly exiting to the NNE of the area by Tuesday afternoon.

The largest wind impacts were felt across eastern VA, northeast NC, and the Lower Eastern Shore, where S to SE wind gusts peaked at 65-75 mph. This downed several trees and power lines, resulting in widespread power outages (300,000+ at one point over eastern portions of the Wakefield forecast area). The heaviest rainfall shifted to along/just left of the track as Isaias interacted with the approaching upper trough. While flooding impacts were limited due to Isaias's fast movement, a widespread 2-5” of rain fell in most areas (locally 6”). The most impressive aspect regarding Isaias’s impacts on our area was the 15* tornadoes that were spawned across portions of eastern VA, NE NC, and the Lower MD Eastern Shore. There were multiple strong (EF2+) tornadoes. The EF-3 tornado in Bertie County was the first EF-3 tornado in the Wakefield forecast area since February 24, 2016. EF-3 tornadoes are quite rare in our forecast area, and are even rarer when associated with tropical cyclones. Storm surge impacts were minor in most surge prone areas, with the exception of the bay side of the Lower Eastern Shore and areas along the north side of the Albemarle/Currituck Sounds (including Back Bay in VA Beach). Moderate to locally major tidal flooding was observed in these areas. See below for additional details on the tornado/tidal flooding aspects of Isaias’s impact on our area.

Tornadoes: As Isaias tracked to the NNE through the area early Tuesday morning, a north-south oriented surface boundary set up on the north side of the storm (just to the east of I-95). On the east side of the boundary, winds were mainly out of the SE with temperatures in the upper 70s-around 80F and surface dew points in the mid to upper 70s. To the west of the boundary, winds were generally out of the north with temperatures and dew points in the upper 60s-around 70F. As is typical with tropical storms, the boundary layer wind fields were extremely strong. Along and just east of the track, there were 60-80 knot south to southeast winds in the 925-850 mb layer. These strong winds allowed for 0-1 km SRH values to climb to extreme levels (400-600 m2/s2) in an area in the vicinity of the surface boundary just to the north and northeast of the center of Isaias where there was some additional backing in the surface wind field (see Environment section for images from 2 am and 5 am). In addition, there was impressive deep-layer shear owing to the presence of the strong upper jet on the east side of the trough. While there was little to no instability on the "cool" side of the boundary, the mid-upper 70s dew points helped contribute to 1000-2000 J/kg of SBCAPE east of the boundary. When one of the outer rain bands of Isaias (already containing mini supercells) moved into the unstable/extremely sheared airmass just to the east of the boundary, this resulted in several tornadoes (2 strong). The tornado reports were generally clustered in a SSW-NNE direction in an area within 75 miles of the location of the boundary in the northeast quadrant of Isaias. This one rain band was responsible for nearly all of the tornadoes. 

Storm Surge/Tidal Flooding: While the fast movement of Isaias prevented widespread moderate-major tidal flooding, strong S to SE winds resulted in moderate (perhaps some locally major) tidal flooding across NE NC north of the Albemarle/Currituck Sounds and Back Bay in VA Beach. Additionally, the strong S to SE winds allowed some water to pile up in the mid to upper Chesapeake Bay early Tuesday morning. Then, when winds shifted to more of a westerly direction on the southern flank of Isaias during the mid to late morning on Tuesday, water quickly got pushed into areas on the bay side of the Lower Eastern Shore (and water levels in these locations rose VERY rapidly). Moderate tidal flooding was observed in most areas from Bishop's Head to Crisfield to Saxis, with locally major flooding in Cambridge (where brief, strong WNW winds created near ideal conditions for water to surge down the Choptank River into Cambridge itself. See the Tidal Flooding section for hydrographs from selected locations.

*All tornado data is preliminary and subject to change.

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