Mesoscale Precipitation Discussion 0638
NWS Weather Prediction Center College Park MD
1002 PM EDT Sat Aug 11 2018

Areas affected...extreme southwestern UT into northwest AZ,
southern NV and far eastern CA

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

Valid 120201Z - 120600Z

Summary...A threat for flash flooding will linger for another
couple of hours across portions of extreme southwestern UT into
northwest AZ, southern NV and far eastern CA. Additional localized
rainfall totals of 1 to 2.5 inches are expected by 06Z.

Discussion...0145Z regional radar showed a small cluster of
thunderstorms over northwestern AZ extending to the NV/UT/AZ
border, all progressing generally to the west. These storms were
outflow dominant but were still producing KESX estimated rain
rates of 1-2 in/hr, mainly in Mohave county. The 01Z SPC
mesoanalysis and the 00Z RAOB from VEF showed lingering MLCAPE of
500-1500 J/kg across the tri-state region with the VEF sounding
showing a fair degree of dry air throughout the column. The dry
air should tend to limit flash flood potential, but lingering CAPE
will likely be enough to support an additional 2-4 hours of robust
convection.

GOES 16 "clean window" IR imagery showed the coldest cloud tops
were generally confined to western Mohave county, coincident with
the strongest updrafts and nearby heaviest rainfall rates. KESX
reflectivity showed an outflow boundary propagating southwestward
across southern Lincoln county, and another bowing westward into
Clark county. Convergence from these mesoscale features into the
remaining instability axis should be enough to continue a short
term flash flood threat with rainfall rates of 1-2 in/hr before
instability is exhausted by 04Z to 06Z. There is good support for
this scenario from the HRRR and the University of Arizona locally
run WRF from 12Z.

Otto

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

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

LAT...LON   37681482 37591409 37351375 36751347 36191350
            35721338 35141331 34791339 34561359 34381397
            34181453 34221499 34591539 35131569 35641548
            36051532 36711536 37251532 37571522