Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
1035 PM MST Thu Aug 20 2020

.UPDATE...Updated aviation Discussion.

&&

.SYNOPSIS...
High pressure will remain dominant over the Four Corners region
through the next several days. But, an increase in humidity,
which will linger through the weekend, will reduce high
temperatures for a few days and support an uptick in storm
activity. Next week, temperatures are expected to increase. Storm
chances will continue for high terrain areas with a lingering
slight chance for some of the lower deserts of central Arizona.

&&

.DISCUSSION...
Thunderstorms are in progress over the typical higher terrain
areas of AZ early this afternoon though more isolated over
southeast AZ (so far). But, there area also storms over northern
Baja, and some of the southern California mountains stretching
from near Mt. San Gorgonio down to the Mexico border. The main
difference from 24 hours ago is the activity over the San Diego
and northern Baja mountains. This is due to additional moisture
moving in from the Gulf of California likely thanks to the
influence of Tropical Storm Genevieve currently centered west-
northwest of Cabo San Lucas with the eye just offshore of the far
southwestern coast of Baja. The Gulf moisture is also evident over
southern AZ with surface dew points running roughly 5-10 degrees
higher than 24 hours ago. Oftentimes, this leads to some boundary
layer cooling which can offset what would otherwise be an increase
in instability. However, the higher terrain often better able to
take advantage of the moisture. As for storm activity the rest of
the afternoon and tonight, the deterministic CAMs and HREF have
been handling current trends relatively well. They are generally
in good agreement on storms struggling to survive as they move
west and southwest off the higher terrain. But, isolated
redevelopment is still depicted. Mainly however, will be areas of
outflow from distant storms. For the Phoenix area, it appears
there may be a couple of main outflows: one from the north and
another from the east/southeast. Such specifics remain highly
uncertain but gusty outflow winds affecting at least some portion
of south-central Arizona are expected. With the wind comes the
potential for blowing dust and this is most likely over Pinal
County. Elsewhere in our forecast area, there is a slight chance
of storms over Joshua Tree N.P.

For Friday, HREF depicts a bit less convective activity over the
higher terrain and in turn less impact for the lower deserts. High
temperatures trend down as well due to the effects of more
humidity, cloud cover, and outflows cooling the lower levels. The
relatively cooler conditions (but still at least normal)
continue over the weekend as the moisture increase is sustained
due to the effects of Tropical Storm Genevieve. Of note,
Genevieve will not have any direct impact on our forecast area as
it tracks further away from land and weakens.

Moisture/PWAT slowly trends down next week and high pressure
strengthens. Thus, temperatures trend upward again with highs
climbing above 110 on the lower deserts beginning Monday. But,
higher terrain areas such as the Mogollon Rim are expected to
continue to have at least some storm activity and steering flow
will have an easterly component for a while. Thus there is
potential for storms to propagate to the lower elevations.

&&

.AVIATION...Updated at 0535Z.

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

Most of the convection has pushed off well to the southeast of the
terminals, and we may see very isolated showers (or storms) thru
about 08z before they dissipate for good. Look for lingering debris
cloudiness overnight with FEW-SCT decks 6-9k feet and SCT to BKN
decks aoa 12k feet. Clouds should tend to dissipate becoming mostly
FEW or SCT after 12z. Most likely convection somewhat diminished
tomorrow following tonight's working over of our atmosphere so no
mention will be made of convection in the TAFs Friday evening, and
will leave out early onset of outflows. Should storms develop more
than expected and outflows become an issue they can be addressed in
later TAF packages. Winds very squirrely and variable with outflows
moving around from all directions so high uncertainty with wind thru
about 09z, but after that winds should generally settle in from an
easterly component, becoming west again after 19z Friday. Should see
some gusts into the teens at least after 21z continuing into the
evening hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

Chances for convection look pretty slim over the western deserts
over the next 24 hours; we should mainly see some mid/high cloud
decks spreading into the area from the southwest this evening with
BKN decks aoa 15k feet. Clouds should become mostly FEW to SCT by
Friday morning. Winds will favor the south during the period at KBLH
with sustained speeds up to 15kt or so at times and maybe a few
gusts above 20kt during afternoon/evening hours. Winds to favor
the southeast at KIPL.

&&

.FIRE WEATHER...
Sunday through Thursday:
No major changes to the outlook...High pressure will persist
across the Desert Southwest this weekend and into next week and
will keep temperatures well above normal. The highest moisture
levels will be on Saturday with the remnants of Hurricane
Genevieve near N Baja and S CA. Afternoon min RHs will be near
20-30% with overnight recovery around 45-60%. Moisture then
decreases by early next week with min RH dipping back into the
teens around Monday with overnight recoveries in the 35-40% range.
The increased moisture this weekend may become a bit more
favorable for thunderstorm activity, even across the lower
deserts, although primary storm chances will stay across high
terrain areas to the east of Phoenix each day. Any storms that
develop have the potential to produce strong gusty, erratic,
winds, lightning and localized flash flooding.

