719
AWUS01 KWNH 281737
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-282330-

Mesoscale Precipitation Discussion 1384
NWS Weather Prediction Center College Park MD
135 PM EDT Mon Sep 28 2026

Areas affected...Desert CA into southern NV, northwestern AZ and
southwestern UT

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

Valid 281734Z - 282330Z

SUMMARY...Increasing thunderstorms are expected into the afternoon
from southern NV into southwestern UT, northwestern AZ and the
desert regions of southern CA. Rainfall intensities are also
expected to increase over time, with 15 minute rates reaching 0.5
to 1.0 inches within the stronger cells. Repeating rounds may
allow for peak rainfall totals of 1 to 2 inches through 00Z.

DISCUSSION...Loops of layered PW imagery from OSPO showed the
advection of tropical moisture in the vicinity of Hurricane Polo
and former TC Odalys streaming northeastward into the southwestern
U.S. with 12Z soundings from VEF and SLC showing total PW values
of 1.1 and 0.9 inches (respectively), and MLCAPE of <500 J/kg with
weak CIN. These PW values are near the 95th percentile to near the
climatological max for late September via SPC sounding
climatology. 17Z visible imagery from GOES West showed clear to
mostly clear skies along the western edge of the anomalous
moisture axis from southern CA into and across the NV/AZ/UT
tri-state region but scattered showers and thunderstorms were
ongoing over NV/UT, to the west of I-15, from Lincoln County NNE
into western UT within the greatest available instability.

SPC mesoanalysis data from 17Z showed that MLCAPE was 500 to 1000
J/kg from eastern San Bernardino County into southeastern NV and a
small portion of western UT but these values are forecast to
increase into the 500-1500 J/kg range with continued daytime
heating. The greatest instability is forecast by the RAP along the
CO River Valley into southern NV and along the western UT/AZ
border. Increased forcing for ascent will arrive through 00Z ahead
of a positively tilted mid-level low/trough approaching from
CA/NV, accompanied by increasingly diffluent flow aloft just ahead
of the trough axis.

As thunderstorms likely increase in coverage over the next 1-3
hours, individual cells are forecast to follow the mean deeper
layer steering flow at 15-30 kt toward the northeast but the setup
will favor repeating rounds of thunderstorms along with brief
training. Hourly rainfall could locally exceed 1 inch as
instability increases later in the day but 15-minute rainfall
values of 0.5 to 1.0 inches could be the larger concern for flash
flood potential. Overlap of high rain rates with low water
crossings, slot canyons, burn scars and/or other areas of poor
drainage will be at most risk for flash flood impacts.

Otto

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

ATTN...WFO...FGZ...LKN...SLC...VEF...

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

LAT...LON   39531303 39091246 38201227 37341226 36881219
            36051228 34581394 34221476 34151595 34851623
            35991591 37231548 38791503 39361399