Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
New Orleans, LA...Louisville, KY...Lexington-Fayette, KY...Baton Rouge, LA...Montgomery, AL...
SPC AC 251306
Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0806 AM CDT Thu Mar 25 2021
Valid 251300Z - 261200Z
...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS WITH A TORNADO
OUTBREAK EXPECTED ACROSS PARTS OF NORTHERN MISSISSIPPI...NORTHERN
ALABAMA...AND SOUTHERN TENNESSEE...
CORRECTED FOR LABEL POSITIONING ON GRAPHIC
A tornado outbreak -- including the threat of a few long-tracked,
violent tornadoes -- is expected today into early this evening over
the Southeast, especially parts of Mississippi, Alabama and
Tennessee. Tornadoes, large to very large hail, and damaging winds
to hurricane force also are possible over a broad area from the
central Gulf Coast to the Ohio Valley and southern Appalachians.
--- Technical Discussion ---
In mid/upper levels, a longwave trough will be maintained over the
western CONUS, as a strong shortwave trough and related speed max
dig south-southeastward across the interior Northwest, and a
formerly basal trough ejects northeastward. The latter trough is
evident in moisture-channel imagery over the southern High Plains to
the Big Bend region of TX, and will move to MO/AR by 00Z. By 12Z,
this feature should reach Lake Erie and OH. This trough will pack
the height gradient to its southeast enough to yield 110-130-kt
250-mb flow and 80-100-kt 500-mb winds over much of the Mid-South,
and Tennessee/Ohio Valleys today and this evening.
The associated surface low-pressure area was analyzed at 11Z across
western AR to northwestern LA, still poorly consolidated around
areas of rain-cooled air. A cold front was drawn from northwestern
LA across southeast TX the middle TX Coast and deep south TX. A
"synoptic" warm front arched from the low-pressure area over
western/middle TN and northern GA. A marine warm front -- denoting
the northern rim of the most fully modified Gulf return flow/warm-
sector air mass -- was drawn over northern LA, central MS, and
southwestern AL, and will move northward over the Mid-South and
Tennessee Valley through the day. The surface cyclone should become
better-defined and deepen as it moves northeastward across AR and
southeastern MO today. By 00Z, the low should reach southern IL in
the MVN/BLV area, with cold front across western TN, central MS, and
southern LA. By 12Z tomorrow, the low should reach western Lake
Erie or adjoining southwestern ON, with cold front across western
PA, the Blue Ridge area of western VA, the western Carolinas,
western GA, to the MOB/PNS vicinity and northwestern Gulf.
Only minor peripheral adjustments were made to the previous outlook
lines for convective/boundary placement on the west side, and
elsewhere, updated analyses and guidance since last issuance. The
core threat area is essentially unchanged for now.
...Southeast to Ohio Valley...
Scattered to numerous thunderstorms are expected from midday through
afternoon over parts of the lower Mississippi Valley region and
Mid-South. This activity will race northeastward while maturing
into cyclic supercells and bow echoes. Tornadoes are the main
concern (some strong to violent). Also, severe nontornadic winds
may exceed 65 kt locally, and sporadic large, damaging hail also is
A relatively undisturbed low-level warm sector with rich moisture
should shift further inland along and south of the marine warm
front, with a very favorable buoyancy/shear pattern over a broad
area. Surface dew points in the upper 60s to low 70s F already are
common over central/southern LA, southern MS and southwestern AL.
This moisture will spread northward over at least the "high" and
"moderate" areas today, where the southern and eastern parts of
strongest deep-layer winds will overlap the greatest low-level
instability. There, diurnal heating will remove MLCINH from midday
onward, contributing to ready thunderstorm development, both in the
destabilizing warm sector and near the cold front. Meanwhile,
expect strengthening deep shear and large, almost ideally shaped
low-level hodographs formed in part by a wide, 60-70-kt LLJ.
Accordingly modified RAOBs and forecast soundings yield peak MLCAPE
of 2000-2500 J/kg, amidst 65-75-kt effective-shear vectors, and
effective SRH values of 400-700 J/kg.
This is an uncommon, upper-echelon parameter space. In such an
environment, any relatively discrete supercells will be capable of
multiple tornadoes, some long-tracked and strong to violent (EF2-5
possible), with considerable destructive potential. A very moist
boundary layer also will reduce potential cold-pool/outflow strength
via less subcloud evaporation, so that even closely spaced storms
may have substantial tornado threats. Forecast wind fields and
model soundings reasonably suggest any sustained supercells and
their tornadoes will be fast-moving (45-55 kt), with individual
tornado paths nearly as long in miles as their duration in minutes.
At any given time, boundary-layer instability should lessen with
northward extent across the Ohio Valley. Nonetheless, warm
advection will help to spread at least weak surface-based buoyancy
over the region prior to cold frontal passage, with similarly
intense deep shear as farther south. As such, the threat for severe
winds and tornadoes in particular will extend into tonight,
spreading up the Ohio Valley and toward the central/southern
Appalachians. Overall severe potential should lessen to "slight"
and "marginal levels" late tonight near the Appalachians as the
mid/upper system and surface low move away from remaining optimal
instability, and flow aloft becomes more boundary-parallel.
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