Mesoscale Precipitation Discussion 0773
NWS Weather Prediction Center College Park MD
210 PM EDT Mon Jul 21 2025

Areas affected...Southeast Arizona, far Southwest New Mexico

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

Valid 211810Z - 220000Z

Summary...Showers and thunderstorms will expand across southern
Arizona trough the afternoon,with rain rates increasing to as much
as 1"/hr. These storms will move slowly, and may repeat in some
areas, leading to 1-2" of rainfall with isolated higher amounts.
This could produce instances of flash flooding.

Discussion...The GOES-E visible satellite imagery late this
morning shows billowing Cu expanding across southeast Arizona,
which when combined with the GOES-E day cloud phase RGB indicates
rapidly expanding and intensifying updrafts from northern Mexico
into Arizona. This activity is building in response to ascent
driven by a shortwave lifting northwest across Chihuahua, Mexico,
and increasing diffluence in the RRQ of a jet streak pivoting into
the Four Corners region. The combined lift of these features is
working upon robust thermodynamics characterized by PWs which were
recently measured via GPS to be 1.5 inches, well above the 90th
percentile for the date, overlapping SBCAPE of 500-1000 J/kg. The
region is pinched between a broad ridge centered over the Gulf
Coast and a trough emerging from the Pacific Northwest, leaving
deep southerly flow across the Southwest. It is this southerly
flow that will allow thermodynamics to become even more impressive
by the afternoon.

As forcing for ascent and thermodynamics maximize atop the area in
the next few hours, convection should rapidly increase in coverage
and intensity. This is supported by simulated reflectivity from
the HRRR, RRFS, and 3kmNAM, as well as the recent HRRR-UA WRF
output. Rainfall rates within this convection may exceed 1"/hr at
times (40% chance from the HREF), with locally even more intense
rain rates producing 15-min rainfall as much as 0.5". With mean
cloud-layer winds remaining southerly at around 10 kts, storms
should move slowly northward, but redevelopment into the higher
thermodynamics is expected resulting in repeating rounds of heavy
rainfall across southern Arizona. Where multiple cells repeat,
total rainfall of 1-2" of rainfall is likely, with isolated higher
amounts possible as reflected by the HRRR-UA WRF and HREF 6-hr PMM.

1-hr FFG across this area is only 0.75 to 1.00 inches, and the
HREF neighborhood exceedance probabilities reach as high as 40%,
with 3-hr FFG exceedance peaking above 60% (due to repeating
rounds). These probabilities suggest that flash flooding is
possible anywhere due to these intense rain rates. However, the
most vulnerable locations will be atop sensitive terrain, recent
burn scars, or urban areas, where any training or slower motion of
these intense rain rates would quickly become runoff leading to
flash flooding.


Weiss

ATTN...WFO...EPZ...PSR...TWC...

ATTN...RFC...FWR...STR...NWC...

LAT...LON   33121030 32970960 32800891 32540842 32120817
            31410809 31090849 31091024 31281134 31501179
            31811209 32331219 32811185 33091112