Jul 20, 2017 0730 UTC Day 3 Severe Thunderstorm Outlook
Updated: Thu Jul 20 07:22:02 UTC 2017 (20170720 0730Z Day 3 shapefile | 20170720 0730Z Day 3 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
0730 UTC Day 3 Outlook Graphic
Day 3 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 376,665 70,295,265 Chicago, IL...Philadelphia, PA...Detroit, MI...Indianapolis, IN...Columbus, OH...
MARGINAL 257,166 22,517,271 Virginia Beach, VA...Omaha, NE...St. Louis, MO...Cincinnati, OH...Louisville, KY...
Probabilistic Graphic
20170720 0730 UTC Day Probabilitic Graphic
Probability of severe weather within 25 miles of a point.
Hatched Area: 10% or greater probability of significant severe within 25 miles of a point.
Day 3 Prob. Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
15 % 376,664 70,353,777 Chicago, IL...Philadelphia, PA...Detroit, MI...Columbus, OH...Baltimore, MD...
5 % 257,232 22,305,362 Indianapolis, IN...Virginia Beach, VA...Omaha, NE...St. Louis, MO...Cincinnati, OH...
   SPC AC 200722

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0222 AM CDT Thu Jul 20 2017

   Valid 221200Z - 231200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER
   MISSISSIPPI VALLEY AREA AND VICINITY SOUTHEAST TO THE MID ATLANTIC
   COAST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AREA...AND INCLUDING PORTIONS OF THE CENTRAL PLAINS...

   ...SUMMARY...
   A broad area of severe weather risk is apparent from portions of the
   northern and central Plains east across the Midwest to the mid
   Atlantic region.

   ...Synopsis...
   Some amplification within the faster west-northwesterly flow field
   over the northern U.S. is expected Day 3/Saturday, as a short-wave
   trough digs southeast out of the Canadian Prairies/northern Plains
   into the upper Mississippi and mid Missouri Valleys and into the
   upper Great Lakes area late in the period.  Ahead of this stronger
   short-wave trough, multiple smaller-scale disturbances will continue
   moving quickly east southeastward across the Midwest and central
   Appalachians and into the mid Atlantic region.

   At the surface, a cold front will shift slowly south across the
   central Plains, while a weak/quasi-stationary front lingers across
   the Midwest and east into the central Appalachians.  These fronts,
   and a weak/reinforcing push of cool air across the upper Mississippi
   valley vicinity, suggest a broad area of convective/local severe
   risk from the central and north central U.S. east the mid Atlantic
   region.

   ...Upper Mississippi Valley and vicinity southeast to the
   mid-Atlantic region...
   Relatively widespread showers and thunderstorms are expected to be
   ongoing across portions of the Day 3 severe risk area at the start
   of the period, which should linger into the afternoon hours. 
   Eventually, local heating/destabilization should fuel a gradual
   increase in convective coverage and intensity, with ample
   west-northwest mid-level flow aiding convective organization.  As a
   result, risk for hail and locally damaging winds will exist across a
   broad zone from the Great Lakes/Midwest across the central
   Appalachians, and into the mid-Atlantic region.  Storms -- and at
   least some severe risk -- will likely continue into the overnight
   hours. 

   A somewhat separate area of strong/locally severe storms may occur
   farther northwest into the upper Mississippi Valley/western Upper
   Great Lakes area, nearer the digging upper system.  Hail and locally
   damaging wind gusts will be possible, mainly during the afternoon
   and evening hours.

   ...Portions of the central Plains vicinity...
   Destabilization along a cool front shifting southward across the
   central Plains should support development of at least isolated
   storms by late afternoon, most likely across the southern
   Nebraska/northern Kansas area and vicinity.  As the belt of stronger
   mid-level flow sags southward into the region as the north-central
   U.S. trough digs southeastward, a few stronger cells may organize
   and pose risk for locally damaging winds and large hail through much
   of the evening.

   ..Goss.. 07/20/2017

   NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0730Z