SPC MD 1337

MD 1337 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR PORTIONS OF AZ...SOUTHERN NV...SOUTHEASTERN CA

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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