Mesoscale Precipitation Discussion 0583
NWS Weather Prediction Center College Park MD
247 PM EDT Thu Aug 02 2018

Areas affected...Mogollon Rim into southeast Nevada and southwest
Utah

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

Valid 021846Z - 030045Z

Summary...Scattered thunderstorms were beginning to develop around
midday across portions of the Middle Colorado River Basin into the
Mogollon Rim region in the Southwest. These storms should become
more numerous in the afternoon, with a chance of flash flooding.
Rain rates of 1-2 in/hr are expected with the strongest storms.

Discussion...GOES-15 visible and IR satellite channels showed a
blossoming of convection around 18Z centered on the confluence of
the NV-AZ-UT borders, and extending southeast into the Mogollon
Rim region. This was occurring as instability was continuing to
gradually increase and convective temperatures were being reached.
By 21Z, the RAP forecasts a nose of higher PWs and CAPE of at
least 1000 j/kg extending up the Colorado River Valley from near
the Mohave Valley into the slot canyon region of southern Utah. It
also forecasts an increase in S-SW flow in the same region in the
next several hours, helping to advect better low-level moisture up
the valley. KICX, TLAS, and KESX VWPs already show around 10-15
knots of inflow in the 850-700mb layer, and this may aid in
sustaining a more concentrated area of convection this afternoon
just downstream (or over southwest Utah and northwest Arizona).
GPS-PW observations near Las Vegas were already registering nearly
1.2 inch PWs, and the 12Z sounding already had a 1.23 inch value.
These are above the 90th percentile on the VEF sounding
climatology; while they are not close to record values, they are
well above average values and should support more widespread
convective coverage as well as heavy rain rates. With moderate
instability in place, thunderstorms should be able to produce 1-2
in/hr rain rates, which could cause flash flooding.

Chances for flash flooding may be highest in southern Utah,
particularly the slot canyon region with narrow basins, where
uni-directional flow on the north side of the ridge could funnel
convection into training patterns. The Mogollon Rim could also be
active with much weaker winds near the center of the ridge, which
may help sustain convection closer to the terrain features for
longer periods of time.

Lamers

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

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

LAT...LON   38471414 38471147 37741079 36541127 35351086
            34501034 33861106 34131252 34881396 35691529
            37481543