Jul 20, 2018 2000 UTC Day 1 Convective Outlook
Updated: Fri Jul 20 20:07:27 UTC 2018 (20180720 2000Z Day 1 shapefile | 20180720 2000Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20180720 2000 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
MODERATE 24,865 3,413,943 Nashville, TN...Evansville, IN...Clarksville, TN...Owensboro, KY...Bowling Green, KY...
ENHANCED 106,878 10,741,355 Memphis, TN...Cincinnati, OH...Louisville, KY...Lexington-Fayette, KY...Huntsville, AL...
SLIGHT 181,342 24,112,082 Indianapolis, IN...Columbus, OH...Wichita, KS...Toledo, OH...Birmingham, AL...
MARGINAL 316,904 31,665,681 Chicago, IL...Detroit, MI...Tucson, AZ...Cleveland, OH...Atlanta, GA...
Probabilistic Tornado Graphic
20180720 2000 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
10 % 21,325 2,244,387 Lexington-Fayette, KY...Clarksville, TN...Jackson, TN...Bowling Green, KY...Richmond, KY...
5 % 53,515 6,864,452 Memphis, TN...Nashville, TN...Louisville, KY...Fort Wayne, IN...Murfreesboro, TN...
2 % 94,806 12,924,542 Indianapolis, IN...Toledo, OH...Cincinnati, OH...Huntsville, AL...Dayton, OH...
Probabilistic Damaging Wind Graphic
20180720 2000 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
30 % 131,431 14,121,650 Memphis, TN...Nashville, TN...Cincinnati, OH...Louisville, KY...Lexington-Fayette, KY...
15 % 180,954 24,176,267 Indianapolis, IN...Columbus, OH...Wichita, KS...Toledo, OH...Birmingham, AL...
5 % 316,438 31,400,817 Chicago, IL...Detroit, MI...Tucson, AZ...Cleveland, OH...Atlanta, GA...
Probabilistic Large Hail Graphic
20180720 2000 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 48,822 5,451,615 Nashville, TN...Louisville, KY...Evansville, IN...Clarksville, TN...Jackson, TN...
45 % 25,360 3,474,490 Nashville, TN...Evansville, IN...Clarksville, TN...Owensboro, KY...Bowling Green, KY...
30 % 55,825 6,609,952 Memphis, TN...Cincinnati, OH...Louisville, KY...Lexington-Fayette, KY...Murfreesboro, TN...
15 % 153,147 14,950,069 Indianapolis, IN...Columbus, OH...Wichita, KS...Birmingham, AL...Little Rock, AR...
5 % 348,399 41,157,973 Chicago, IL...Detroit, MI...Cleveland, OH...Atlanta, GA...Toledo, OH...
   SPC AC 202007

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0307 PM CDT Fri Jul 20 2018

   Valid 202000Z - 211200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR
   WESTERN/MIDDLE TN...WESTERN/CENTRAL KY...FAR SOUTHERN IN...

   ...SUMMARY...
   The greatest threat for severe thunderstorms reamins across portions
   of the Ohio and Tennessee Valley regions into the Mid-South today
   into tonight, with damaging gusts, large hail and a few tornadoes
   possible.

   The primary change with this outlook is to reduce probabilities in
   areas where significant redevelopment is not expected in the wake of
   ongoing convection, and also in areas where the potential for
   substantial destabilization now appears limited. Some adjustment
   eastward has been made for the Slight Risk into portions of
   WV/western VA, where some damaging wind threat may spread in from
   the east. 

   To the southwest of the ongoing convection across the Ohio Valley,
   redevelopment of scattered severe thunderstorms is still expected
   across the Enhanced and Moderate Risk areas, though the timing and
   exact location of initiation remains uncertain. A very favorable
   combination of instability and effective shear continues to support
   significant hail, damaging wind, and a few tornadoes with any
   development over the next several hours. Multiple rounds of severe
   convection remain possible, with additional storms possible into
   late this evening spreading out of the central Plains into portions
   of the MS Valley.

   ..Dean/Dial.. 07/20/2018

   .PREV DISCUSSION... /ISSUED 1128 AM CDT Fri Jul 20 2018/

   ...Mid MS/OH/TN Valleys...
   Morning water vapor loop shows a significant upper low tracking
   southeastward across southern WI.  A strong mid level jet lies to
   the south of the low, extending from the northern Plains into MO. 
   As this upper wind max overspreads the warm sector over parts of the
   Mid MS, OH, and TN Valleys, the risk of rather widespread severe
   thunderstorms will develop this afternoon into tonight.  A very
   moist and unstable air mass has developed across this region with
   pockets of MUCAPE in the 5000-6000 J/kg range.  Steep low and mid
   level lapse rates combined with strong effective shear values will
   result in a conditional risk of severe storms throughout the region.
   The primary forecast uncertainty is the timing and location of
   initiation.

   12z model guidance shows wide diversity in where/when the most
   intense convective clusters will occur later today.  Due to this
   uncertainty, few changes have been made to the ongoing outlook. 
   Discrete supercell development is possible along/ahead of the
   approaching cold front due to shear vectors orthogonal to the front
   and favorable shear magnitudes.  Any storms that remain discrete
   will pose a risk of very large hail and a tornado or two.  It
   appears likely that multiple clusters of storms will congeal into
   fast-moving bowing structures posing a risk of rather widespread
   damaging winds.  This threat will likely spread as far east as
   eastern KY/TN overnight.

   ...KS/MO/OK/AR this evening...
   A surface boundary will extend from western KS into southwest MO and
   northwest AR later today.  Given the intense heat to the south of
   boundary, and a corridor of high dewpoints immediately north of the
   boundary, a zone of high CAPE values will result.  There is
   relatively good consensus of 12z models that show clusters of storms
   forming over KS late this afternoon and tracking along this corridor
   though the evening.  Damaging winds and large hail will be possible
   if this scenario unfolds.

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z