Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
158 PM MST Thu Jul 22 2021

.SYNOPSIS...
A low pressure system will drift slowly towards the Desert Southwest
over the next few days. With abundant Monsoon moisture in place,
thunderstorms will be likely, particularly across portions of
Arizona. The strongest storms will be capable of producing heavy
rainfall and areas of flash flooding. Temperatures will also be well
below normal beginning Friday. Warmer and drier conditions are
anticipated next week as the low pressure system lifts northward
into the Great Basin.

&&

.DISCUSSION...
Water vapor imagery clearly shows a well-defined westward moving
upper low over the Rio Grande Valley of Texas this afternoon.
The leading edge of the influence of this feature is moving across
New Mexico, approaching a mid-level anticyclone positioned over
northern Arizona. Some modest drying was indicated in morning
Phoenix soundings (including a special 16Z sounding), especially
aloft, but overall there's plenty of moisture available to support
thunderstorm development this afternoon. SPC mesoanalysis showed a
rich axis of moisture (PW values near 2 inches) oriented across
the lower Colorado River Valley, with similar values spreading
into southeast California. More ample heating today is supporting
a very unstable air mass across all desert locations, with MLCAPE
values of 1500-2000+ J/kg in place. Thunderstorm activity should
remain fairly isolated through this afternoon, but any
thunderstorm that develops has the potential to quickly intensify,
with heavy rainfall, gusty winds, and small hail a threat.
Blowing dust will also be possible across southwest Arizona and
southeast California in vicinity of any thunderstorms.

Almost all CAM guidance continues to suggest thunderstorm
potential will increase markedly this evening into tomorrow. The
first wave of activity will likely develop across western New
Mexico into the White Mountains of eastern Arizona. Although
specific details are different amongst various model signals,
there's still a very strong signal overall that multiple episodes
of heavy rain will develop across southern Gila County and move
into the lower deserts overnight tonight, and again redeveloping
Friday afternoon. This setup has the potential for significant
rainfall over the higher terrain and lower deserts of south-
central Arizona. Daily rainfall records will be in jeopardy on
Friday (including 0.41 inches at Phoenix). The WPC Excessive
Rainfall Outlook has been upgraded to Moderate for Friday for
southern Gila County, with significant flash flooding potential
expected.

Scattered showers and thunderstorms are likely to continue during
the day Saturday into early Sunday. Overall rainfall totals are
expected to range between 2-4" across the higher terrain east of
Phoenix, to 1-2" across the deserts of Maricopa and northwest
Pinal Counties. Locally higher totals are likely. Rainfall
amounts of these magnitudes have the potential to produce very
dangerous flash flooding, particularly in normally dry washes and
even along mainstem rivers such as the Agua Fria, Hassayampa and
Tonto Creek. Model consensus indicates rain totals drop off
considerably west of Maricopa County.

The one silver lining to all of the clouds and rain is temperatures
will be well below normal through at least Monday, which will reduce
the Heat Risk. High temperatures in Phoenix will likely remain below
the 100 degree mark for 4 consecutive days, Friday through Monday.
The last time this happened during July or August was in August of
2014, when 6 consecutive days below 100 degrees were recorded.

Ensembles are in good agreement warmer and drier conditions will
return to the Desert Southwest by Tuesday as the low pressure system
gets pulled rapidly northward into the Great Basin.

&&

.AVIATION...Updated at 1910Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds have become mostly light and variable across the metro with
winds starting to favor more of a south to southwest component.
Expect winds to continue to favor more of a westerly component
going through the early afternoon hours with some gusts upwards of
15-20 kts possible at times later this afternoon. Chances for
showers and thunderstorms will increase (20-40%) going into the
late evening and overnight hours across the Phoenix area, though
the exact timing and evolution of this convection remains
uncertain. Any thunderstorms that do develop will be capable of
producing gusty erratic outflow winds with HREF guidance showing a
30-50% chance of outflow winds exceeding 30 kts. Chances for more
widespread precipitation increase (50-60%) going through tomorrow
morning and into the afternoon across the Phoenix area. Cigs are
expected to fall to around 7-8 kft tonight with cigs falling to
around 6 kft or perhaps slightly lower with any showers and
thunderstorms that pass through.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds will generally favor a south to southeast component at the
terminals through the period. Speeds are expected to remain elevated
around 8-12 kts with some gusts to around 20 kts this
afternoon/evening, especially from any outflow winds from Baja.
Chances for showers and thunderstorms remain low for this afternoon
(~20%) but can't rule out some development near the terminals.
However, overall confidence remains too low to mention in the TAFs
at this time. Cloud bases are expected to mostly remain aoa 10 kft
through the TAF period.

