Flash Flood in Arizona, September 1, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
140 AM MST Wed Sep 1 2021

.SYNOPSIS...
Scattered to widespread showers and thunderstorms will occur today
over the entire region before mainly being confined to southwest
and south central Arizona tonight into Wednesday. Locally heavy
rainfall capable of producing flash flooding is expected, but
localized damaging wind gusts will also be possible. A Flash Flood
Watch is in effect through late tonight for southeast California
and Yuma and until late Wednesday night for the rest of southwest
and south central Arizona. A drying trend will begin on Thursday,
but lingering chances for isolated storms will remain over the
Arizona high terrain into the weekend. Well below normal
temperatures are expected through Thursday and warming back near
or just below normal by the weekend.

&&

.DISCUSSION...
The atmosphere is very moisture laden across the Southwest. The
01.00Z KPSR and KTUS soundings had 2.0" of PWAT, which is near
record levels for late Aug/early Sep. Upper level low near KSAN
and an associated jet streak ahead of it provided favorable
divergent flow across our western CWA Tuesday leading to intense
thunderstorms and heavy rain. Since then, the favorable dynamics
have shifted north leading to drier mid levels and an encroaching
capping inversion (seen on the 01.00Z KSAN sounding). Over our
eastern CWA, the main plume of deep moisture is slowly moving
east. Short term model trends suggest continue showers with
perhaps a few thunderstorms for the Phoenix area into southern
Gila County through the rest of the morning. While rainfall rates
will generally be 0.1-0.5"/hr, we may still see thunderstorms
develop that put out 1"+/hr rates. Given current trends and data,
cancelled the western portions of the FFA. The rest of the FFA
will continue, keeping in-line with previous forecasts and in
coordination with neighboring WFOs and WPC's latest ERO.

Drier air will gradually be filtering into the forecast area from
the northwest today as the low near KSAN lifts through the Four
Corners, but primarily in the mid and upper levels. Boundary layer
moisture will still remain a bit elevated, with PWAT values
decreasing from 1.5-2.0" to 0.5-1.5". There may be just enough
influence from the right-entrance region of the departing jet
streak and forcing from the mid level trough axis, interacting
with a pocket of 500-1000 MUCAPE (per HREF), to cause showers and
storms to develop around across central Arizona later this
afternoon into the overnight hours. Both SREF and HREF data
suggest this potential, with NBM PoPs of 10-40% into Thursday
morning.

A general warming and drying trend arrives for Thursday to
Saturday. There will remain low end PoPs for the mountains of AZ
but otherwise little rain chance with a southwest flow. Late in
the weekend and into early next week, GEFS/EPS/CMC support a ridge
building over the West Coast. While this means continued warm
temperatures (700 mb temps ~90th percentile), we may also see
influence of a backdoor front moving down the Front Range, through
New Mexico, and into eastern Arizona. Easterly flow develops in
response, with a modest moisture influx. Low end NBM PoPs look
quite reasonable. While ensemble cluster analysis suggests most
members keep a generally dry or low-key forecast next week, there
are about 20-30% of members that cluster into a wetter scenario
for Sunday-Monday. Another possibility is that no moisture really
materializes and temperatures continue to warm - NBM is giving a
10% (Phoenix area) to 20% (SE CA/SW AZ) chance of hitting 110+ deg
F starting Sun/Mon and continuing through next week.

