Mesoscale Precipitation Discussion 0675
NWS Weather Prediction Center College Park MD
230 PM EDT Thu Aug 16 2018

Areas affected...Southern/Eastern CA...Southern NV...Southern
UT...AZ...Western NM

Concerning...Heavy rainfall...Flash flooding likely

Valid 161830Z - 170030Z

Summary...Numerous showers and thunderstorms will be developing
over the next several hours. Heavy rainfall rates and slow-moving
cells will likely result in flash flooding going through the
afternoon hours.

Discussion...The latest GOES-16 Geocolor RGB imagery shows
convection just beginning to initiate over the higher terrain of
far eastern CA, southern NV, central AZ and western NM as strong
solar insolation continues which is contributing to a
destabilizing boundary layer. Already, MLCAPE values are on the
order of 2000 to 2500 j/kg over southwest AZ and the instability
axis expands across southern/eastern CA and southern NV that very
closely aligns with a tight mid level moisture gradient seen in
Simple WV RGB imagery.

The PWATS across the Southwest U.S. are quite anomalous and
notably more so than 24 hours ago, with values of 1.8 to 2.0
inches seen across central/southern AZ and the deserts of southern
CA. This moisture is about 2 to 2.5 standard deviations above
normal and will be conducive to much more efficient convection
where rainfall rates will be enhanced.

Over the next several hours, convection will continue to initiate
over the higher terrain of the broader Southwest U.S. and expand
in coverage with slow-moving cells and rainfall rates that are
likely to reach over 2 inches/hr within the stronger convective
cores. Based on the latest suite of 12Z hires guidance, and HREF
probabilities, the more concentrated activity should tend to be
over areas of southern CA and AZ, but convection will be
developing also farther north across southern NV, southern UT and
western NM as well.

In all areas, flash flooding is expected to be a concern this
afternoon given the anomalous moisture profiles, weak steering
flow, and proximity of terrain which will locally anchor and focus
convection even further. Expect rainfall totals at least locally
of as much as 3 to 4 inches going through 00Z. Flash flooding will
be likely over any impacted burn scar areas, washes and
slot-canyons as well as areas well downstream from these locations.

Orrison

ATTN...WFO...ABQ...EPZ...FGZ...GJT...HNX...LKN...LOX...PSR...
SGX...SLC...TWC...VEF...

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

LAT...LON   38501250 38351129 37771033 36910921 36150840
            35150779 33700780 32700851 32000974 31741077
            31751183 31751220 31961287 32251393 32471493
            32551633 32861695 33621724 34001752 34031788
            34201823 34681826 35401752 36221709 37411683
            37781665 38421598 38461494 37931433 37791342