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Severe Thunderstorms, Heavy Rainfall, And Flooding Possible Over The Plains; Typhoon Bualoi Approaches The CNMI

A storm system will bring the potential for severe thunderstorms capable of damaging winds, large hail, and isolated tornadoes over the south central U.S. into Monday. Heavy rainfall may also lead to flash flooding. The system will shift toward the eastern U.S. on Tuesday. Over the western Pacific, Typhoon Bualoi will intensify and impact the northern Marianna Islands over the next couple of days. Read More >

Overview

On the morning of April 19th, a deep upper trough was centered over the Mississippi River Valley, with the associated area of low pressure at the surface centered just west of the Appalachians. Much of the eastern United States was in the warm sector, and showers and thunderstorms (some severe) were ongoing from South Carolina to northern Florida. Winds were SE-SSE over much of the Mid-Atlantic region, with 850-500 mb winds largely out of the SSW-SW as the upper trough was slowly moving to the ENE. Temperatures warmed into the 70s throughout the area by early afternoon before an area of rain (with a few embedded severe t-storms) quickly moved across the CWA from south to north. After the area of rain exited the CWA, temperatures rebounded back into the low-mid 70s from central VA to northern NC by late afternoon-early evening (with dew points in the mid-upper 60s). This allowed for modest surface-based instability to develop over the area (SBCAPE/MLCAPE values of 500-1000 J/kg on average) while LCLs remained low due to the relatively small T/Td spread. At the same time, a line of thunderstorms with embedded supercells rapidly tracked to the NE across NC towards our CWA.

As the line entered our area from the SW during the early evening, winds were still out of the SSE as the strong (~992 mb) surface low was centered over the Appalachians (directly to our west). Winds veered to the S then SSW from 925-850 mb, with 45-60 kt winds just 1-2 km above the surface. This rapid increase in winds from the surface to the 925-850 mb layer (combined with veering of the winds from the surface-850 mb) resulted in 0-1 km SRH values of 300-400 m2/s2 over much of the area. As the aforementioned upper trough moved eastward (and a closed upper low formed over TN/AL by evening), mid-level winds increased to nearly 70 kt over the area (but wind profiles remained largely unidirectional from 850-500 mb). See the Environment section for more details (and a few RAP analysis soundings from selected locations) from 6-9 PM on the evening of the 19th.  The combination of modest surface-based instability, 300-400 m2/s2 of 0-1 km SRH, and low LCLs allowed for several tornadoes to touch down from the VA Piedmont to central/SE VA to interior NE NC as the line quickly moved across the area from SW to NE. With strong mean-layer winds, the storms moved as fast as 70 mph at times! All of the tornadoes over our CWA occurred from 6:30-8:30 PM. As the line approached the Lower Eastern Shore during the late evening hours, a stable marine layer in place (especially near the Atlantic coast) likely played a part in preventing any additional tornado development. In addition, this stable marine layer likely contributed to the weakening of the line as it approached the Atlantic coast.  While the main line of t-storms exited the area by 10 PM, more showers (w/ embedded t-storms) continued through the overnight hours across SE VA/NE NC before the trailing cold front crossed the area from SW to NE on the morning of the 20th (as the area of low pressure began occluding). There were a few reports of street flooding across far SE VA/NE NC. In total, *14* tornadoes were confirmed across the area on the 19th, making this the day with the highest number of tornadoes over our CWA since February 24, 2016.

See the Radar section for close-up images (including dual-pol variables) of several of the embedded supercells that produced tornadoes. There were even a few tornado debris signatures that were observed, and a couple of these are shown as well.

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

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