Dec 20, 2017 1300 UTC Day 1 Convective Outlook
Updated: Wed Dec 20 12:41:53 UTC 2017 (20171220 1300Z Day 1 shapefile | 20171220 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20171220 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
MARGINAL 58,806 9,717,064 Atlanta, GA...Birmingham, AL...Montgomery, AL...Columbus, GA...Sandy Springs, GA...
Probabilistic Tornado Graphic
20171220 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
2 % 19,436 689,323 Hattiesburg, MS...Meridian, MS...Laurel, MS...
Probabilistic Damaging Wind Graphic
20171220 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
5 % 58,950 9,728,133 Atlanta, GA...Birmingham, AL...Montgomery, AL...Columbus, GA...Sandy Springs, GA...
Probabilistic Large Hail Graphic
20171220 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
No Risk Areas Forecast
   SPC AC 201241

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0641 AM CST Wed Dec 20 2017

   Valid 201300Z - 211200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY FROM
   SOUTHEASTERN MS TO NORTHERN GA...

   ...SUMMARY...
   Isolated strong to severe wind gusts, and perhaps an isolated
   tornado, will be possible today from southeastern Mississippi into
   northern Georgia.

   ...MS/AL/GA today...
   A midlevel shortwave trough over AR this morning will progress
   eastward to the southern Appalachians by this evening and off the
   Atlantic coast overnight, well downstream of another trough digging
   southeastward over the Great Basin.  A weak surface cyclone will
   precede the midlevel trough from northern MS to northern GA through
   this afternoon, while a feed of mid-upper 60s boundary-layer
   dewpoints will be maintained south of the low track.  Some surface
   heating is expected within cloud breaks in the warm sector, which
   will combine with the low-level moisture to support surface-based
   CAPE of 500-1000 J/kg.

   A belt of strong low-midlevel flow (40-50 kt at 850 mb and 70 kt at
   500 mb) will be maintained today along the I-20 corridor from MS to
   GA.  However, low-level flow will veer/weaken with time and
   hodographs will become largely straight within the corridor of
   somewhat stronger buoyancy.  As has already been observed this
   morning, convection along the cold front should maintain cellular
   characteristics given weakening forcing for ascent with time and
   southward extent, and substantial cross-frontal, deep-layer
   flow/shear vectors.  The net result will be an environment
   supportive of a broken band of storms (either supercells or short
   bowing segments) capable of producing isolated strong outflow gusts,
   prior to the threat diminishing this evening.  An isolated tornado
   may occur through late morning or midday from southern MS into
   adjacent parts of AL, where low-level shear will be somewhat
   stronger in the area of richest low-level moisture.

   ..Thompson/Peters.. 12/20/2017

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