Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
459 PM MST Sun Aug 15 2021

.UPDATE...
Updated 00Z aviation discussion.

&&

.SYNOPSIS...
Abundant monsoon moisture will remain entrenched across the Desert
Southwest into the middle part of the week maintaining daily
chances for showers and thunderstorms. The primary threats with
any storms will be localized heavy rainfall that may cause flash
flooding. Localized strong winds and some blowing dust will also
be possible for desert locations. A gradual decrease in moisture
during the latter half of the week should reduce rain chances
across the lower deserts as early as Thursday night. Temperatures
will remain below normal for most of Arizona, while near normal
temperatures are mostly expected for southeast California.

&&

An anomalously moist airmass continues to dominate the region today.
This morning's PSR 12Z sounding registered an anomalously high PW of
2.19" and mean W of 16.5 gm/kg. The objective analysis shows the UL
high centered over the NE CA/NW NV border with pronounced mid level
ENE flow along its S flank which is introducing some moderate mid
level drying, with an area of mid-level elevated mean RH sliding
into S AZ/N MX. Vort max's and inverted troughs are over this same
borderland NW MX/N Baja area with an additional connected trough
over NW Chihuahua/SSW NM. The disturbances were supporting a
small area of rain/showers over Imperial Cty and modest isolated
thunderstorms over S-SE AZ. On the NE flank of the high was
another shortwave trough pushing SSEwrd into N CO/N UT. An area of
isolated thunderstorms was developing ahead of this trough in N
AZ and CO. The midday ACARs mean sounding confirmed some
pronounced mid level drying centered at H5 with PW down to 1.11",
although elevated lower mid level and BL moisture were still
supporting SB-MU CAPE in the low 1000s with modest CIN. The
slight-moderate instability was held in check with a strong CAP
just below H7.

Early afternoon METSAT showed decreasing cloudiness with partly-
mostly sunny conditions across much of the deserts with mostly
fair weather CU. The HREF members are in much better agreement for
this afternoon and evening's storm prospects. They are picking up
on the short term drying and decreasing instability trend by
agreeing on keeping storms rather isolated this afternoon and
evening and confined mainly to the high terrain of N-E AZ, due to
the approaching shortwave to the N, and the deserts of S-SE AZ due
to the inverted troughs. The NBM has also locked onto the trend
as well with only 10-20% POPs depicted for this afternoon and into
Monday. Some isolated storms to the north may try to impact the
far N portions of La Paz Cty from near Parker to Alamo Lake/State
Park. Any isolated thunderstorms that manage to form later today
will still be capable of producing localized flash flooding across
the deserts or near Phoenix due to the wet soils.

Beginning later tomorrow and through the middle of the week the
shortwave trough and associated vort max's up north will push Swrd
into NM and E-AZ as PW once again recovers and inversion CAPS are
weakened through the column. The HREF members are now fairly
bullish on either a scattered, or scattered to numerous, line of
organized thunderstorms forming along the Rim on Monday afternoon
and descending S into Phoenix and the lower deserts Monday
evening. These storms will be capable of producing strong winds,
heavy rainfall, and localized flooding as potential threats. HREF
also calls for a >70% chance of 35 mph wind gusts Monday evening
in and around Phoenix. In preparation for this system the Flash
Flood Watch for S-Cent AZ has been extended through 2 AM MST on
Monday night.

The active weather pattern should continue into at least
Wednesday as the inverted trough that will exit to our southwest
today, likely comes back around into our region on Wednesday. This
weak cut off low (what was the inverted trough) looks to get
picked up in the flow as an anomalously strong trough dives
through the Pacific Northwest into the Great Basin. There is good
model evidence of a fairly strong upper level jet entering
Arizona from the south Tuesday night into early Wednesday while
there may also be increasing jet forced ascent from the trough
diving into the Great Basin. With plenty of moisture likely still
in place across our region, Tuesday night into Wednesday could end
up being another fairly widespread (modest) rain event across
south-central and eastern Arizona.

For the late week and weekend model uncertainty increases, but
the majority of the ensembles show the deep trough to our north
and then to our northeast becoming the dominant feature. With
broad troughing likely taking over our region, we should be under
a drying westerly flow by Thursday, likely lasting through next
weekend. Models are in better agreement showing this drier air
working into our region Thursday into Friday, likely ending or
significantly reducing our rain chances over the lower deserts on
Thursday with only very limited rain chances over the Arizona
high terrain.

Temperatures this week are likely to be fairly stable with the
western deserts mostly around seasonal normals each day. For the
south-central Arizona deserts, including the Phoenix area, below
normal temperatures are likely to continue with highs mostly in
the mid 90s to around 100 degrees.

&&

.AVIATION...Updated at 2359Z.

