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AWUS01 KWNH 232316
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-240510-

Mesoscale Precipitation Discussion 0668
NWS Weather Prediction Center College Park MD
715 PM EDT Tue Jul 23 2019

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

Concerning...Heavy rainfall...Flash flooding possible

Valid 232310Z - 240510Z

SUMMARY...Scattered showers and thunderstorms will continue to
develop and expand just a bit more in coverage over the next
couple of hours. Slow cell motions and heavy rainfall rates will
continue to foster concerns for flash flooding into the evening
hours.

DISCUSSION...The latest GOES-17 visible satellite imagery shows
several pockets of showers and thunderstorms evolving generally
over the higher terrain of southern CA and also from east-central
to northwest AZ including the Mogollon Rim. Strong solar
insolation throughout the afternoon has led to locally focused
areas of stronger instability with MLCAPE values of as much as
1000 to 1500 j/kg. This is especially the case over southern AZ
and parts of southern CA where surface dewpoints are higher, and
moisture in general is a bit more anomalous given PWAT values near
2 standard deviations above normal.

Over the next several hours, there will be additional smaller
scale pockets of heavy showers and thunderstorms as instability
peaks and then gradually begins to wane going toward the evening
hours. A weak MCV near the junction point of CA/NV/AZ is still
helping to provide at least some small scale forcing/ascent that
will continue to invigorate some new areas of convection, and
especially over areas of northwest AZ, and likely edging into
southeast NV and possibly southwest UT. Much of the activity
elsewhere is highly orographic in nature and will continue to be
into the evening hours.

Identifying where the most organized convection will be over the
next few hours is a bit of a challenge, but the hires model suite
of guidance and visible satellite trends would tend to suggest
areas of southeast AZ and locally across the Mogollon Rim, where
there appears to be a bit more instability and/or orographic
forcing depending on the location. The CU field over many of these
areas currently suggests additional convective initiation will
soon occur.

The low-levels of the column remain relatively dry, but the mid
and upper levels are rather moist. Convection again will have to
moisten up the low levels a bit, but once this occurs, the more
sustainable convective cells will be capable of generated rather
enhanced rainfall rates given greater efficiency.

Rainfall rates may reach as high as 1.5 inches/hr within the
stronger and more sustainable cores, and some localized storm
totals may exceed 2 inches going through the evening hours. This
may result in some additional areas of flash flooding, and
especially over the more sensitive dry washes, slot-canyons and
steeper sloped terrain near and downstream of the convection.

Orrison

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

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

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

LAT...LON   37981369 37621275 36531165 35251052 34690994
            33940934 32700922 31610939 31201022 31271115
            31621212 32111275 32851330 33161425 33011475
            32981510 33401552 33261586 32751574 32551614
            32971672 33611683 33891685 34521687 35221664
            36001578 36941506 37631461


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