Mar 25, 2021 1300 UTC Day 1 Convective Outlook
Updated: Thu Mar 25 13:06:32 UTC 2021 (20210325 1300Z Day 1 shapefile | 20210325 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20210325 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
HIGH 25,134 1,727,832 Huntsville, AL...Decatur, AL...Madison, AL...Tupelo, MS...Florence, AL...
MODERATE 53,574 6,787,893 Memphis, TN...Nashville, TN...Birmingham, AL...Jackson, MS...Chattanooga, TN...
ENHANCED 77,339 6,137,074 Evansville, IN...Jonesboro, AR...Owensboro, KY...Bowling Green, KY...Hattiesburg, MS...
SLIGHT 134,774 16,283,611 New Orleans, LA...Atlanta, GA...Cincinnati, OH...Louisville, KY...Lexington-Fayette, KY...
MARGINAL 160,534 23,986,779 Indianapolis, IN...Columbus, OH...Pittsburgh, PA...Akron, OH...Columbus, GA...
Probabilistic Tornado Graphic
20210325 1300 UTC Day 1 Tornado Probabilities Graphic
Probability of a tornado within 25 miles of a point.
Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Day 1 Tornado Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SIG SEVERE 156,147 14,656,083 Memphis, TN...Nashville, TN...Birmingham, AL...Jackson, MS...Huntsville, AL...
30 % 25,037 1,728,418 Huntsville, AL...Decatur, AL...Madison, AL...Tupelo, MS...Florence, AL...
15 % 53,505 6,784,097 Memphis, TN...Nashville, TN...Birmingham, AL...Jackson, MS...Clarksville, TN...
10 % 77,438 6,186,401 Chattanooga, TN...Evansville, IN...Jonesboro, AR...Owensboro, KY...Bowling Green, KY...
5 % 108,488 13,144,234 Atlanta, GA...Cincinnati, OH...Louisville, KY...Lexington-Fayette, KY...Baton Rouge, LA...
2 % 167,864 23,876,105 Indianapolis, IN...Columbus, OH...New Orleans, LA...Akron, OH...Mobile, AL...
Probabilistic Damaging Wind Graphic
20210325 1300 UTC Day 1 Damaging Wind Probabilities Graphic
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.
Day 1 Wind Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SIG SEVERE 155,779 14,525,922 Memphis, TN...Nashville, TN...Birmingham, AL...Jackson, MS...Huntsville, AL...
45 % 79,025 8,499,688 Memphis, TN...Nashville, TN...Birmingham, AL...Jackson, MS...Huntsville, AL...
30 % 76,827 6,100,137 Chattanooga, TN...Evansville, IN...Jonesboro, AR...Owensboro, KY...Bowling Green, KY...
15 % 134,929 16,354,403 New Orleans, LA...Atlanta, GA...Cincinnati, OH...Louisville, KY...Lexington-Fayette, KY...
5 % 160,716 24,016,128 Indianapolis, IN...Columbus, OH...Pittsburgh, PA...Akron, OH...Columbus, GA...
Probabilistic Large Hail Graphic
20210325 1300 UTC Day 1 Large Hail Probabilities Graphic
Probability of hail 1" or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
Day 1 Hail Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SIG SEVERE 80,877 5,795,240 Memphis, TN...Jackson, MS...Huntsville, AL...Tuscaloosa, AL...Jackson, TN...
30 % 81,196 5,819,672 Memphis, TN...Jackson, MS...Huntsville, AL...Tuscaloosa, AL...Jackson, TN...
15 % 97,215 8,359,048 Nashville, TN...Birmingham, AL...Shreveport, LA...Chattanooga, TN...Clarksville, TN...
5 % 128,820 14,011,969 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

   ...SUMMARY...
   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 ---

   ...Synopsis...
   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
   possible.

   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.

   ..Edwards/Leitman.. 03/25/2021

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