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AWUS01 KWNH 241818
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-250000-

Mesoscale Precipitation Discussion 0669...Corrected
NWS Weather Prediction Center College Park MD
217 PM EDT Wed Jul 24 2019

Corrected for Additional information

Areas affected...Southern CA...Southern NV...Western
AZ...Southwest UT

Concerning...Heavy rainfall...Flash flooding likely

Valid 241800Z - 250000Z

SUMMARY...Increasing monsoonal moisture today will be driving
expanding coverage of heavy showers and thunderstorms going
through the afternoon hours. Slow cell motions and intense
rainfall rates of as much as 2 inches/hr will likely result in
some areas of flash flooding.

DISCUSSION...The latest GOES-17 visible satellite imagery is
showing an expanding area of agitated CU/TCU across the higher
terrain of far southern CA, southeast NV and western AZ. A few
heavier showers and thunderstorms have begun to initiate and over
the next several hours, it is expected that a much more widespread
area of at least scattered showers and thunderstorms will evolve
as a weak 500/700mb layer trough axis and embedded vort over far
northwest Mexico and southwest AZ advances west-northwest this
afternoon into areas of southern CA and adjacent areas of southern
NV. Secondarily, another smaller scale vort center is noted over
far northwest AZ which will be drifting northwest into far
southwest UT.

Al of this energy will be interacting with an increasingly
unstable boundary layer due to strong diurnal heating, and also
coupled with an increasingly moist boundary layer as low-level
monsoonal moisture advances northward from the Gulf of California.
Already there is a pool of moderate instability with 1500+ j/kg of
MUCAPE focused over central and southern AZ, and far southern CA.
The latest CIRA-LPW data sets along with morning RAOB soundings
confirm a rather deep pool of moisture which is expected to allow
for convection with efficient rainfall processes. As it is, the
PWATs across the region are near or over 2 standard deviations
above normal, and as convection evolves, the expectation is that
some of the rainfall rates will exceed 2 inches/hr.

The latest hires model guidance tends to focus much of the
convective threat near the Mogollon Rim and up across northwest
AZ, but given the proximity of the aforementioned energy and
monsoonal moisture pooling farther to the west across southern NV,
southern CA and to an extent over southwest UT, the guidance is
probably a bit underdone for these areas, and especially given the
localized orographic forcing that will be in place to help
initiate and anchor some of the convection.

While the convection will be scattered in nature, it will tend to
be slow moving, and will be capable of producing excessive
rainfall amounts given the high short-term rainfall rates. Expect
storm totals locally to be as much as 2 to 4 inches with the
stronger and more anchored convective cores. Flash flooding will
be likely for these areas, and especially when considering more
sensitive areas of steep and sloped terrain, dry washes, burn
scars and slot-canyons. Will continue to closely monitor.

Orrison

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...FGZ...LKN...LOX...PSR...SGX...SLC...TWC...VEF...

ATTN...RFC...CBRFC...CNRFC...

LAT...LON   38211451 38051298 37331204 35891164 35181088
            34510969 33890926 33300935 33311022 33041161
            32791246 32451286 32251330 32181399 32391481
            32651597 32611638 32941681 33721694 33991725
            34121768 34501787 34741770 35141689 35821638
            36431661 37281652 37851579


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