&&

.FIRE WEATHER...
Sunday through Thursday:

After a very wet period, isolated to scattered thunderstorms
will continue Sunday into next week with the potential for
locally heavy rainfall. Storm activity is anticipated to spread
as far west as southeast California, especially on Sunday and
Monday with most storms having good potential for wetting rain
(0.10" or more). Tuesday will see gradual drying conditions, but
enough moisture will linger for continued daily thunderstorm
chances on Wednesday and Thursday, especially across the higher
terrain. Apart from thunderstorms, winds will favor familiar
diurnal patterns.

Min RH levels are expected to be in the 30-45% range across south-
central Arizona (45-60% higher terrain of south-central AZ) and 25-
35% across southeast California and southwest Arizona through
Monday. Max RH levels, outside of storms, will be 55-70% most places
(60-90% higher terrain of south-central AZ). Values will fall into
a more typical range for the summer by Tuesday, with min RH
readings ranging from 15-20 percent in southeast California to
20-35% in south-central Arizona.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch from 5 PM MST this afternoon through Saturday
     evening for AZZ534-537>546-548>551-553>555-559.

     Flash Flood Watch through Saturday evening for AZZ547-552-
     556>558-560>563.

CA...None.
&&

DISCUSSION...Rogers/Hirsch
AVIATION...Smith
FIRE WEATHER...Rogers/Kuhlman

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1212 PM MST THU JUL 22 2021

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

1030 PM     HAIL             2 SW PARADISE VALLEY    33.51N 111.97W
07/21/2021  E0.50 INCH       MARICOPA           AZ   PUBLIC

            *** DELAYED REPORT *** SOCIAL MEDIA POST
            SHOWS ACCUMULATING HAIL ALONG NORTH 53RD ST.
            BETWEEN EAST PALOMINO RD. AND EAST
            WONDERVIEW RD. WITH SIZES ESTIMATED TO BE
            SIMILAR TO NEARBY REPORTS.

1030 PM     HAIL             2 WSW SCOTTSDALE        33.49N 111.96W
07/21/2021  E0.50 INCH       MARICOPA           AZ   PUBLIC

            *** DELAYED REPORT *** RESPONSE TO SOCIAL
            MEDIA POST REPORTED HAIL AT NORTH 56TH ST.
            AND EAST OSBORN RD.

1030 PM     HAIL             3 W SCOTTSDALE          33.49N 111.97W
07/21/2021  E0.50 INCH       MARICOPA           AZ   PUBLIC

            *** DELAYED REPORT *** SOCIAL MEDIA POST
            SHOWS HANDFUL OF HAIL WITH SIZE ESTIMATED
            FROM VIDEO NEAR EAST INDIAN SCHOOL RD. AND
            NORTH 50TH ST.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
1213 PM MST THU JUL 22 2021

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

0357 PM     HAIL             AVONDALE                33.44N 112.36W
07/21/2021  E0.25 INCH       MARICOPA           AZ   PUBLIC

            *** DELAYED REPORT *** PEA-SIZED HAIL WITH
            WIND GUSTS ESTIMATED TO 35-40 MPH.

&&

GL

Special Weather Statement
National Weather Service Phoenix AZ
306 PM PDT Thu Jul 22 2021

CAZ562-566-567-222300-
Imperial County West CA-Imperial County Southwest CA-
Imperial Valley CA-
306 PM PDT Thu Jul 22 2021

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHWESTERN IMPERIAL COUNTY
UNTIL 400 PM PDT...

At 305 PM PDT, Doppler radar was tracking a strong thunderstorm 8
miles west of Mt. Signal, or 10 miles south of Plaster City, moving
north at 10 mph.

Winds in excess of 40 mph along with blowing dust and reduced
visibilities down to two miles will be possible with this storm.

Locations impacted include...
Plaster City, Ocotillo, Mt. Signal, Dixieland and Coyote Wells.

This includes the following highways...
CA Interstate 8 near mile marker 1, and between mile markers 3 and
30.
CA Route 98 between mile markers 1 and 26.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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 3280 11610 3286 11593 3276 11564 3266 11559
      3262 11610 3279 11611
TIME...MOT...LOC 2205Z 161DEG 8KT 3264 11579

Iniguez

Special Weather Statement
National Weather Service Phoenix AZ
343 PM MST Thu Jul 22 2021

AZZ538-539-559-222330-
Sonoran Desert Natl Monument AZ-Tonopah Desert AZ-Gila Bend AZ-
343 PM MST Thu Jul 22 2021

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 430 PM
MST...

At 342 PM MST, Doppler radar was tracking a strong thunderstorm over
Gila Bend, moving north at 20 mph.

Wind gusts up to 50 mph will be possible with this storm.

Locations impacted include...
Gila Bend, Gila Bend Auxiliary Field, Bosque and Cotton Center.

This includes the following highways...
AZ Interstate 8 between mile markers 109 and 131.
AZ Route 238 between mile markers 1 and 10.
AZ Route 85 between mile markers 1 and 11, and between mile markers
119 and 137.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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 3279 11242 3271 11276 3310 11292 3319 11261
TIME...MOT...LOC 2242Z 162DEG 19KT 3291 11268

Iniguez