Mesoscale Precipitation Discussion 0532
NWS Weather Prediction Center College Park MD
514 PM EDT Tue Jul 24 2018

Areas affected...Sierra Nevada Mountains into western Nevada

Concerning...Heavy rainfall

Valid 242113Z - 250200Z

SUMMARY...An increased threat for flash flooding will develop
later this afternoon into the evening hours from the Sierra Nevada
Mountains into western Nevada. Rainfall rates up to 1 or 1.5 in/hr
may cause excessive runoff through 02Z.

DISCUSSION...12Z soundings from locations across Nevada into
northern California showed precipitable water values in the
90-95th percentile, ranging from 0.94 inches at REV to 1.24 inches
at VEF. While some drying has been noted as seen in the special
18Z REV sounding with a precipitable water value of 0.75 inches,
standardized anomalies remain +1.5 to +2.5 across the region.
Mostly clear skies up until this point have allowed for increasing
CAPE and visible satellite imagery from GOES 16 showed increasing
towering cumulus for a few locations across the Sierra Nevada
range.

With continued daytime heating and CAPE production amid the
anomalous moisture environment, convective coverage should
increase into the late afternoon. Per observed and forecast
soundings, deep layered flow is unidirectional from the
south-southwest with weak wind speeds (less than 20 kt) located
below about 300 or 400 mb. Upper level flow is stronger over
northern Nevada, but the concern is convection in its incipient
stages which should be shallower and be contained within the
portion of the troposphere containing weaker wind speeds, with the
potential for slow movement/training across the higher terrain.
Eventual development into portions of western Nevada will be
possible near and beyond 00Z as highlighted by locally run WRF
simulated reflectivity from the University of Arizona.

The flash flood threat across California into Nevada is expected
to remain scattered but localized issues will develop especially
if overlap occurs with burn scar locations.

Otto

ATTN...WFO...HNX...LKN...REV...STO...VEF...

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

LAT...LON   40581918 40291832 39691759 38921724 37761739
            37271791 36781806 36451807 36151820 36081848
            36311870 36731900 37371949 37972001 38782049
            39572061 40492002