Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
435 PM MST Tue Jul 20 2021

.UPDATE...Updated Aviation Discussion...

&&

.SYNOPSIS...
Abundant monsoon moisture will continue to bring daily chances
for at least isolated to scattered showers and thunderstorms
across much of Arizona with a tendency to mostly favor the higher
terrain of eastern Arizona, especially for the first half of the
week. Near normal temperatures are favored through midweek,
followed by below normal by late in the week. More scattered to
possibly widespread storms are expected for the second half of the
week and into the weekend as a more organized weather system is
expected to affect the region. Storms will be capable of producing
both locally heavy rain, strong winds, and blowing dust.

&&

.DISCUSSION...
Showers and thunderstorms scooted across the Phoenix metro and
southwest AZ/southeast CA this morning, dropping 0.10 to 0.25" of
rain over a few fortunate spots, in response to a passing
inverted trough and nearly 1500 j/kg of CAPE. Often, morning
convection will spoil storm chances for the afternoon and the
morning HREF showed a relatively quiet day for our forecast area.
However, latest radar trends indicate that may not be the case
with multiple thunderstorms now developing southwest, east, and
north of the Phoenix metro. Storms forming over the higher terrain
is not surprising but does cause some pause as to whether or not
we can get more storms into the lower desert today than previously
thought. The latest aircraft soundings show less inhibition with
a weaker midlevel cap and much more instability than guidance
suggested...but those soundings may not be adequately sampling the
storm environment either. Lower desert storm chances for this
afternoon are ~10-25%.

Overall, water vapor analysis shows a ridge of high pressure
centered over the Colorado Rockies with a fairly moist air mass
stretching from tropical Mexico through the Intermountain West.
The H5 ridge center will continue to weaken while settling close
to the Four Corners region over the next day or two, providing
stronger southeasterly flow through southern Arizona. This
position will keep the monsoon in gear with chances for
thunderstorms seemingly every day. PV anomalies associated with a
modest midlevel jet streak over western TX and New Mexico are
expected to move through the region later this week, potentially
increasing chances for thunderstorms perhaps as early as Wednesday
evening, but more likely Thursday and/or Friday.

Aside from the aforementioned 1500 j/kg CAPE, the 12z KPSR
sounding also showed a whopping 2.13" in precipitable water,
exceeding the daily maximum for date according to SPC Sounding
Climatology. Daytime mixing should lower PWs somewhat, but not by
all that much, as the latest aircraft soundings out of Sky Harbor
also measured 1.98" PW. These moisture levels are much more
typical of a late August tropical airmass rather than July 20th.
This ultimately means that any thunderstorms that do develop will
have the potential for producing heavy rain that could cause flash
flooding. To illustrate, a thunderstorm that developed rapidly
over the Bradshaw Mountains around noon dumped nearly 3.00" of
rain over one gauge within one hour, and exceeded rainfall rates
of 1.00"/hr over a few other rain gauges! That does not mean every
storm will dump that much rain but it does not take nearly that
much rain to cause flooding in our arid environment. The threat
for flash flooding is anticipated to increase daily through the
end of this week as storm chances and coverage are also forecast
to increase.

Temperatures through Wednesday are expected to be near seasonal
norms to slightly above normal across western deserts before
dropping several degrees as the storm chances increase and the
upper low moves into the area late week. Latest NBM has Wednesday
as the warmest day and Friday and Saturday as the coolest. NBM is
even supporting a 30% chance for sub-90 degree highs Friday and
Saturday in Phoenix. Temperatures should warm back up during early
next week as the pattern continues to shift with ensemble
guidance supporting a transition to increased south to southwest
flow and a gradual drying trend.

&&

.AVIATION...Updated at 2335Z.

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

As expected, storm activity today has been fairly limited with the
current activity well west of Phoenix. For the rest of the
afternoon/evening, expect little if any aviation impacts with
prevailing light westerly winds. Can't completely rule out a minor
outflow during the evening hours, potentially shifting winds, but
chances are pretty low (less than 20%). Tonight, expect a more
typical diurnal shift to easterly winds.

As far as Wednesday is concerned, expecting easterly winds to
persist into the early afternoon hours with a typical diurnal
switch to westerly during the mid/late afternoon hours. Prospects
for convective activity impacting the terminals look better for
Wed evening (30-40%) with an even better chance for easterly
outflows (60-70%). Current timing of a potential wind switch to
easterly Wednesday evening is somewhere in a 01-03Z time period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Generally quiet conditions will prevail with winds favoring the
south at both KIPL and KBLH through Wednesday afternoon. Can't
completely rule out a few isolated showers or storms Wednesday
afternoon, mainly across southwest Arizona, but it is unlikely to
impact the terminals.

&&

.FIRE WEATHER...
Friday through Tuesday:
No major changes to the forecast...Lower than normal afternoon
temperatures and ample monsoonal moisture will persist. Storm
activity is anticipated to spread as far west as southeast
California with most storms having good potential for wetting rain
(0.10" or more). The best potential for more widespread storm
activity will likely be Thursday through Friday and into Saturday
as an upper level disturbance moves through the region. Threats
from storms will continue to involve locally heavy rainfall and
flash flooding along with strong outflow winds and localized
downbursts within storms. Significant flooding may be experienced
on previously burned areas of Gila County, particularly during the
second half of the week. Min RH levels are expected to be in the
25-40% range across south-central Arizona (30-50% higher terrain
of south- central AZ) and 20-30% across southeast California and
southwest Arizona. Max RH levels, outside of storms, will be
45-70% most places (60-90% higher terrain of south-central AZ).
Apart from thunderstorms, winds will favor familiar diurnal
patterns.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

DISCUSSION...AD/Benedict
AVIATION...Kuhlman
FIRE WEATHER...Smith/Kuhlman

Special Weather Statement
National Weather Service Phoenix AZ
423 PM MST Tue Jul 20 2021

AZZ533-210015-
Central La Paz AZ-
423 PM MST Tue Jul 20 2021

...SIGNIFICANT WEATHER ADVISORY FOR EAST CENTRAL LA PAZ AND MARICOPA
COUNTIES UNTIL 515 PM MST...

