Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
954 PM MST Mon Aug 16 2021

.UPDATE...
Updated 06Z aviation discussion and Flash Flood Watch for south
central Arizona extended until 5 AM MST Tuesday.

&&

.SYNOPSIS...
Active monsoon weather will persist over much of Arizona into the
middle part of the week creating daily chances for showers and
thunderstorms. The best chance of thunderstorms through midweek
will be this afternoon and evening. The primary threats with any
storms will be localized heavy rainfall that may cause flash
flooding and strong winds. A steeper decrease in moisture starting
Wednesday should reduce rain chances across the area as early as
Wednesday afternoon, but especially by Thursday. Much drier
conditions are then anticipated by the weekend with temperatures
eventually warming back to normal readings.

&&

.DISCUSSION...
Convection is off to an earlier than normal start today with
midday radar and METSAT imagery showing areas of developing
thunderstorms from NE of Tucson to the White Mountains/W NM and
along the Rim NE of Phoenix. The storms SE-E of Phoenix and W-
Cent NM formed along a positively tilted H3 trough axis aloft
while the Rim storms were being triggered by a series of vort max
disturbances grouped near a weak H5 E-W trough and thermal/cold
trough embedded in the northerly flow over N AZ-NW NM. The
anomalously moist airmass remains in place across the region today
to complement the aforementioned triggers. The UL high has not
moved much and is centered over Cent-CA at H5 with moist NE flow
aloft and Nrly flow to the N. The latest ACARs mean soundings
maintained elevated PW of 1.91" owing to a moist column from the
BL through mid levels/H5. Also H7-H5 steering/flow remained
NEerly. Also, MU CAPE had jumped up to 2,381 j/kg with modest CIN.

Fairly decent chances for Monsoon storm activity will persist
through Wednesday for Phoenix and the deserts. However the very
best chance of thunderstorms, including an SPC Day 1 slight risk
of severe convection, through Wed will occur early evening when
NBM PPI/POPs spike up to 70%. Models agree on the area of
disturbances to N-NE of the area to shift SWrd into S-Cent AZ
including Phoenix and the lower deserts this afternoon and
evening. As a result An organized area of storms is expected to
fire along the rim with scattered to possibly/occasionally
numerous storms descending through Phoenix and the lower deserts
this evening and tonight.

These storms will be capable of producing strong winds, heavy
rainfall, and localized flash flooding, and hail as potential
threats. The latest HREF continues to call for a >=70% chance of
35 mph wind gusts this evening in and around Phoenix and a
>=10-20% chance of locally severe wind gusts of 60-75 mph.

The latest HREF probability of 3-hr QPF >1" for this evening has
increased to >=50% with WPC Day 1 ERO in the slight range for
Phoenix and the deserts. This generally puts the risk of rainfall
exceeding flash flood guidance at 10-20%. In preparation for this
system the Flash Flood Watch for S-Cent AZ continues through 2 AM
MST on Monday night. Although there remains 40-50% chance for
lingering scattered showers and storms to last until dawn for
Phoenix and the deserts. The HREF members are in good agreement on
the timing of scattered thunderstorms in the early evening for
Phoenix and the lower deserts. Although the FV3 is a slight
outlier with a bit larger coverage for storms even though the
other members already have pretty good coverage.

The shortwave trough should mostly exit east of the region on
Tuesday with the flow aloft weakening over the area. Guidance
mostly points toward only modest storm chances during the day
Tuesday, mostly focused over the high terrain. Can't rule out some
of the high terrain storms making their way into the lower
deserts, but the steering flow will be quite weak and shifting out
of the west later in the day. Chances for showers and isolated
storms should then increase Tuesday night into Wednesday morning
from Phoenix and areas to the east of Phoenix as a unseasonably
strong trough digs southeastward into the Great Basin and a fairly
strong vort max tracks northward into southern Arizona later on
Wednesday. Increasing difluent flow aloft should provide some
broad scale ascent, possibly combining with potential vorticity
from the incoming vort max sometime on Wednesday. Overall, the
majority of the EPS and GEFS members support an area of rain
developing across southern into central Arizona sometime Tuesday
night into Wednesday morning, though instability will be fairly
limited and elevated. Forecast PWATs of 1.6-1.9" would support
some heavy rainfall, so an extension of the current Flash Flood
Watch may be needed.