&&

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

&&

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

CA...None.

&&

DISCUSSION...AJ
AVIATION...CB
FIRE WEATHER...Sawtelle/Benedict

Special Weather Statement
National Weather Service Phoenix AZ
906 PM MST Thu Aug 20 2020

AZZ537-538-540-210430-
Buckeye/Avondale AZ-Tonopah Desert AZ-Northwest Valley AZ-
906 PM MST Thu Aug 20 2020

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 930 PM
MST...

At 906 PM MST, Doppler radar was tracking a strong thunderstorm near
White Tank Mountain Park, or near Waddell, moving south at 10 mph.

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

Locations impacted include...
Glendale, Surprise, Goodyear, Buckeye, El Mirage, Sun City West,
Waddell, Litchfield Park, Surprise Stadium, White Tank Mountain Park,
Luke AFB and Beardsley.

This includes the following highways...
US Highway 60 between mile markers 137 and 144.
AZ Route 303 between mile markers 105 and 120.

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 3371 11257 3369 11233 3346 11234 3348 11267
TIME...MOT...LOC 0406Z 014DEG 8KT 3360 11248

INS

Special Weather Statement
National Weather Service Phoenix AZ
929 PM MST Thu Aug 20 2020

AZZ553-554-559-210515-
Sonoran Desert Natl Monument AZ-Northwest Pinal County AZ-
West Pinal County AZ-
929 PM MST Thu Aug 20 2020

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA AND PINAL COUNTIES UNTIL
1015 PM MST...

At 928 PM MST, Doppler radar was tracking a strong thunderstorm near
Estrella Sailport, or 25 miles east of Gila Bend, moving southwest at
10 mph.

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

Locations impacted include...
Estrella Sailport, Mobile and Ak-Chin Village.

This includes AZ Route 238 between mile markers 20 and 40.

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 3315 11226 3316 11212 3301 11211 3291 11223
      3291 11242 3306 11238
TIME...MOT...LOC 0428Z 029DEG 7KT 3303 11226

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
936 PM MST THU AUG 20 2020

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

0755 PM     TSTM WND DMG     4 E AHWATUKEE FOOTHILLS 33.31N 111.99W
08/20/2020                   MARICOPA           AZ   TRAINED SPOTTER

            MULTIPLE TREES REPORTED DOWN. THE LARGEST
            WAS 6+ INCH DIAMETER.

&&

DH

Special Weather Statement
National Weather Service Phoenix AZ
941 PM MST Thu Aug 20 2020

AZZ559-210515-
Sonoran Desert Natl Monument AZ-
941 PM MST Thu Aug 20 2020

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA AND PINAL COUNTIES UNTIL
1015 PM MST...

At 941 PM MST, Doppler radar was tracking a strong thunderstorm near
Kaka, or 35 miles southeast of Gila Bend, moving south at 25 mph.

Wind gusts of 50 to 55 mph will be possible with this storm.

Locations impacted include...
Kaka.

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 3269 11233 3268 11220 3251 11221 3250 11221
      3251 11243 3251 11244
TIME...MOT...LOC 0441Z 020DEG 20KT 3256 11228

INS

Special Weather Statement
National Weather Service Phoenix AZ
1007 PM MST Thu Aug 20 2020

AZZ559-210600-
Sonoran Desert Natl Monument AZ-
1007 PM MST Thu Aug 20 2020

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 1100 PM
MST...

At 1007 PM MST, Doppler radar was tracking a strong thunderstorm 12
miles west of Kaka, or 22 miles northeast of Ajo, moving south at 25
mph.

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

This storm will remain over mainly rural areas of Maricopa County.

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.

This storm 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 3272 11275 3268 11239 3251 11241 3251 11278
TIME...MOT...LOC 0507Z 017DEG 21KT 3255 11252

INS

Special Weather Statement
National Weather Service Phoenix AZ
1039 PM MST Thu Aug 20 2020

AZZ539-559-210615-
Sonoran Desert Natl Monument AZ-Gila Bend AZ-
1039 PM MST Thu Aug 20 2020

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 1115 PM
MST...

At 1039 PM MST, Doppler radar was tracking a strong thunderstorm 12
miles north of Ajo, moving west at 20 mph.

Wind gusts of 50 to 55 mph will be possible with this storm.

This storm will remain over mainly rural areas of Maricopa County.
This includes AZ Route 85 between mile markers 19 and 32.

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 3272 11308 3270 11275 3251 11278 3251 11310
      3252 11310
TIME...MOT...LOC 0539Z 087DEG 18KT 3257 11290

CK