At 423 PM MST, Doppler radar was tracking a strong thunderstorm 19
miles southeast of Salome, or 31 miles west of Tonopah, moving
northwest at 10 mph.

Half inch hail and wind gusts of 50 to 55 mph will be possible with
this storm.

Locations impacted include...
Salome, Vicksburg, Brenda, Vicksburg Junction, Wenden and Harcuvar.

This includes the following highways...
AZ Interstate 10 between mile markers 29 and 68.
US Highway 60 between mile markers 31 and 63.
AZ Route 72 between mile markers 40 and 49.

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.

&&

LAT...LON 3350 11333 3341 11352 3346 11366 3347 11394
      3367 11405 3387 11376 3389 11354
TIME...MOT...LOC 2323Z 149DEG 10KT 3353 11348

JS

Special Weather Statement
National Weather Service Phoenix AZ
430 PM MST Tue Jul 20 2021

AZZ534-538-210015-
Aguila Valley AZ-Tonopah Desert AZ-
430 PM MST Tue Jul 20 2021

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 515 PM
MST...

At 430 PM MST, Doppler radar was tracking a strong thunderstorm 9
miles south of Aguila, or 23 miles southwest of Wickenburg, moving
north at 20 mph.

Half inch hail and wind gusts of 50 to 55 mph will be possible with
this storm.

Locations impacted include...
Aguila.

This includes US Highway 60 between mile markers 78 and 90.

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.

&&

LAT...LON 3372 11302 3371 11318 3400 11332 3400 11308
TIME...MOT...LOC 2330Z 164DEG 17KT 3382 11312

JS

Special Weather Statement
National Weather Service Phoenix AZ
447 PM MST Tue Jul 20 2021

AZZ552-556-558-210030-
Tonto Basin AZ-Superior AZ-Pinal/Superstition Mountains AZ-
447 PM MST Tue Jul 20 2021

...SIGNIFICANT WEATHER ADVISORY FOR GILA AND PINAL COUNTIES UNTIL 530
PM MST...

At 447 PM MST, Doppler radar was tracking a strong thunderstorm over
Top-Of-The-World, or 8 miles northeast of Superior, moving west at 5
mph.

Pea size hail and wind gusts of 50 to 55 mph will be possible with
this storm.

Locations impacted include...
Superior and Top-Of-The-World.

This includes the following highways...
US Highway 60 between mile markers 226 and 240.
AZ Route 177 between mile markers 166 and 167.

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.

&&

LAT...LON 3344 11094 3330 11091 3325 11109 3335 11116
      3345 11113
TIME...MOT...LOC 2347Z 079DEG 6KT 3337 11100

JS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
512 PM MST TUE JUL 20 2021

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

0446 PM     TSTM WND GST     4 SW VICKSBURG          33.71N 113.81W
07/20/2021  M56 MPH          LA PAZ             AZ   MESONET

            MESONET STATION EW4845 SALOME.

&&

JT

Special Weather Statement
National Weather Service Phoenix AZ
515 PM MST Tue Jul 20 2021

AZZ533-210100-
Central La Paz AZ-
515 PM MST Tue Jul 20 2021

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL LA PAZ COUNTY UNTIL 600
PM MST...

At 515 PM MST, Doppler radar was tracking a strong thunderstorm near
Brenda, or 34 miles southeast of Parker, moving northwest at 10 mph.

Half inch hail and wind gusts of 50 to 55 mph will be possible with
this storm.

Locations impacted include...
Quartzsite, Salome, Vicksburg, Bouse, Wenden and Harcuvar.

This includes the following highways...
AZ Interstate 10 between mile markers 13 and 33.
US Highway 60 between mile markers 31 and 33, and between mile
markers 51 and 61.
AZ Route 72 between mile markers 24 and 44.

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.

&&

LAT...LON 3367 11397 3353 11400 3350 11412 3350 11435
      3374 11432 3397 11407 3398 11383 3391 11358
      3382 11354 3376 11358 3372 11365 3376 11378
      3374 11392
TIME...MOT...LOC 0015Z 137DEG 10KT 3367 11404

JS

Special Weather Statement
National Weather Service Phoenix AZ
521 PM MST Tue Jul 20 2021

AZZ547-556-558-210100-
Tonto Basin AZ-Rio Verde/Salt River AZ-
Pinal/Superstition Mountains AZ-
521 PM MST Tue Jul 20 2021

...SIGNIFICANT WEATHER ADVISORY FOR GILA...MARICOPA AND PINAL
COUNTIES UNTIL 600 PM MST...

At 521 PM MST, Doppler radar was tracking a strong thunderstorm 10
miles southeast of Apache Lake, or 13 miles north of Superior, moving
northwest at 10 mph.

Pea size hail and wind gusts of 50 to 55 mph will be possible with
this storm.

Locations impacted include...
Apache Lake.

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.

&&

LAT...LON 3353 11100 3341 11103 3342 11134 3369 11120
TIME...MOT...LOC 0021Z 113DEG 10KT 3348 11111

JS