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

Aside from a 20% chance of convective outflows this evening,
minimal aviation impacts are expected until early Monday evening
when another round of strong to severe thunderstorms are possible.
Light and variable or south-southwest winds below 10 knots will
persist at the Phoenix TAF sites until easterly flow returns
around 05-06Z. There is only about a 10% chance of storms at any
terminal tonight if any weak northeasterly outflows push into the
lower deserts due to a low-level stable layer. Westerly flow
should return by late Monday morning through the afternoon hours,
with a 40% chance of strong to severe thunderstorms during the
evening. The best timing of outflows and storms is between 01-04Z
Tuesday, with a 70% chance of winds exceeding 35 knots and at
least a 10% chance of winds exceeding 50 knots. BKN-OVC ceilings
in these storms should generally remain above 6-8 kft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Minimal aviation concerns are expected through Monday morning,
with gusty south-southeast winds expected on Monday afternoon.
Winds will generally favor a southeast component at KIPL with
speeds generally remaining below 10 knots aside from gusts up to
20 knots on Monday afternoon. At KBLH, winds will generally favor
a southeast to southerly component with speeds below 10 knots
aside from gusts up to 25 knots on Monday afternoon. FEW-SCT
ceilings should remain above 10 kft.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Elevated moisture levels and a threat of showers and thunderstorms
will continue through at least Wednesday. A drying trend should
then set in late in the week with dwindling chances for wetting
rains on Thursday and likely little if any rain chances by Friday.
The strongest storms early in the week will be capable of
producing localized flash flooding, particularly in the higher
terrain north and east of Phoenix. Minimum humidities mostly
between 30-45% early in the period will gradually lower to 20-30%
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 through late Monday night for AZZ534-537>563.

CA...None.
&&

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

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
604 PM MST SUN AUG 15 2021

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

0241 AM     FLASH FLOOD      1 E AVONDALE            33.44N 112.33W
08/14/2021                   MARICOPA           AZ   MESONET

            CORRECTS PREVIOUS TSTM WND DMG REPORT FROM 1
            E AVONDALE. **DELAYED REPORT** COUNTY FLOOD
            GAGE AT BUCKEYE RD/AGUA FRIA RIVER EXCEEDED
            ALARM STAGE (0.72 FT) REACHING 0.79 FT AT
            1932 CFS. FLOWING WATER LEAD TO CLOSURE OF
            LOWER BUCKEYE RD, ONE MILE SOUTH OF THE
            GAGE, BETWEEN 4TH ST AND 127TH AVE.

&&

AD

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
610 PM MST SUN AUG 15 2021

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

1050 PM     HEAVY RAIN       4 SE DEER VALLEY        33.63N 112.07W
08/13/2021  E2.50 INCH       MARICOPA           AZ   PUBLIC

            ***DELAYED REPORT*** HEAVY RAIN LED TO A
            PARTIAL ROOF COLLAPSE AT MOON VALLEY BIBLE
            CHURCH. NO INJURIES WERE REPORTED. TIME
            ESTIMATED FROM HEAVIEST RAINFALL AND
            RAINFALL ESTIMATED FROM RADAR AND NEARBY
            MESONET OBSERVATIONS.

&&

SDB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
640 PM MST SUN AUG 15 2021

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

0645 PM     TSTM WND DMG     4 NW EAST MESA          33.46N 111.72W
08/11/2021                   MARICOPA           AZ   TRAINED SPOTTER

            **DELAYED REPORT** FLIPPED OVER PLANE AND
            SHEET ROOFING DAMAGE FROM THUNDERSTORM WIND
            GUSTS REPORTED. DAMAGE WAS CORROBORATED VIA
            PILOT-TO-TOWER AUDIO COMMUNICATION.

&&

AD

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
654 PM MST SUN AUG 15 2021

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

1055 PM     TSTM WND DMG     3 E EAST MESA           33.42N 111.63W
08/13/2021                   MARICOPA           AZ   PUBLIC

            ***DELAYED REPORT*** 40 FOOT TREE FELL
            THROUGH A 5 FOOT CINDERBLOCK WALL AND ONTO A
            CHICKEN COOP. NO INJURIES REPORTED. WINDS
            ESTIMATED 55 TO 65 MPH. TIME ESTIMATED FROM
            RADAR.

&&

SDB

Special Weather Statement
National Weather Service Phoenix AZ
749 PM MST Sun Aug 15 2021

AZZ560-563-160345-
Southeast Gila County AZ-San Carlos AZ-
749 PM MST Sun Aug 15 2021

...Strong thunderstorms will impact portions of Gila County through
845 PM MST...

At 749 PM MST, Doppler radar was tracking strong thunderstorms along
a line extending from 8 miles south of Cedar Creek to 12 miles
southwest of Fort Apache to 15 miles south of Seven Mile. Movement
was west at 25 mph.

HAZARD...Wind gusts of 50 to 55 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Hilltop and Sawmill.

This includes US Highway 60 between mile markers 269 and 297.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with these storms and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 3347 11017 3344 11020 3344 11032 3340 11035
      3340 11042 3354 11064 3379 11069 3381 11066
      3383 11032 3360 11004 3360 10999 3347 11000
TIME...MOT...LOC 0249Z 077DEG 21KT 3377 11014 3365 11012 3357 11001

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...55 MPH

SDB