Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
237 PM MST Wed Jun 30 2021

.SYNOPSIS...
Scattered showers and thunderstorms will be common over the
higher terrain and more isolated on the lower elevations the rest
of the week. Strong windS, localized blowing dust, and localized
heavy rain/flooding will be the main impacts. A slow decline in
storm activity begins by early next week. Temperatures will be
near, to slightly below, normal the rest of this week before
increasing a bit early next week.

&&

.DISCUSSION...
Strong convection developed early today, before 18Z, over the
northern AZ high terrain, with the strongest activity in the
Prescott area. An MCV also developed in southern San Bernadino
county CA and moved east through La Paz county this morning with
some embedded strong storms and moderate rain. The MCV is still
pushing east into portions of Maricopa county early this
afternoon, with an outflow pushing out ahead of it, but the
convection with it has mostly dissipated. Going forward through
the rest of the afternoon, the atmospheric flow pattern, dynamics,
and outflows may play a significant role in how convection
evolves.

Water vapor satellite shows a prominent upper level circulation
over the Great Basin and weak anticyclonic circulation over
northern Baja California. An enhanced gradient between the two
features may have aided the MCV this morning. A general divergent
flow pattern is seen over the central and eastern AZ with very
week steering flow. 850-300mb mean flow from RAP mesoanalysis is
around 5-10 kts out of the west to southwest. With the week
steering flow, storm motion will be slow and likely driven mostly
by outflows. With slow storm motions and non-existent shear,
ordinary storm modes are expected. With SBCAPE values upwards of
1000-1500 J/kg and PWATS around 1.2-1.5" analyzed over central AZ,
intense saturated updrafts are expected. As has already been seen
along the Rim and in Yavapai county. Strong storms may be brief,
suffering from waterloading, but still could produce significant
impacts. DCAPE is high, around 1000-1300 J/kg, over south-central,
so strong outflows will be a threat through the afternoon. 12Z HREF
probability of >35 mph is around 50-60% with max wind gusts around
45-55 mph, which seems a little low, but could be attributed more
to the moist environment. With the elevated moisture, slow storm
motions, and high instability, localized flooding impacts may be
the biggest concern this afternoon, especially with any storms
that become anchored to mountain peaks and ridgelines and/or
develop over burn scars. Rain rates may be a little tricky at
lower elevations, with HREF showing upwards of 0.5-1.0"/hr. A
gage near Crown King in Yavapai county measured ~1.0" in 10
minutes. So there is evidence of the potential for higher
rain rates.

A special 19Z sounding was launched out of Phoenix and shows a
virtually uncapped surface environment, with ~1000 J/kg SBCAPE,
~1.4" PWATS, and 10.6 g/kg mixing ratio. Forcing is the only thing
missing at this point. Storms so far have been limited to higher
terrain, but given the sounding, a strong outflow may be enough to
fire off storms over the lower deserts. Activity, in general is
not expected to last long past sunset this evening, dissipating
with the loss of heating.

.PREVIOUS DISCUSSION...
For Thursday, some sort of inverted trough may finally move deeper
into AZ - possibly aided by the sheared out mid/upper level
remnants of Enrique (weakens to a Depression today over southern
Gulf of California) and the westward advance of high pressure
centered to our east. The track, or even presence, of any such
perturbations remains quite uncertain. But, there may also be a
little bit more moisture to work with (associated with Enrique's
influence). Ensemble data suggests a little bit more moistening
yet on Friday. As the moisture availability increases, the
potential for storms to produce heavy rain also increases. That
being said, the best precipitable water values look to be over
southwest AZ as opposed to south-central AZ. The threat would tend
to be pretty localized as well. Look for temperatures to be near
average.

A slow downtrend in moisture (PWAT) begins as early as Saturday or
as late as Monday. Meanwhile, the upper high centered over Texas
is advertised to become centered over Arizona over the weekend.
With weak steering flow and decreased instability, that would
tend to hinder storm development on the lower elevations. But, if
enough moisture lingers and steering flow isn't too weak, there
could be a period of relatively better chances for the the
south-central AZ deserts. But, the high isn't anticipated to be
super strong so we're not anticipating an excessive heat event
through Tuesday.

&&

.AVIATION...Updated at 2040Z.

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

Thunderstorm activity across southern Yavapai County will likely
help to initiate new convection across Gila County this afternoon.
These storms are then expected to produce an outflow boundary
that will propagate into the lower deserts, including the Phoenix
area. There is a 50% chance of the outflow boundary reaching KPHX
with easterly winds up to 35 kt. Conditions will also remain
favorable for additional isolated storms to develop along the
boundary and near subsequent boundary interactions in the Valley.
There is also a slight (10%) chance of a dust storm this evening.
Otherwise, gusty winds and heavy rain will be the primary hazards.

