Aug 18, 2017 0600 UTC Day 1 Convective Outlook
Updated: Fri Aug 18 05:59:54 UTC 2017 (20170818 1200Z Day 1 shapefile | 20170818 1200Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20170818 1200 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 38,115 35,696,643 New York, NY...Philadelphia, PA...Baltimore, MD...Washington, DC...Newark, NJ...
MARGINAL 319,694 36,513,795 Oklahoma City, OK...Kansas City, MO...Virginia Beach, VA...Omaha, NE...Tulsa, OK...
Probabilistic Tornado Graphic
20170818 1200 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 % 32,882 30,698,008 New York, NY...Philadelphia, PA...Baltimore, MD...Newark, NJ...Jersey City, NJ...
Probabilistic Damaging Wind Graphic
20170818 1200 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 % 38,115 35,696,643 New York, NY...Philadelphia, PA...Baltimore, MD...Washington, DC...Newark, NJ...
5 % 318,665 36,508,859 Oklahoma City, OK...Kansas City, MO...Virginia Beach, VA...Omaha, NE...Tulsa, OK...
Probabilistic Large Hail Graphic
20170818 1200 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
5 % 183,168 23,718,210 Baltimore, MD...Washington, DC...Oklahoma City, OK...Kansas City, MO...Omaha, NE...
   SPC AC 180559

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Fri Aug 18 2017

   Valid 181200Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE LOWER
   HUDSON VALLEY SOUTH INTO CENTRAL VIRGINIA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR THE MIDDLE
   ATLANTIC STATES INTO CONNECTICUT AND NEW YORK...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE MIDDLE
   AND LOWER MISSOURI VALLEYS INTO KANSAS AND OKLAHOMA...

   ...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 of the middle to lower Missouri Valley southward
   into Kansas and Oklahoma.

   ...Synopsis...
   A broad area of cyclonic mid-level flow aloft will extend from the
   northern Plains into the Northeast.  A lead shortwave trough will
   move from the Upper Great Lakes across the Northeast with surface
   low over eastern Ontario by 00Z.  A well-defined surface trough east
   of the Appalachians will focus shower/thunderstorm development in
   advance of a cool front forecast near the Appalachian spine by early
   evening.  

   Farther west, an amplifying shortwave trough will move from the
   eastern Dakotas into the southern Great Lakes during the period.  A
   weaker surface boundary will extend from western MN southward across
   KS and trailing west across the OK/TX Panhandles. 

   ...Middle Atlantic states northward into NY/CT...
   Scattered early-day cloud cover, associated with a plume of
   mid-level moisture from the TN Valley into the Mid-Atlantic states,
   will give way to strong heating from PA/NJ southward.  A
   moisture-rich airmass sampled by Thursday evening RAOBS (reference
   the Dulles sounding) will become moderately to strongly unstable by
   mid afternoon from the Mid-Atlantic to southern NY.  Models show the
   strongest mid-level flow will generally remain north of central VA
   with effective shear increasing from 25-40 kt with northward extent.
    
   The combination of weak ascent associated with a mid-level vorticity
   maximum (forecast to move from IN to PA during the day), and a weak
   cap and strong heating in the lee of the higher terrain, will favor
   scattered to numerous storms developing during the afternoon from
   central VA northward into the lower Hudson Valley.  Forecast
   soundings show MLCAPE 1500-3000 J/kg and 70s degrees F boundary
   layer dewpoints.  A mixed mode of multicells and perhaps a couple of
   supercells will favor isolated gusts around 60 mph---conducive for
   scattered wind damage.  A marginal risk for a tornado is possible
   from the Chesapeake Bay vicinity north into the lower Hudson Valley.
    Forecast hodographs are largest over the Delaware River Valley
   north into southeast NY during the late afternoon/early evening.  A
   gradual weakening in storm intensity is expected by the early-mid
   evening as the activity moves towards the coast. 

   ...Middle to lower MO Valley into KS/OK...
   A more conditional severe risk will likely exist across portions of
   this region during the afternoon/evening.  It seems to reason the
   greatest risk for thunderstorms and isolated severe will focus near
   1) a residual frontal zone forecast to advance northward through the
   Ozarks and into the lower MO Valley by early evening and/or 2) in
   the vicinity of the northeastern portion of a central plains cool
   front.  With that stated, mesoscale dependencies related to
   overnight thunderstorm clusters across the central Plains lend
   relatively large uncertainty on the initiation and subsequent
   development of concentrated thunderstorm areas.  However, the
   eastward advection of a steep 700-500 mb lapse rate plume into
   OK/KS/western MO and sufficient 500-mb northwesterly flow will
   result in an environment supportive of organized severe storms. 
   Will reevaluate for a possible inclusion of higher severe
   probabilities in later outlook(s).

   ..Smith/Gleason.. 08/18/2017

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z