Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
1119 PM MST Sun Aug 15 2021

.UPDATE...
Updated 06Z 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 0620Z.

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

Convective impacts through 07-08Z, particularly for the East
Valley terminals, and another round of strong to severe storms
early Monday evening will be the main aviation impacts. Easterly
10-20 knot outflows are expected to move through all terminals in
the East Valley by 0630Z, with 35-50 knot gusts expected within
thunderstorms and very isolated gusts in excess of 50 knots
possible. The best chances of storms will continue be in the East
and central Valley including KIWA, KSDL, and KIWA, but we have
left VCTS out of KDVT for now due to low confidence. There is a
10-20% chance of thunderstorm development in the West Valley,
particularly if an additional outflow from the north moves in
after 08Z. Otherwise, light and variable or easterly flow will
generally prevail overnight into tomorrow morning, with westerly
flow returning by late Monday morning through the afternoon.

Scattered to widespread thunderstorms are likely at all Phoenix
terminals by early Monday evening, with a 60% chance of strong to
severe thunderstorms during the evening. The best timing of
outflows and thunderstorms is between 01-04Z Tuesday, with a
70-80% chance of winds exceeding 35 knots and a 10-20% chance of
winds exceeding 50 knots. Very isolated destructive winds
approaching 70 knots will be possible. 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 and
evening. Winds will generally favor a southeast component at KIPL
with speeds generally remaining below 10 knots aside from 15-20
knot gusts 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 and
evening. 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

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

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

AZZ552-558-560>563-160500-
Southeast Gila County AZ-San Carlos AZ-Dripping Springs AZ-
Globe/Miami AZ-Superior AZ-Pinal/Superstition Mountains AZ-
918 PM MST Sun Aug 15 2021

...A strong thunderstorm will impact portions of Gila and Pinal
Counties through 1000 PM MST...

At 917 PM MST, Doppler radar was tracking a strong thunderstorm near
Cutter, or near Globe, moving southwest 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...
Globe, Superior, Miami, Cutter, Central Heights-Midland City,
Top-Of-The-World and Claypool.

This includes the following highways...
US Highway 60 between mile markers 225 and 245, and between mile
markers 248 and 256.
AZ Route 70 between mile markers 253 and 268.
AZ Route 77 between mile markers 152 and 170.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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

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

&&

LAT...LON 3328 11049 3312 11077 3310 11086 3324 11094
      3316 11106 3316 11108 3332 11115 3348 11061
TIME...MOT...LOC 0417Z 066DEG 23KT 3333 11072

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

SDB

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