Aug 18, 2017 1300 UTC Day 1 Convective Outlook
Updated: Fri Aug 18 12:55:08 UTC 2017 (20170818 1300Z Day 1 shapefile | 20170818 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20170818 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 65,335 37,529,802 New York, NY...Philadelphia, PA...Baltimore, MD...Washington, DC...Kansas City, MO...
MARGINAL 292,391 33,680,635 Virginia Beach, VA...Omaha, NE...Tulsa, OK...Wichita, KS...Raleigh, NC...
Probabilistic Tornado Graphic
20170818 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 % 33,265 32,057,398 New York, NY...Philadelphia, PA...Baltimore, MD...Washington, DC...Newark, NJ...
Probabilistic Damaging Wind Graphic
20170818 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
15 % 53,130 37,273,923 New York, NY...Philadelphia, PA...Baltimore, MD...Washington, DC...Kansas City, MO...
5 % 289,933 32,952,399 Virginia Beach, VA...Omaha, NE...Tulsa, OK...Wichita, KS...Raleigh, NC...
Probabilistic Large Hail Graphic
20170818 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
15 % 26,945 1,813,234 Kansas City, MO...Independence, MO...Columbia, MO...Blue Springs, MO...Raytown, MO...
5 % 170,470 22,322,005 Baltimore, MD...Washington, DC...Omaha, NE...Tulsa, OK...Wichita, KS...
   SPC AC 181255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Fri Aug 18 2017

   Valid 181300Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   FROM VA TO SOUTHEASTERN NY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   ACROSS SOUTHWESTERN MN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING INTO
   EARLY TONIGHT ACROSS PARTS OF NORTHERN MO...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   MN AND MO SLIGHT RISK AREAS...AND EXTENDING ACROSS NORTHERN OK TO
   THE NORTHEASTERN TX PANHANDLE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   MID ATLANTIC SLIGHT RISK AREA...

   ...SUMMARY...
   Scattered strong to localized severe gusts capable of wind damage
   are forecast from the Chesapeake Bay vicinity northward into
   southern New York.  Isolated severe thunderstorms are possible
   across portions southwestern Minnesota and northern Missouri.

   ...VA to NY this afternoon...
   A low-amplitude shortwave trough over the OH Valley will progress
   east-northeastward to PA/NY by this evening, along with a cold front
   trailing southward from the primary cyclone over Ontario/Quebec. 
   Surface heating in cloud breaks and boundary layer dewpoints in the
   low-mid 70s will contribute to moderate buoyancy (MLCAPE ranging
   from 1500 J/kg in NY to nearly 3000 J/kg in VA) in a corridor
   immediately ahead of the cold front.  Though midlevel lapse rates
   will remain poor, an increase in midlevel flow to 30-40 kt (and
   effective bulk shear near 35 kt) with minimal convective inhibition
   will allow a combination of short line segments and supercells to
   form along the front this afternoon.  Damaging winds will be the
   main threat, though an isolated tornado may occur in the zone of
   slightly stronger vertical shear from eastern PA/NJ into
   southeastern NY.

   ...MO/KS through tonight...
   A pronounced midlevel trough over the Dakotas/northeastern NE this
   morning will move southeastward over IA/MO by this evening, and IL
   overnight.  A diffuse surface cold front will accompany the midlevel
   trough, and low-level moisture now in OK will spread northeastward
   today across eastern KS and western MO in the wake of weakening
   convection and associated outflow in OK this morning.  Some
   convection could develop late this afternoon/evening in the zone of
   ascent preceding the midlevel trough and surface boundary,
   especially across northern MO.  Ascent will be weak, however, and
   lingering clouds could slow surface heating some into this
   afternoon, so details of storm initiation become more uncertain with
   southwestward extent into KS.  Strong buoyancy (MLCAPE of 3000 J/kg
   or greater), midlevel lapse rates greater than 8 C/km, and effective
   bulk shear of 35-40 kt will favor a conditional threat for initial
   supercells capable of producing large hail and damaging winds, with
   the more probable area being across MO on the northeastern edge of
   the returning moisture/buoyancy.

   ...Southwestern MN this afternoon/evening...
   The embedded midlevel vorticity center over ND this morning will
   move over southwestern MN this afternoon.  Residual low-level
   moisture (near 60 F dewpoints) and ascent with the vort max could
   result in some re-invigoration of storm cluster in ND by this
   afternoon in MN.  Moderate buoyancy and cool midlevel temperatures
   (-13 to -14 C at 500 mb) will support a threat for large hail with
   mainly multicell clusters (given effective bulk shear near 25 kt). 
   The storms should weaken slowly by late evening/early tonight as
   they approach northern IA.

   ..Thompson/Dial.. 08/18/2017

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z