Aug 20, 2017 1630 UTC Day 1 Convective Outlook
Updated: Sun Aug 20 16:27:15 UTC 2017 (20170820 1630Z Day 1 shapefile | 20170820 1630Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20170820 1630 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 39,558 2,021,614 Des Moines, IA...Sioux City, IA...Waterloo, IA...Ames, IA...West Des Moines, IA...
MARGINAL 185,788 18,611,680 Chicago, IL...Omaha, NE...Lincoln, NE...Aurora, IL...Springfield, MO...
Probabilistic Tornado Graphic
20170820 1630 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
No Risk Areas Forecast
Probabilistic Damaging Wind Graphic
20170820 1630 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
15 % 33,049 1,718,643 Des Moines, IA...Waterloo, IA...Ames, IA...West Des Moines, IA...Cedar Falls, IA...
5 % 189,767 18,799,891 Chicago, IL...Omaha, NE...Lincoln, NE...Aurora, IL...Springfield, MO...
Probabilistic Large Hail Graphic
20170820 1630 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
15 % 28,041 1,086,312 Sioux City, IA...Ames, IA...Ankeny, IA...Urbandale, IA...Mason City, IA...
5 % 161,945 17,797,017 Chicago, IL...Omaha, NE...Lincoln, NE...Des Moines, IA...Aurora, IL...
   SPC AC 201627

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1127 AM CDT Sun Aug 20 2017

   Valid 201630Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS UPPER MS VALLEY
   TO CENTRAL GREAT PLAINS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS IL/IN...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS MO/AR...

   ...SUMMARY...
   A swath of scattered severe storms with large hail and damaging
   gusts are possible over parts of Iowa and vicinity late tonight.

   ...IA and vicinity...
   An MCV over east-central IA should drift east across northern IL/IN
   through tonight. A few multicell clusters are ongoing to the south
   and west of this MCV within weak low-level warm advection over parts
   of the Lower MO Valley. Isolated damaging winds and severe hail are
   possible with any of this activity, mainly near the IL/IN and MO/AR
   border areas. 

   In the wake of this morning activity, gradual recovery of low-level
   moisture is expected from KS to the Mid-MO Valley. A robust elevated
   mixed layer should largely inhibit surface-based storm development
   through this evening. A cluster of elevated storms is expected late
   tonight from northeast NE into IA in a corridor of strengthening
   warm advection to the north of the outflow-reinforced baroclinic
   zone. Rather steep mid-level lapse rates of 7.5-8.5 C/km from
   700-500 mb will yield large buoyancy with a broad plume of MUCAPE
   above 2000 J/kg. On the periphery of 25-35 kt 500-mb westerlies,
   effective shear of 30-40 kt should support a few embedded supercell
   and/or bowing structures as an MCS likely matures over parts of IA.
   This scenario appears sufficiently probable to warrant an upgrade to
   Slight risk for severe hail/wind.

   ..Grams/Picca.. 08/20/2017

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z