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AWUS01 KWNH 311535
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-312130-

Mesoscale Precipitation Discussion 0692
NWS Weather Prediction Center College Park MD
1134 AM EDT Wed Jul 31 2019

Areas affected...Western Arizona, Southwest Utah, Southeast Nevada

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

Valid 311530Z - 312130Z

Summary...Coverage of slow moving thunderstorms will increase
through the afternoon. Rain rates will likely become enhanced
during peak heating, and may approach 2"/hr at times. These rain
rates will have the potential to produce flash flooding.

Discussion...Showers and thunderstorms are beginning to increase
in southerly Monsoon flow around the periphery of a ridge of high
pressure to the east. An MCV evident on satellite and radar is
slowly lifting northward embedded in this flow, and will act to
enhance mesoscale ascent through the afternoon. In addition, large
scale ascent is being provided through a broad RRQ to a jet streak
lifting to the northeast, as well as a 700mb convergent trough
axis positioned across western Arizona. These features together
combined with afternoon surface heating to erode morning MLCINH
will drive increased thunderstorm coverage today.

In addition to this deep layer lift, the thermodynamic environment
is highly favorable for heavy rainfall. PWATs from the regional
12Z U/A soundings were 1.31" at VEF, 1.07" at FGZ, and 1.97" at
PSR, all above the 90th percentile for the date. This anomalous
moisture will persist, and may increase through the day as
low-level flow remains southerly, directly from pooled 2.25" PWAT
near the Gulf of California. This enhanced PWAT will combine with
MUCape progged to reach 2000-3000 J/kg on latest RAP forecasts,
suggesting an environment highly favorable for heavy rainfall,
with efficient rain rates likely, noted by LI remaining modest
near -6C.

As the MCV lifts northward, convection will likely expand ahead of
it, but storm motions will remain quite low. RAP forecast Corfidi
vectors and 0-6km mean winds are generally 5 kts or less, with
some anti-parallel direction noted of the two suggesting
backbuilding potential. Storm mergers are also likely due to the
nearly stationary storm motions in a region of weak bulk shear.
With HREF probabilities showing a moderate to high risk for 1"/hr
rain rates, these mergers could briefly push rates up to 2"/hr,
which is well above the hourly FFG across the area. The potential
for this is further evidenced by HREF v2 exceedance probabilities
for 3-hrly FFG peaking near 40% this afternoon.

There is good model consensus that convection will produce 1-2" of
rainfall, with several models including recent HRRR runs, ARW, and
HREF PMM suggesting pockets of over 3". While the convection will
remain scattered in nature, any slow moving thunderstorm will have
the potential to produce flash flooding, but it will be the storm
interactions or subtle backbuilding which would likely produce
these higher rainfall totals.

Weiss

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

ATTN...WFO...FGZ...PSR...SLC...VEF...

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

LAT...LON   37591409 37531362 37181324 36671300 36031287
            35261275 34291280 33641287 33191330 32981379
            33631429 35201470 35811489 36471504 37191482



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