Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
600 PM MST Wed Aug 11 2021

.UPDATE...Updated Aviation discussion.

&&

.SYNOPSIS...
Upper level disturbances will produce favorable conditions for
showers and isolated thunderstorms across the region most days.
Thunderstorms are favored to move through areas west of Phoenix
later today. Primary threats with any storms remains strong winds
blowing dust, and flash flooding the primary threats. Rain
chances decrease towards the end of the workweek, although
do not completely diminish, and then increase again for this
weekend. Persistent humid conditions and periods of cloudy skies
will help keep afternoon temperatures near or slightly below
normal most days.

&&

.DISCUSSION...
A weak upper low continues to meander around central AZ and helped
drive another round of morning convection in the Phoenix area. The
circulation has since lifted just northeast of the CWA this
afternoon, but there are lingering impacts from the rain that
fell. Particularly, significant runoff has collected into streams
like Sycamore creek, with flow into the Verde river, and Tonto
creek. A Flood Warning is in effect for these areas for the next
few hours.

Scattered convection has flared up in a few areas this afternoon,
including southeast AZ, the high terrain north of Phoenix, and a
cluster of storms in southern Yuma county. There is also isolated
convection in western Maricopa county. The strongest storms have
been in southern Yuma county where where MLCAPE is analyzed around
2000-2500 J/kg and PWATs are around 2 inches. Shear is pretty much
non-existent in the this area and across much of the region,
resulting in slow-moving ordinary pulse storms. Storms in Yuma
county have been aided by colliding outflows. Strong winds,
blowing dust, and torrential rain have been observed and will
remain the primary concern through the rest of the afternoon.

The central and west Valley as well as portions of western
Maricopa have missed out on storm activity the past few days and
it is no surprise that's is where some of the isolated convection
has been flaring up. It is evident in the CU field that these
areas are less stable than area that saw the rain this morning.
The environment still seems conditional, but a strong enough
outflow, outflow collisions, and/or cell mergers may be sufficient
to fully realize the available CAPE and moisture in these areas.

With the high moisture content, any storm will be capable of
1-2"/hr rain rates, and locally higher for brief periods, which
could lead to localized flash flooding. Strong gusty winds may
also generate blowing dust, in addition to lightning. The overall
severe threat is fairly low, but with a cell merger or outflow
collision, it cannot be completely ruled out. CAMs are indecisive
on whether additional convection will be possible overnight and/or
Thursday morning, but with the upper low still in close proximity
and moisture levels still elevated, it cannot be ruled out,
especially in areas that miss out on convection this
afternoon/evening.

&&

.PREVIOUS DISCUSSION...
As the influence of the aforementioned low wanes Thursday,
afternoon storms are favored to develop over the more traditional
areas, such as the Mogollon Rim, with the possibility for activity
to move off the higher terrain. Confidence for a completely dry
day or two in the CWA is low, but Friday seems poised to be the
quietest period of the forecast, as a H5 ridge strengthens over
the Southwest late this week. A moist atmosphere and embedded
upper level waves and disturbances within the synoptic circulation
will keep storm chances in the forecast with PoPs somewhere in
the 15-50% most days through at least Tuesday. Temperatures are
forecast to remain 2-5 degrees below seasonal normal across south-
central AZ while warming a bit more into the 105-110 degree range
along the Colorado River and southeastern California.

&&

.AVIATION...Updated at 0054Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
A low pressure system will continue to meander across central
Arizona through Thursday, promoting favorable conditions for
additional showers and thunderstorm activity this evening.
Numerous outflow boundaries are evident on the TDWR and will
continue to interact this evening, resulting in scattered
convection across the Valley. After 03z, uncertainty increases
considerably. Any storm that develops will be capable of producing
occasional lightning, brief, heavy rain, winds up to 45 kt and
ceilings as low as 6k ft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Isolated thunderstorms will remain a possibility overnight,
particularly at KBLH. Any storms that develop will be capable of
producing occasional lightning, brief, heavy rain, and winds up to
45 kt. Probabilities are generally around 20 percent tonight
lowering to 10 percent Thursday.

&&

.FIRE WEATHER...
Friday through Tuesday:
Shower and storm activity may be a little more quiet on Friday but
conditions will remain favorable for activity most days. Any
storms or showers that develop will likely produce heavy rain that
could lead to flash flooding. RH values also remain elevated with
periodic influxes of moisture and below normal temperatures in
most places. Min RH will fall into the 25-35% range most days
while overnight recoveries 50+ percent for most areas. Winds will
favor typical diurnal/terrain influences most days.

&&

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

&&

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

DISCUSSION...Benedict
PREVIOUS DISCUSSION...12
AVIATION...Hirsch
FIRE WEATHER...12

Special Weather Statement
National Weather Service Phoenix AZ
514 PM MST Wed Aug 11 2021

AZZ538>540-554-559-120100-
Sonoran Desert Natl Monument AZ-Buckeye/Avondale AZ-
Tonopah Desert AZ-Gila Bend AZ-West Pinal County AZ-
514 PM MST Wed Aug 11 2021

...Strong thunderstorms will impact portions of Maricopa and Pinal
Counties through 600 PM MST...

At 512 PM MST, Doppler radar was tracking multiple strong
thunderstorms over southwest Maricopa County.

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

SOURCE...Radar indicated.

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

Locations impacted include...
South Goodyear, Buckeye, Gila Bend, Mobile and Rainbow Valley.

This includes the following highways...
AZ Interstate 8 between mile markers 112 and 150.
AZ Route 238 between mile markers 1 and 34.
AZ Route 85 between mile markers 1 and 3, and between mile markers
119 and 146.

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.

&&

LAT...LON 3301 11291 3336 11302 3326 11224 3283 11216
      3278 11257
TIME...MOT...LOC 0012Z 255DEG 12KT 3303 11250

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

AJ

Special Weather Statement
National Weather Service Phoenix AZ
549 PM MST Wed Aug 11 2021

AZZ541-543-544-546-548-549-120130-
Fountain Hills/East Mesa AZ-Scottsdale/Paradise Valley AZ-
Cave Creek/New River AZ-East Valley AZ-Central Phoenix AZ-
North Phoenix/Glendale AZ-
549 PM MST Wed Aug 11 2021

...A strong thunderstorm will impact portions of Maricopa County
through 630 PM MST...

At 547 PM MST, Doppler radar was tracking a strong thunderstorm near
Salt River Indian Community, or near Paradise Valley, moving east at
10 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. Dangerous driving conditions.

Locations impacted include...
Scottdale, East Phoenix, north Mesa, Tempe, Fountain Hills, Paradise
Valley and Salt River Indian Community.

This includes the following highways...
AZ Route 87 between mile markers 175 and 188.
AZ Route 202 between mile markers 3 and 5, and between mile markers
11 and 22.
AZ Route 101 between mile markers 38 and 51.

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.

&&

LAT...LON 3344 11200 3361 11200 3366 11173 3343 11169
TIME...MOT...LOC 0047Z 265DEG 10KT 3353 11186

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

AJ

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