Valid 242000Z – 251200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE
TENNESSEE VALLEY REGION…
The threat for tornadoes, large hail, and damaging winds will be
focused this afternoon and evening across the Tennessee Valley.
As a convective increase is gradually underway over the Tennessee
Valley area, short-term severe potential is increasing. As such,
aside from some trimming on the western fringes of the risk areas
near and west of the Mississippi River, no substantive changes are
being made. Potential for a couple of tornadoes, along with
damaging winds and hail, remains evident.
Meanwhile, thunder areas are being removed from the north-central
U.S., where earlier lightning has dissipated, and from the Pacific
Northwest, where only sporadic flashes at best are now anticipated.
.PREV DISCUSSION… /ISSUED 1115 AM CDT Tue Mar 24 2020/
…TN Valley this afternoon/evening…
A midlevel shortwave trough over AR/MO this morning will move
eastward to the TN Valley by this evening, as an associated surface
cyclone in OK develops eastward along a rain-reinforced boundary
near the southern TN border. Along and south of this surface
boundary, dewpoints will remain in the mid 60s through the
afternoon, beneath midlevel lapse rates of 7-8 C/km per 12z
soundings. The net result with daytime heating in cloud breaks will
be moderate buoyancy (MLCAPE 1000-1500 J/kg) within the warm sector,
in advance of the surface cyclone/midlevel trough.
In the wake of the elevated convection this morning, additional
thunderstorm development is expected by mid afternoon in the
vicinity of southwestern TN/northern MS, south of the ongoing
elevated storms in the left-exit region of the midlevel jet over
northeast AR. Storms will subsequently spread eastward across
northern AL and southern middle TN through the evening. Strong
deep-layer vertical shear with long hodographs and effective bulk
shear of 50-60 kt, as well as low-level hodograph curvature with 0-1
km SRH near 200 m2/s2, will favor supercells. Low-level shear could
be enhanced some along the residual rain-cooled boundary that will
likely shift northward into southern middle TN by mid afternoon.
Farther south, some vertical mixing of moisture could introduce
somewhat larger (15-20 F) temperature-dewpoint spreads from central
MS into central AL this afternoon. Thus, the most favorable
corridor for tornadic storms will be in the zone of greater
low-level shear and retained low-level moisture near the rain-cooled
boundary, closer to the TN border this afternoon/evening. The
favorable near-storm environment, in combination with an expected
discrete/cluster storm mode, may support a strong tornado or two.