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Outlook for Thursday, May 28

Outlook Summary

Within a very extensive swath of marginal severe-storm potential, two areas appear to have the most-concentrated hail/wind threats today: parts of Mississippi/Alabama and southern Tennessee, as well as southwest Texas near the Rio Grande.

Outlook Images

overview

tornado 2%

wind 15%

hail 15%

Detailed Outlook

SPC AC 281254

Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Thu May 28 2020

Valid 281300Z - 291200Z

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF MISSISSIPPI/ALABAMA AND SOUTHERN TENNESSEE…AS WELL AS SOUTHWEST TEXAS NEAR THE RIO GRANDE

### SUMMARY

Within a very extensive swath of marginal severe-storm potential, two areas appear to have the most-concentrated hail/wind threats today: parts of Mississippi/Alabama and southern Tennessee, as well as southwest Texas near the Rio Grande.

Synopsis

The upper-air pattern from the CONUS westward across the northeastern Pacific will remain one characterized by split flow and cutoff lows, albeit with a deamplifying trend beginning in this period. The Ozarks-area cyclone and attached trough are forecast to devolve into a weakening, open-wave perturbation. By 12Z tomorrow, the resulting shortwave trough should extend from the lower Ohio Valley, northern MS, northern LA and southeast TX, somewhat in phase with a convectively induced/enhanced vorticity maximum over northeastern MX. This will occur as a northern-stream trough – evident in moisture-channel imagery from the MB-ON border to the Dakotas – pivots to the James Bay region, Lake Superior and the upper Mississippi Valley. Strong synoptic-scale ridging will shift eastward across the AZ, the Great Basin and northern Rockies.

At the surface, a low was analyzed over east-central IA, with diffuse, quasistationary front across eastern MO, central AR, and southeast/south-central TX. A secondary cold front was organizing from the low southwestward across southwestern OK to northwest TX and southeastern NM. The secondary front will overtake the northward-retreating leading boundary over south TX this afternoon and evening.

An extensive area of at least marginal, localized severe potential is evident from the interior Mid-Atlantic to the Gulf States and southeastern NM, with a northward break, then near parts of the central Rockies in low-level upslope flow. This discussion will focus on two relative concentrations of severe threat within the broader swath.

TX near Rio Grande

Widely scattered thunderstorms are expected to develop today over the western and northern parts of the outlook area. Thunderstorms regionally will be forced by a combination of lift from surface heating, the southward-moving baroclinic zone, and orographic forcing over the Big Bend region and northern Coahuila. Additional convection is possible this evening farther east as the frontal zone impinges on richer inflow-layer moisture. Large hail and severe gusts will be the main concerns.

Height rises are expected as the Ozarks cyclone/trough weaken and move away from the area. Nonetheless, associated northwesterly flow aloft and strong directional shear will persist, despite weak low-level winds. This will contribute to 30-40-kt effective-shear magnitudes, though the lack of more robust low-level flow will limit hodograph size. Steep midlevel lapse rates, along with some moisture return through an air mass affected by prior days' convective complexes, will combine to yield MLCAPE in the 2000-3000 J/kg range across much of this region. More-robust moisture return is probable this evening over south-central TX and the lower Rio Grande Valley, which will be available to any convection that can develop anew along the combined frontal zone, or evolve upscale from diurnally initiated convection from Coahuila. That contingency appears increasingly probable based on more-recent guidance and the observational trends in the lower Rio Grande Valley, so the outlook has been expanded on the east side.

MS/AL

Widely scattered to scattered thunderstorms should develop this afternoon and move northeastward across this region, offering sporadic strong-severe gusts and large hail. Activity should form atop a weakly capped, strongly heated and increasingly moist boundary layer. Peak preconvective MLCAPE may reach 1000-1500 J/kg, despite modest midlevel lapse rates, with a well-mixed subcloud layer supporting maintenance of gusts/hail to the surface. The outlook area generally represents a relative max in expected convective coverage within the broader moist plume occupying the low levels to the southeast of the ejecting cyclone/trough, along with associated modest increase in deep shear. A cyclonically curved belt of relatively strong mid/upper winds will contribute to 30-40-kt effective-shear magnitudes and favorable upper-level ventilating flow, in support of convective organization. Weak low-level winds and lack of stronger buoyancy will be limiting factors.

..Edwards/Smith.. 05/28/2020

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National Risk Overview

Thursday, May 28
TORNADO: 2%
HAIL: 15%
WIND: 15%
Friday, May 29
TORNADO: 2%
HAIL: 5%
WIND: 15%
Saturday, May 30
ANY SEVERE: 5%
Sunday, May 31
ANY SEVERE: low / uncertain
Monday, June 1
ANY SEVERE: low / uncertain
Tuesday, June 2
ANY SEVERE: low / uncertain
Wednesday, June 3
ANY SEVERE: low / uncertain
Thursday, June 4
ANY SEVERE: low / uncertain

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