Severe Storm Outlook Narrative (AC)
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ACUS11 KWNS 141505
SPC MCD 141504

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 for graphic product...


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