AWUS01 KWNH 220928
FFGMPD
AZZ000-UTZ000-221527-

Mesoscale Precipitation Discussion 1511
NWS Weather Prediction Center College Park MD
527 AM EDT Thu Oct 22 2026

Areas affected...AZ into southern UT

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

Valid 220927Z - 221527Z

Summary...Convection streaming northward across central/northern
Arizona into southern Utah will pose an isolated to scattered
flash flood threat through 15Z. While quick cell motions have
generally kept hourly totals under 0.5 inches, high short duration
rates over increasingly saturated ground will drive runoff
concerns, with the potential for linear training bands to produce
locally significant impacts in sensitive terrain.

Discussion...A broad plume of deep tropical moisture is streaming
northward across the Southwest ahead of a mid/upper level low
tracking into California. This moisture advection has pushed PW
values to near or at record levels for mid to late October across
a wide corridor. Despite being near the typical diurnal minimum in
instability, thermodynamic profiles remain unusually buoyant, with
CAPE exceeding 1000 J/kg across portions of Arizona and hovering
near 500 J/kg into southern Utah.

Convective activity across Arizona is expected to persist and
gradually increase in coverage and intensity through the early
morning hours. This will be driven by strengthening large scale
forcing for ascent from the west as the upper low approaches,
combined with the eastward progression of the low level moisture
transport axis into central Arizona. Convective elements
developing within this corridor will continue to stream rapidly
north northeastward into southern Utah.

To this point, fast storm motions to the north-northeast have
largely kept hourly rainfall totals in check, generally below 0.5
inches. However, short duration rainfall rates remain quite high
within the convective cores. Furthermore, antecedent rainfall over
the past 24 hours has left conditions increasingly saturated and
hydrologically sensitive across the region. Consequently, even
quick moving cells will be capable of generating localized flash
flood concerns. As convective coverage expands and instability
gradually ticks upward this morning, cumulative rainfall totals
will steadily mount over Arizona and southern Utah.

The deep unidirectional southerly flow supports localized training
and backbuilding, particularly across central Arizona at the nose
of the low level moisture transport axis and along the sharpest
instability gradient. Several recent HRRR/RRFS runs have indicated
this potential, highlighting a corridor between Phoenix and
Flagstaff favored for training convection capable of producing
rates exceeding 1"/hr. Even in southern UT where hourly rates
remain under 1 inch, recent HRRR runs depict some FFG exceedance
developing by 10-12Z, with spatial coverage expanding through mid
morning. While the FFG exceedance footprint is somewhat delayed
across Arizona, HRRR TLE exceedance probabilities climb by 12Z as
linear training bands may begin to organize.

Overall, at least isolated to scattered flash flooding is
considered likely through 15Z. Given the broader synoptic
evolution and diurnal heating, this threat will likely continue
beyond 15Z and may ultimately peak later in the day. Locally
significant flash flood impacts are possible, especially across
complex terrain, slot canyons, and sensitive drainage basins.

Chenard


ATTN...WFO...FGZ...GJT...PSR...SLC...TWC...VEF...

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

LAT...LON   38521171 38421056 38230994 37780936 37010927 
            35410937 34360935 33410948 33321014 33401072 
            33431181 33701262 34851307 35801325 37771314 

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