&&

.AVIATION...Updated at 0536Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Current light -SHRA activity that is currently impacting the Phoenix
area is expected to continue off-and-on thru the rest of tonight and
into early Wed. Main concern is that some lower CIGS could develop
at times late tonight/early Wed (in the 3-5K foot range) if shower
activity does become heavier. An isolated TS cannot be completely
ruled out, but confidence is far too low to include TSRA/VCTS in any
of the TAFs at this point. Wind directions during this period are a
bit uncertain, but likely will favor a light southerly direction
until around daybreak, then favor a westerly direction thereafter.
Gradual improvement in the CIGS is expected during the late
morning/early afternoon hours on Wed as drier air moves into the
region from the west. Some redevelopment of shower/TS activity is
possible across the northern portion of the Phoenix area, but
confidence is too low to include any mention of VCSH/VCTS in the
TAFs at this point. Wed evening will feature clearing skies with
a more typical diurnal easterly wind returning by mid/late
evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

Drier air moving in from the south will begin to allow skies to
clear and shower/TS activity to move well off to the north by late
tonight. Dry conditions are then forecast for Wed with just some SCT-
BKN mid-level CIGS in the TAFs. Once the TS activity ends, winds at
KIPL will become mainly southeasterly thru late afternoon on
Wednesday, then switch to westerly during the evening hours on Wed.
Winds to remain mainly southerly at KBLH thru the remainder of the
TAF period.

&&

.FIRE WEATHER...
Friday through Tuesday:
Rain chances will be near zero for all but the far eastern
mountains of Arizona Friday. Those rain chances will slowly creep
back west through the weekend and remain into next week. For now,
minimum humidity values will drop back into the 15-35% for this
weekend and into early next week with modest overnight recovery.
Winds will generally follow diurnal trends aside from stronger
gusts in thunderstorm outflows.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through late tonight for AZZ533-534-537-538-
     540>558-560>563.

CA...None.
&&

DISCUSSION...Iniguez
AVIATION...Percha/AD
FIRE WEATHER...Iniguez

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
951 PM MST TUE AUG 31 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0453 PM     TSTM WND DMG     1 SSW WIEST             33.04N 115.45W
08/31/2021                   IMPERIAL           CA   LAW ENFORCEMENT

            CORRECTS PREVIOUS TSTM WND DMG REPORT FROM 1
            SSW WIEST. POWER POLE DOWN AND BLOCKING
            TRAFFIC. TIME ESTIMATED FROM RADAR.

&&

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
957 PM MST TUE AUG 31 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0925 PM     FLASH FLOOD      2 WNW CALEXICO          32.69N 115.53W
08/31/2021                   IMPERIAL           CA   LAW ENFORCEMENT

            COLE ROAD AT DOGWOOD ROAD CLOSED DUE TO
            FLOODING.

&&

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1030 PM MST TUE AUG 31 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0630 PM     TSTM WND DMG     3 E SEELEY              32.80N 115.62W
08/31/2021                   IMPERIAL           CA   LAW ENFORCEMENT

            ***DELAYED REPORT*** POWER LINES DOWN ALONG
            WITH APPROXIMATELY 20 POWER POLES DOWN OR
            LEANING OVER ALONG EVAN HEWES HIGHWAY NEAR
            FORRESTER ROAD. TIME ESTIMATED FROM RADAR.

&&

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1039 PM MST TUE AUG 31 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0654 PM     FLASH FLOOD      10 SSE WINONA           33.12N 115.85W
08/31/2021                   IMPERIAL           CA   LAW ENFORCEMENT

            ***DELAYED REPORT*** WATER AND DEBRIS ACROSS
            SR 86 AT WILLOW WASH MAKING THE ROAD
            IMPASSABLE.

&&

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1206 AM MST WED SEP 1 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1144 PM     HEAVY RAIN       WENDEN                  33.82N 113.53W
08/31/2021  M1.93 INCH       LA PAZ             AZ   MESONET

            A MARICOPA COUNTY FLOOD CONTROL DISTRICT
            GAUGE ALONG HIGHWAY 60 AT THE CENTENNIAL
            WASH MEASURED 1.93 INCHES IN ABOUT 1 HOUR
            AND 20 MINUTES AS OF 1144PM. THE HEAVIEST 1
            HOUR AMOUNT WAS 1.81 INCHES BETWEEN 1028PM
            AND 1128PM.

&&

AJ