SPC AC 130039

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0739 PM CDT Tue Sep 12 2017

   Valid 130100Z - 131200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS EVENING
   ACROSS PORTIONS OF THE SAN JOAQUIN VALLEY...

   ...SUMMARY...
   Isolated strong thunderstorm development still appears possible this
   evening across parts of the San Joaquin Valley, mainly to the west
   and southwest of Fresno and Merced, accompanied by some risk for
   localized potentially damaging wind gusts.

   ...01Z Outlook Update...

   ...California...
   Confidence in thunderstorm development along the coastal ranges to
   the south of the San Francisco Bay area within the next few hours is
   diminishing.  But persistent attempts at deepening convective
   development are evident along the mountains west/southwest of Fresno
   and Merced, where the boundary layer remains hot (temps in the mid
   90s F) and deeply mixed.  If a thunderstorm can emerge from this
   activity prior to 02-03Z, 30+ kt cyclonic flow in the 700-500 mb 
   layer may still contribute to a risk for potentially damaging wind
   gusts before diurnal surface cooling begins to stabilize the near
   surface environment more appreciably.

   Otherwise, the initiation of some thunderstorm activity near and
   south of San Francisco Bay with the gradual inland progression of
   the offshore closed low now appears more probable late tonight
   (09-12Z).  This may be accompanied by a risk for small hail and
   perhaps gusty winds, but severe weather potential appears limited.

   Elsewhere, scattered vigorous thunderstorm activity persists along
   the Sierra Nevada, southward into the Mojave Desert.  Isolated
   stronger cells along the Sierra Nevada may be accompanied by some
   small hail, while activity within the hot and deeply mixed boundary
   layer across the Mojave Desert may be accompanied by locally strong
   surface gusts.  But this activity is likely to diminish in intensity
   during the 02-04Z time frame, if not before.

   ..Kerr.. 09/13/2017

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