491
AWUS01 KWNH 222315
FFGMPD
NVZ000-CAZ000-230500-

Mesoscale Precipitation Discussion 0979
NWS Weather Prediction Center College Park MD
713 PM EDT Fri Aug 22 2025

Areas affected...Portions of the Sierra Nevada

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

Valid 222313Z - 230500Z

SUMMARY...Scattered slow-moving areas of showers and thunderstorms
over the Sierra Nevada late this afternoon and early this evening
may cause some localized concerns for flash flooding.

DISCUSSION...The mid-afternoon GOES-W Day Cloud Phase RGB
satellite imagery along with LightningCast data shows convection
initiating and gradually increasing in coverage over portions of
the Sierra Nevada. The convection that is initiating is in
response to strong diurnal heating with increasing instability and
terrain-driven circulations in a rather moist monsoonal regime.

SBCAPE values have risen to over 1000 K/kg along the western
flanks of the Sierra Nevada involving eastern CA with surface data
showing modest upslope flow into the higher terrain. The latest
RAP analysis generally suggests about 10 to 15 kts of southerly
flow in the 850/700 mb layer from Lake Tahoe southward down
Bishop.

Coinciding with this is a rather moist mid-level column with
CIRA-ALPW showing elevated moisture in the 700/500 mb layer and
this is supporting PW anomalies across the region of 2.0 to 2.5
standard deviations above normal. Deep layer southerly flow around
the western periphery of the subtropical ridge over the Southwest
U.S. is facilitating the poleward transport of monsoonal moisture
up across these areas, and this is expected to help drive a threat
for scattered heavy shower and thunderstorm activity going through
the evening hours.

Overall, the latest hires model guidance is rather ill-defined
with the convective footprint. However, the satellite and radar
trends suggest concerns for showers and thunderstorms to be
locally anchored near the higher terrain this evening with some
backbuilding characteristics which is further supported by weak
propagation vectors and the orographic focus that is in place for
development.

Rainfall rates with the stronger storms may reach 0.75 to 1.0
inch/hour, with potential for much of this to fall in as little as
30 minutes. Spotty storm totals of 1.5 to 2.5 inches will be
possible where any repeating cell-activity occurs. Given the
setup, some localized areas of flash flooding will be possible
going through this evening, and this will include potential for
some burn scar impacts.

Orrison

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

ATTN...WFO...HNX...REV...VEF...

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

LAT...LON   39371907 39131865 38311816 37671790 37091777
            36501775 36361800 36391833 36691869 37431910
            38031941 38741982 39181969