Mesoscale Precipitation Discussion 0636
NWS Weather Prediction Center College Park MD
405 PM EDT Sat Aug 11 2018

Areas affected...Southwest Utah, Northwest Arizona, Far Southeast
Nevada

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

Valid 112004Z - 120200Z

Summary...Scattered thunderstorms will continue to develop early
this afternoon along the higher terrain ridges of southwest Utah
and northwest Arizona. A greater concentration of storms is
possible in far southwest Utah and near the UT-AZ border. Over the
entire region, the storms may produce rain rates in excess of 1
in/hr, which could lead to flash flooding.

Discussion...19Z RAP analysis places a bubble of stronger
instability very near the confluence of the NV-AZ-UT borders, with
MLCAPE values above 1000 j/kg. Convective inhibition was beginning
to erode further east initially, from around the ranges of central
Utah south into Arizona. Convection was already developing along
the terrain ridges in Utah and Arizona, and this should continue
as the atmosphere destabilizes further in the next hour or two.
The available instability should support fairly strong convection,
with the potential for locally heavy rainfall. There was a minimum
in the deep layer (850-300mb) mean wind from northern Clark
County, NV into western Garfield County, UT near the center of a
mid-level ridge axis. Just to the south over northern Arizona, the
mid-level flow was actually above 20 knots out of the NE/ENE. This
may allow for faster storm propagation in portions of Arizona,
which could limit flash flood potential despite generally higher
precipitable water and CAPE values. The best overlap of the slower
storm motions and stronger instability, therefore, is close to the
UT-AZ border region. This would be the area most favored for flash
flooding, and the 12Z HREF has the greatest probabilities for over
1 inch of rainfall in the same area. KICX radar already shows some
slow moving storms in southwest Utah, and this may continue
through the afternoon hours as convection gradually spreads west
into the region of greater instability.

Lamers

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

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

LAT...LON   39401485 39381413 38931341 39201247 38881177
            38011168 36621203 35281163 34711201 34641294
            34921398 35541501 36961540 38121494