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AWUS01 KWNH 242359
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-250515-

Mesoscale Precipitation Discussion 0672
NWS Weather Prediction Center College Park MD
758 PM EDT Wed Jul 24 2019

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

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

Valid 242358Z - 250515Z

SUMMARY...Increasing monsoonal moisture today will be driving
expanding coverage of heavy showers and thunderstorms throughout
much of the evening.  The combination of slow moving cells and
deep moisture will result in some flash flooding due to ntense
rainfall rates of as much as 2 inches/hr.

DISCUSSION...The latest GOES-17 visible satellite imagery showed
an expanding area of showers and thunderstorms that were most
numerous across the higher terrain of far southern CA, southeast
NV and western AZ. A few heavier showers and thunderstorms have
begun to move off the mountains bringing the risk of flash
flooding to lower elevations.  In addition, a weak 500/700mb layer
trough axis and embedded vorticity over far northwest Mexico and
southwest AZ will be advancing west-northwest into areas of
southern CA and adjacent areas of southern NV during the evening.
Secondarily, another smaller scale vorticity center is noted over
far northwest AZ which will be drifting northwest into far
southwest UT.

All of this energy will be interacting with an unstable boundary
layer given the 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 the RAP showing MUCAPE of 1000
to 1500 J per kg focused over central and southern AZ, and far
southern CA. A 22Z sounding from 1Y7 had a Precipitable Water
value of 2.03 inches and nearly 1100 J per kg of CAPE already
present.  The sounding also showed the column from around 700 mb
to 400 mb being nearly saturated and low level mean RH at roughly
50 percent.  Expectation is that this kind of environment could
support rainfall rates which exceed 2 inches/hr.  Farther to the
north, Precipitable Water values were not as high but forecast
sounding still supported the idea of some intense rainfall rates.

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 was
still mostly likely a bit underdone for these areas... 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.

Bann

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

ATTN...WFO...FGZ...HNX...PSR...SGX...SLC...TWC...VEF...

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

LAT...LON   38311238 37611134 36581198 35331088 34180952
            32890965 33841195 34001343 33481441 32521482
            32791660 33571707 34421726 35321769 36091671
            37621454 38251303


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