871
AWUS01 KWNH 111847
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-120040-

Mesoscale Precipitation Discussion 0862
NWS Weather Prediction Center College Park MD
246 PM EDT Sun Aug 11 2024

Areas affected...northwestern AZ, southeastern NV and
central/southern UT

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

Valid 111843Z - 120040Z

SUMMARY...Thunderstorm coverage will expand over the next few
hours leading to scattered flash flood concerns across portions of
northwestern AZ, central/southern UT into southeastern NV.
Rainfall rates of 1-2 inches in 30-60 minutes are expected.

DISCUSSION...Monsoon convection had already initiated as of 1815Z
across northern AZ into southwestern UT. A mid-level low off of
the Baja Peninsula and ridge over western NM has allowed anomalous
moisture to funnel northward into the Desert Southwest with 12Z
soundings from VEF and FGZ displaying precipitable water values
between 150 to 175 percent of normal along with a lack of
appreciable dry air aloft. Northwestern AZ being positioned on the
periphery of the Baja low and NM high while placed south of the
westerlies in northern UT, was also within a region of weak wind
shear with 15 kt or less of tropospheric winds and 10 kt or less
deeper layer mean flow, which will support slow moving storms. SPC
mesoanalysis data from 18Z showed MLCAPE of 500-1500 centered over
northwestern AZ where mostly clear skies were giving way to
thunderstorm development along the higher terrain.

Expectations are for thunderstorm coverage to increase across
northwestern AZ into southern UT as daytime heating continues to
expand the coverage of available instability. Activity may also
enhance ahead of a well-defined MCV tracking northwestward from
southern AZ. Thunderstorm development along the southern end of
the Wasatch Range is also expected to continue where mean westerly
winds are stronger (~15 kt) but matching similarly to 700 mb wind
vectors, which may promote areas of training. Subsequent outflow
boundary interactions and storm mergers may allow for convection
to merge with isolated convection that develops across adjacent
portions of southern NV.

Rainfall rates are expected to peak in the 1-2 inch range per
30-60 minutes time, resulting in concerns for flash flooding. The
favorable setup for thunderstorms combined with wet antecedent
conditions (many areas displaying 200+ percent of average rainfall
over the past week) will lead to an increased threat for flash
flooding compared to a typical monsoon convective day. The flash
flood threat is likely to continue well into the evening hours.

Otto

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

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

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

LAT...LON   39671289 39561146 39041078 38181068 37201085
            35921133 34591129 34211168 34021293 34581433
            36191588 37341549 38781462 39371379