AWUS01 KWNH 171404
FFGMPD
AZZ000-UTZ000-NVZ000-171900-

Mesoscale Precipitation Discussion 0891
NWS Weather Prediction Center College Park MD
1004 AM EDT Sat Aug 17 2024

Areas affected...southeast Nevada, northwest Arizona, southern
Utah

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

Valid 171403Z - 171900Z

Summary...Showers and thunderstorms will expand across the eastern
Great Basin and into the Four Corners through the morning.
Rainfall rates are expected to increase to 0.5-1"/hr, leading to
hourly rainfall accumulations of up to 0.75". Flash flooding is
possible.

Discussion...The GOES-E WV imagery this morning shows a potent
shortwave lifting across far southern NV, accompanied by expanding
reflectivity associated with deepening showers and thunderstorms.
The environment into which this shortwave is lifting is becoming
increasingly favorable for heavy rainfall as reflected by 3-hr
MUCAPE change from the SPC RAP reaching +200 J/kg combined with a
slow surge of PWs of 1-1.3" lifting into the northern High Deserts
of NV and far SW UT. Further evidence of the amplifying
environment is rising lightning-cast probabilities coincident with
increasing glaciation noted in the GOES-E day cloud phase RGB, and
it is likely that the next few hours will feature a rapid
expansion and intensification of reflectivity across the region.

As the shortwave continues to pivot northward, it will combine
with increasing bulk shear and enhanced diffluence within the RRQ
of a strengthening jet streak arcing into the Intermountain West.
Together this will drive strong deep layer ascent, which will work
across robust thermodynamics as daytime destabilization occurs in
conjunction with a persistent surge of low level moisture from the
south, and 850-700mb moisture flux anomalies are progged to reach
nearly +5 sigma according to the SREF. This will not only cause
expanding convective coverage, but intensification of moist
updrafts leading to rainfall rates which have a 10-20% chance of
exceeding 1"/hr according to the HREF. Mean 850-300mb winds will
remain progressive through the afternoon at 20-30 kts or more,
reducing the residence time of any individual cell, but hourly
rainfall of 0.5-0.75 inches is still likely in some areas as noted
by the HRRR 15-min rainfall product and UA WRF hourly rain
accumulations. Additionally, Corfidi vectors aligned to the mean
0-6km wind suggest repeating or training rounds of storms is
likely, which could produce 1-2" of rain with locally more than 3"
possible (HREF neighborhood probabilities for 3"/6hrs 10-15%).

Despite the fast motion, these intense rainfall rates, combined
with any training, could quickly overwhelm soils across the area.
Recent rain has increased the 0-10cm soil moisture percentiles to
as high as 90% in some areas according to NASA SPoRT, increasing
the vulnerability of what is already a sensitive region to heavy
rain rates due to the many slot canyons, dry washes, and recent
burn scars across the area. Any heavy rain rates moving across
these areas could quickly result in rapid runoff and flash
flooding.

Weiss


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

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

LAT...LON   40291288 40121105 39141042 37781083 37081144 
            36471263 36091461 36111579 36611625 37231615 
            37991574 39151493 39931420 
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