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Mesoscale Discussion 1337
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   Mesoscale Discussion 1337
   NWS Storm Prediction Center Norman OK
   0116 PM CDT Tue Jul 18 2017

   Areas affected...Portions of AZ...southern NV...southeastern CA

   Concerning...Severe potential...Watch unlikely 

   Valid 181816Z - 182045Z

   Probability of Watch Issuance...5 percent

   SUMMARY...The risk for strong storms will be developing through the
   afternoon and then continue into the evening. While a few isolated
   marginally severe storms cannot be ruled out, Watch issuance will
   not be needed.

   DISCUSSION...The 12Z morning soundings indicate 90th-percentile to
   near-record precipitable water values across AZ and southern NV,
   based on the SPC Sounding Climatology. This is in association with a
   plume of rich, monsoon-related moisture that extends well inland
   from the Gulf of California. With Tucson and Phoenix soundings
   indicating around 1.8-inch PW this morning, surface dewpoints
   continue holding in the 60s across the lower elevations with
   widespread insolation and diurnally increasing vertical mixing. This
   is supporting the development of around 1000-2000 J/kg of MLCAPE in
   many areas, despite somewhat modest mid-level lapse rates associated
   with the rich, deep moisture. In response to limited capping and
   diurnally strengthening orographic circulations, cumulus fields are
   becoming increasingly agitated over the Mogollon Rim and surrounding
   mountains, with isolated storm development ongoing -- somewhat
   earlier than development during typical diurnal cycles.

   While the limited mid-level lapse rates will tend to temper the
   overall magnitude of convective-scale accelerations and downdraft
   strength, widespread convective coverage is anticipated. As a
   result, merging cold pools yielding small-scale instances of
   convective upscale growth, will have the potential to support
   enhancements to storm-scale, vertical circulations capable of
   supporting strong wind gusts. This would especially be the case in
   areas where surface heating is strongest. Present indications that
   this may be within a corridor from central AZ to the lower Colorado
   River Valley vicinity, as building convection spreads westward and
   southwestward with time. Also, a slight enhancement to mid-level
   flow may linger on the north side of an MCV crossing southern AZ,
   potentially encouraging some loose organization to convective
   activity.

   Through this evening, locally strong wind gusts will be possible,
   and a few isolated, marginally severe wind gusts cannot be ruled
   out. Small hail could also occur -- especially over the mountains.
   However, the limited magnitude of low- to mid-level winds per VAD
   wind profiles, and modest mid-level lapse rates will tend to
   minimize the severe potential.

   ..Cohen/Hart.. 07/18/2017

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...TWC...FGZ...PSR...VEF...

   LAT...LON   36571461 36571202 35271004 33060927 31760970 31801095
               33511291 34841492 35751535 36571461 

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Page last modified: July 18, 2017
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