WOW, they just expanded the HIGH RISK both west and east some. Now includes Birmingham.
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0731 AM CDT Wed Mar 17 2021
Valid 171300Z - 181200Z
...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON INTO TONIGHT FROM NORTHEAST LA/SOUTHEAST AR ACROSS CENTRAL MS AND AL...
...SUMMARY... A significant tornado outbreak, with long-track, intense tornadoes is expected to begin this afternoon across parts of Louisiana and Arkansas, and then spread eastward and peak this evening into tonight across Mississippi and Alabama.
...Synopsis... A compact, intense shortwave trough over the TX Panhandle this morning will progress eastward over the Red River Valley today and the lower MS Valley tonight. This midlevel trough will be preceded by a surface cyclone that will move from northeast OK across northern AR/southern MO to the confluence of the MS/OH Rivers near the end of the period. This process will result in strengthening mid-upper flow over the lower MS Valley, as well as increasingly strong low-level southerly flow and moisture transport. The net result will be a broad warm sector centered on the lower MS Valley, capable of supporting multiple rounds of storms/supercells today into tonight.
...Short-term, ongoing convection... There are several zones of ongoing convection this morning, though most of this convection is on the fringes of a broad warm sector. The primary short-term threats will be focused with warm advection storms, now rooted near the surface warm front, from southeast AR across northern MS/adjacent southwest TN through about midday. More than adequate deep-layer shear and SRH, in a slowly warming/moistening environment, will support a continued threat for all hazards with this convection through about midday. Farther west, an extensive (but broken) band of convection extends from northeast TX into western AR in association with a bore-like wave propagating eastward in the warm sector. These storms will persist through the morning, with primary threats for damaging winds and large hail. Farther north, a loose cluster of elevated storms continues to spread east-northeastward from northeast OK toward southwest MO, in a zone of strong low-level warm advection with MUCAPE of 1000-2000 J/kg. Despite the elevated nature of the storms, there will still be the potential for downdrafts to reach the surface with strong/damaging gusts through the morning.
...Primary tornado outbreak this afternoon into tonight... The ongoing convection across eastern AR/western TN/northern MS will tend to reinforce the surface warm front near the TN/MS border through early afternoon. The northeast TX and eastern OK bands of storms will overspread the Ark-La-Tex and the remainder of central AR through mid-late morning, with the potential to produce damaging winds, large hail, and a couple of tornadoes. In the wake of this convection, some air mass recovery is expected across AR, back to the surface cyclone near the AR/OK/MO border intersection by midday to early afternoon. Thereafter, surface-based thunderstorm development is expected along the wind shift from near Shreveport to Fort Smith, and storms will spread eastward across AR/LA through the afternoon/evening.
The destabilizing warm sector with warming temperatures and mid-upper 60s dewpoints, will support MLCAPE up to 2000 J/kg, and strengthening vertical shear will be favorable for a broken band of supercells capable of producing tornadoes, large hail, and damaging winds. A couple of strong tornadoes will be possible this afternoon/evening across LA/AR.
Farther east, the most dangerous part of the severe weather outbreak is expected to evolve today into tonight from central MS into central AL. Here, there will be the potential for scattered supercell development in the open warm sector by midday, in an environment with MLCAPE near or above 2000 J/kg, effective bulk shear near 50 kt, and effective SRH near 200 m2/s2. All hazards will be possible with these warm sector storms during the afternoon. By late afternoon and into early tonight, a low-level jet segment will strengthen to 50-60 kt across MS/AL and the midlevel trough approaches from the west, contributing to very strong low-level shear (0-1 km SRH of 400-500 m2/s2). Buoyancy will be slow to decrease after sunset and with eastward extent based on the prevalence of upper 60s dewpoints, while very favorable wind profiles will maintain the threat for long-track, intense tornadoes with both warm sector supercells, as well as supercells within the broken band along and ahead of the surface wind shift progressing eastward across MS by early tonight. West central GA appears to be the eastern edge of the primary severe threat area through tonight.
Tornado Watch Number 28 NWS Storm Prediction Center Norman OK 840 AM CDT Wed Mar 17 2021
The NWS Storm Prediction Center has issued a
* Tornado Watch for portions of Southwest Arkansas Northwest Louisiana Far eastern Texas
* Effective this Wednesday morning and afternoon from 840 AM until 100 PM CDT.
* Primary threats include... A few tornadoes possible Scattered damaging winds likely with isolated significant gusts to 75 mph possible Isolated large hail events to 1.5 inches in diameter possible
SUMMARY...A squall line across eastern TX should continue east across parts of the Sabine Valley into Louisiana and southern Arkansas through midday. Damaging wind gusts will be the main hazard initially, but the tornado risk should increase later this morning.
The tornado watch area is approximately along and 45 statute miles east and west of a line from 10 miles east of El Dorado AR to 55 miles southwest of Natchitoches LA. For a complete depiction of the watch see the associated watch outline update (WOUS64 KWNS WOU8).
REMEMBER...A Tornado Watch means conditions are favorable for tornadoes and severe thunderstorms in and close to the watch area. Persons in these areas should be on the lookout for threatening weather conditions and listen for later statements and possible warnings.
OTHER WATCH INFORMATION...CONTINUE...WW 25...WW 26...WW 27...
AVIATION...Tornadoes and a few severe thunderstorms with hail surface and aloft to 1.5 inches. Extreme turbulence and surface wind gusts to 65 knots. A few cumulonimbi with maximum tops to 500. Mean storm motion vector 26035.