Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 141505
SWOMCD
SPC MCD 141504
UTZ000-AZZ000-141700-

Mesoscale Discussion 1644
NWS Storm Prediction Center Norman OK
1004 AM CDT Thu Sep 14 2017

Areas affected...southern UT...northern AZ

Concerning...Severe potential...Watch possible

Valid 141504Z - 141700Z

Probability of Watch Issuance...40 percent

SUMMARY...Storms will continue to slowly organize this morning as
the airmass destabilizes.  Isolated severe thunderstorms with an
accompanying hail/wind threat are possible by late morning/midday.

DISCUSSION...Early morning water-vapor imagery shows a compact
mid-level low moving east across the Mojave Desert and approaching
the Grand Canyon.  A band of convection has developed from
southeastern NV arcing southeast into northwest AZ.  9-km CAPPI
radar imagery has shown thunderstorms intensify during the past
30-60 minutes from near Saint George, UT to 65 mi south of Saint
George in northwest AZ.

The implied strong forcing for ascent as indicated by the
eastward-migrating mid-level vorticity maximum will continue to
support longevity with the thunderstorm band.  12Z 500-mb upper-air
data indicated a 50+ kt speed max over southern CA and this feature
is forecast to move into north-central AZ by early afternoon.  With
gradual heating of the boundary layer expected through midday,
low-level lapse rates will steepen.  As a result, the risk for
strong to severe gusts will increase in tandem with the steepening
low-level lapse rates.  Large hail may occur with the stronger
updrafts which will primarily favor the southern 2/3 of the
discussion area where buoyancy is forecast to be highest.

..Smith/Hart.. 09/14/2017

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

ATTN...WFO...GJT...FGZ...SLC...VEF...

LAT...LON   37311391 37891336 38331151 37831019 36861015 36041144
            35801319 36751357 37311391




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