Aug 21, 2018 1730 UTC Day 2 Convective Outlook
Updated: Tue Aug 21 17:23:22 UTC 2018 (20180821 1730Z Day 2 shapefile | 20180821 1730Z Day 2 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20180821 1730 UTC Day 2 Outlook Graphic
Day 2 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
MARGINAL 174,206 18,950,841 Boston, MA...Virginia Beach, VA...Norfolk, VA...Chesapeake, VA...Richmond, VA...
Probabilistic Graphic
20180821 1730 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 2 Prob. Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
5 % 174,197 18,949,901 Boston, MA...Virginia Beach, VA...Norfolk, VA...Chesapeake, VA...Richmond, VA...
   SPC AC 211723

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1223 PM CDT Tue Aug 21 2018

   Valid 221200Z - 231200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   NEW ENGLAND...THE COASTAL CAROLINAS/MID-ATLANTIC
   STATES...SOUTH-CENTRAL HIGH PLAINS...AND THE INTERMOUNTAIN WEST...

   ...SUMMARY...
   Isolated strong to severe thunderstorms may occur Wednesday across
   portions of New England, the coastal Carolinas/southeast Virginia,
   south-central High Plains, and Intermountain West.

   ...Synopsis...
   A deep upper trough will be in place over much of the East on
   Wednesday, as a series of embedded shortwaves move through portions
   of the Ohio Valley, Mid-Atlantic, and Northeast. A surface low will
   move northeastward across Quebec as a trailing cold front sweeps
   through much of the East. Further west, an upper trough will move
   slowly eastward across the Intermountain West as a low-amplitude
   ridge remains in place over the southern Plains. 

   ...Southeast ID...Eastern UT...WY...Northwest CO...
   Steep midlevel lapse rates and sufficient moisture are expected to
   result in MLCAPE values of 500-1500 J/kg by afternoon across
   portions of the Intermountain West, depending on elevation and the
   extent of diabatic heating that can occur. As the upper trough
   begins to eject eastward, midlevel flow in the base of the trough is
   expected to increase, resulting in a corresponding increase in
   effective shear into the 35-45 kt range, sufficient for organized
   storm structures including the potential for a few supercells and/or
   bowing segments, capable of locally severe hail and wind. Severe
   probabilities have been confined to western WY for now, though there
   is some potential for the threat to spread across the state through
   the evening as initial convective development spreads eastward.

   ...South-central High Plains...
   Similar to Tuesday, moist upslope flow along the southern fringe of
   stronger midlevel westerlies will support a focused area of severe
   potential across the south-central High Plains. Moderate instability
   and sufficient effective shear will support a couple of supercells
   and/or bowing segments capable of hail and locally severe wind
   gusts.  

   ...Carolinas northward into New England...
   Ample moisture will be in place ahead of the cold front on Wednesday
   afternoon from the eastern Carolinas northeastward through the
   Mid-Atlantic into portions of New England. However, widespread early
   day cloudiness will tend to limit the potential for significant
   destabilization, and updrafts will struggle to intensify within an
   environment characterized by only modest low-level convergence and
   weak midlevel lapse rates. The highest relative threat still appears
   to be across two areas: portions of New England where convergence
   along the cold front may be somewhat stronger in closer proximity to
   the surface low, and across eastern VA/NC/SC, where warmer boundary
   layer temperatures will support somewhat greater buoyancy. Effective
   shear of 30-40 kt will support some modestly organized storm
   structures with the strongest updrafts, with localized damaging
   winds being the primary threat.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:  <2%     - None
   Wind:      5%     - Marginal
   Hail:      5%     - Marginal

   ..Dean/Dial.. 08/21/2018

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