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AWUS01 KWNH 310821
FFGMPD
AZZ000-NVZ000-CAZ000-311420-

Mesoscale Precipitation Discussion 0691
NWS Weather Prediction Center College Park MD
421 AM EDT Wed Jul 31 2019

Areas affected...central to southwestern AZ to the Colorado Valley

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

Valid 310820Z - 311420Z

Summary...A localized flash flood threat will continue across
portions of southwestern into central AZ through mid-morning.
Hourly rainfall rates of 1-2 in/hr will be possible within the
greatest areas of stalled or slow moving convection with the
potential for an additional 2-3 inches through 15Z.

Discussion...GOES 17 1-min infrared imagery through 08Z showed the
coldest cloud tops in the region were located over western
Maricopa County, in association with an outflow boundary which
emanated from a burst of convection over I-10 around 06Z. The
outflow has since pushed south and west into central La Paz and
northeastern Yuma Counties, igniting new convection in the
presence of 1000-1500 MLCAPE via the 07Z SPC mesoanalysis page.
New convective cells were also observed via KIWA reflectivity
across other portions of western AZ.

A larger low-mid level circulation was clearly evident from the
last few images of visible imagery combined with recent RAP
analyses while being partially evident from recent IR imagery over
the northern Gulf of CA. This circulation is forecast to slowly
drift north over the next 6 hours with ~15 kt of 850-700 mb
southerly flow forecast to help sustain renewed low level moisture
flux into southwestern AZ and southeastern CA.

Area RAP analysis soundings showed LFC-EL mean winds of less than
5 kt, located where the greatest instability remained in
southwestern/western AZ. Recent GPS observations showed PWAT
values were hovering between 1.7 and 2 inches which will support
heavy rain rates of 1-2 in/hr in the near term. Beyond 12Z, low
level moisture advection up the Colorado Valley will help to
increase CAPE into southern NV through 15Z with the potential for
additional slow moving thunderstorms to develop in the 12Z-15Z
window from western AZ into far southern NV, focusing on numerous
low level boundaries and smaller scale circulations from earlier
convection. This idea is roughly supported in a few of the
experimental HRRR runs, but confidence on the exact details
remains a bit uncertain. Maximum additional rainfall totals of 2-3
inches through 15Z will be possible.

Otto

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

ATTN...WFO...FGZ...PSR...TWC...VEF...

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

LAT...LON   35731403 35731348 35311258 34601210 33061248
            32051264 31751274 32291462 32611488 34071478
            35341469


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