Mesoscale Precipitation Discussion 0571
NWS Weather Prediction Center College Park MD
836 AM EDT Wed Jul 02 2025

Areas affected...High Deserts of California and Nevada

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

Valid 021235Z - 021800Z

Summary...Showers and thunderstorms blossoming across the High
Deserts of the Southwest will continue through the morning.
Despite fast motion, rainfall rates of 0.5"/hr could result in
instances of flash flooding.

Discussion...The regional radar mosaic this morning shows rapidly
expanding showers and embedded thunderstorms blossoming across
southeast CA and southern NV. These storms are developing despite
modest instability (SPC RAP analysis 500 J/kg of MUCAPE) but with
a steady increasing trend noted. This instability is overlapping
with PWs that are as high as 0.8 to 1.2 inches, or above the 90th
percentile for the date. Offsetting the modest thermodynamics,
forcing for ascent is increasing downstream of a strengthening
upper trough digging across CA leaving height falls and spokes of
vorticity/PVA across the Southwest, collocated with a low-level
convergence axis on the periphery of an 850mb moisture surge out
of Arizona. This ascent is helping to cause the rapid increase in
convective coverage this morning.

Rainfall rates have been estimated via KESX to be as high as
0.5"/hr at times, which despite rapid storm motions on 850-300mb
mean winds of 20-25 kts from the south, has produced 3-hr rainfall
according to MRMS of up to 0.75 inches. As the ascent continues to
maximize through the morning downstream of the upper trough, the
low-level flow surges higher moisture northeast, and instability
climbs through the morning, activity should expand and intensify.
Although the CAMs are struggling with the coverage and intensity
of ongoing convection, the environment suggests that storms should
continue for several more hours, and the HRRR-forced UA WRF
indicates rain rates will continue at as much as 0.5"/hr through
this aftn with a slow northward advance of heavy rainfall. At the
same time, convection will likely build repeatedly into the
greater thermodynamics over the High Deserts of CA, with lines of
storms training south to north producing locally as much as 1.5"
of rain.

These impressive rain rates will move across soils that are
saturated above the 90th percentile with respect to 0-40cm depths
according to NASA SPoRT. While impacts due to this rainfall are
likely to be isolated and focused across any sensitive terrain
features or urban areas, where any longer-duration training can
occur through the next several hours, instances of flash flooding
are possible.


Weiss

ATTN...WFO...HNX...LKN...PSR...SGX...SLC...VEF...

ATTN...RFC...RSA...STR...NWC...

LAT...LON   38391576 38031475 37381394 36081394 34941409
            34231431 33861470 33831530 33981583 34391647
            34911710 35691768 36621780 37941705