Drier air aloft will begin working into the area from the west
southwest later in the day Wednesday, but chances for showers and
storms are likely to continue through Wednesday evening across
eastern Arizona. The drier air should really be noticeable
starting Thursday with PWATs lowering to around 1.25", which
should completely end rain chances across the lower deserts. May
see a few isolated showers and storms across the high terrain on
Thursday, but NBM PoPs are only 15-25%. Ensembles then support
continued dry westerly flow aloft through the upcoming weekend,
likely even ending rain chances over most of the high terrain.
Temperatures will also begin to warm later this week, but we may
not see highs back to normal readings until maybe the weekend.

&&

.AVIATION...Updated at 0454Z.

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

Thunderstorms impacting the Phoenix terminals over the next few
hours will cause strong to severe winds, heavy rain, frequent
lightning, and possibly blowing dust. A line of storms currently
moving into eastern Maricopa County will cause northeast 25-35
knot outflows, with a 10-20% chance for winds to exceed 50 knots
within thunderstorms at any terminal. Very isolated 60-70 knot
gusts will be possible as well, with a Severe Thunderstorm Watch
in effect until 07Z. The outflow should move into the East Valley
including KIWA by 0515Z, KPHX by 0545Z, and clear the West Valley
by 0615-0630Z. Thunderstorms should generally end by 08Z, with
showers possible through 10Z.

Light and variable winds are expected overnight once the shower
activity ends, with perhaps a brief 1-2 hour period of southerly
or southwest winds behind any organized lines of storms.
Otherwise, winds will eventually settle upon easterlies at 5-10
knots late in the overnight hours before veering to westerly
between 19-21Z with a brief period of southerly winds for a
couple hours ahead of that switch. Another round of showers and
thunderstorms will be possible again tomorrow evening or during
the overnight hours into Wednesday morning, but the evolution of
tonight's convection or lack thereof will strongly affect our
storm chances tomorrow. Thus, for now we have not included any
convection in the extended TAF for KPHX and will reevaluate for
the next package. Ceilings will generally remain above 6-8 kft
aside from brief decreases to 4-5 kft within thunderstorms.

Southeast California/Southwest Arizona including KIPL and KBLH:

Gusty winds this evening and again on Tuesday afternoon will be
the main aviation impacts through the TAF period. In addition,
isolated to scattered thunderstorms capable of producing blowing
dust and strong to severe winds will be possible from 07-09Z
over La Paz and Yuma Counties, but should eventually dissipate
before crossing the Colorado River into southeast California. At
KBLH, winds will generally favor a southerly component with speeds
around 10-15 knots aside from gusts up to 20 knots tonight and
25-30 knots on Tuesday afternoon. Winds at KIPL will have more
southeast headings, with gusts up to 25 knots expected. FEW-SCT
cloud decks will stay above 10-15 kft aside from any thunderstorms
in southwest Arizona overnight tonight.

&&

.FIRE WEATHER...
Wednesday through Sunday:
The threat for showers and thunderstorms will continue through
Wednesday before a drying trend sets in late in the week with
dwindling chances for wetting rains on Thursday and likely little
if any rain chances by Friday. Minimum humidities mostly between
30-45% on Wednesday will gradually lower to 15-30% late in the
period. Temperatures will remain below normal across much of
Arizona through the period, while the western deserts see near
normal readings.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch until 5 AM MST Tuesday for AZZ534-537>563.

CA...None.
&&

DISCUSSION...Sawtelle/Kuhlman
AVIATION...Hopper
FIRE WEATHER...Kuhlman

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1151 PM MST MON AUG 16 2021

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

1134 PM     DUST STORM       2 SE CITRUS PARK        33.52N 112.42W
08/16/2021                   MARICOPA           AZ   DEPT OF HIGHWAYS

            VISIBILITY NEAR ZERO ON LOOP 303 SOUTHBOUND
            NEAR BETHANY HOME ROAD.

&&

LJH

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1155 PM MST MON AUG 16 2021

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

1131 PM     TSTM WND GST     5 SSE AVONDALE          33.37N 112.31W
08/16/2021  M58 MPH          MARICOPA           AZ   MESONET

            MESONET STATION FW4391 AVONDALE.

&&

SDB

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