Southeast California/Southwest Arizona including KIPL and KBLH:

Overall little aviation impacts expected through the period for
KIPL and KBLH. Thunderstorm activity is generally expected to
remain well east across Arizona. Otherwise, southerly winds will
predominate at both KIPL and KBLH.

&&

.FIRE WEATHER...
Saturday through Wednesday:
High pressure is expected to become centered over Arizona over the
weekend. A slow decline in atmospheric moisture is also
anticipated to begin by early next week. Thus, the best potential
for storms with locally heavy rain looks to be this weekend.
Otherwise, a familiar Monsoon season pattern will be in place with
storms primarily focused over the higher terrain producing
outflows that spread to the lower deserts and in turn generate
isolated new storms. Temperatures will start off this period at,
or slightly below, normal followed by a a little bit of warming.
Min RH values will generally be in the 20-30% range through Sunday
before decreasing to 15-25% on Tuesday. Max RH values will follow
a similar trend but with decreases starting sooner. For instance,
Max RH values will be 40-60% most places (less eastern Riverside
County) through Monday morning before decreasing to 30-50% most
places by Tuesday morning. Apart from thunderstorms, wind patterns
will follow familiar diurnal patterns.

&&

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

&&

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

DISCUSSION...Benedict
PREVIOUS DISCUSSION...AJ
AVIATION...Hirsch
FIRE WEATHER...AJ/Benedict

Special Weather Statement
National Weather Service Phoenix AZ
1035 AM MST Wed Jun 30 2021

AZZ533-301830-
Central La Paz AZ-
1035 AM MST Wed Jun 30 2021

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN LA PAZ COUNTY UNTIL
1130 AM MST...

At 1035 AM MST, Doppler radar was tracking a strong thunderstorm 10
miles northwest of Harcuvar, or 26 miles south of Alamo Lake, moving
east at 30 mph.

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

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

This includes the following highways...
AZ Interstate 10 between mile markers 47 and 51.
US Highway 60 between mile markers 40 and 72.
AZ Route 72 between mile markers 30 and 49.

LAT...LON 3378 11403 3418 11387 3425 11337 3367 11336
      3361 11371
TIME...MOT...LOC 1735Z 285DEG 24KT 3391 11374

Hirsch

Special Weather Statement
National Weather Service Phoenix AZ
206 PM MST Wed Jun 30 2021

AZZ552-558-302130-
Superior AZ-Pinal/Superstition Mountains AZ-
206 PM MST Wed Jun 30 2021

...SIGNIFICANT WEATHER ADVISORY FOR GILA AND PINAL COUNTIES UNTIL 230
PM MST...

At 205 PM MST, Doppler radar was tracking a strong thunderstorm near
Top-Of-The-World, or near Superior. This storm was nearly stationary.

Winds in excess of 40 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 224 and 239.
AZ Route 177 between mile markers 165 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.

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 3320 11091 3321 11093 3324 11095 3319 11103
      3330 11117 3340 11099 3329 11085
TIME...MOT...LOC 2105Z 042DEG 4KT 3330 11100

LJH

Special Weather Statement
National Weather Service Phoenix AZ
213 PM MST Wed Jun 30 2021

AZZ552-558-302145-
Superior AZ-Pinal/Superstition Mountains AZ-
213 PM MST Wed Jun 30 2021

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

At 212 PM MST, Doppler radar was tracking a strong thunderstorm 10
miles south of Apache Lake, or 11 miles northwest of Superior. This
storm was nearly stationary.

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

This storm will remain over mainly rural areas of Maricopa and Pinal
Counties.

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 3335 11124 3346 11130 3357 11111 3340 11103
TIME...MOT...LOC 2112Z 239DEG 4KT 3344 11121

LJH

Special Weather Statement
National Weather Service Phoenix AZ
419 PM MST Wed Jun 30 2021

AZZ547-549-552-555>558-562-010000-
Fountain Hills/East Mesa AZ-Mazatzal Mountains AZ-Tonto Basin AZ-
Apache Junction/Gold Canyon AZ-Globe/Miami AZ-Superior AZ-
Rio Verde/Salt River AZ-Pinal/Superstition Mountains AZ-
419 PM MST Wed Jun 30 2021

...SIGNIFICANT WEATHER ADVISORY FOR GILA...MARICOPA AND PINAL
COUNTIES UNTIL 500 PM MST...

At 418 PM MST, Doppler radar was tracking strong thunderstorms along
a line extending from near Tonto National Monument to 7 miles north
of Queen Valley. Movement was southeast at 15 mph.

Winds in excess of 40 mph will be possible with these storms.

Locations impacted include...
Apache Junction, Superior, Gold Canyon, Roosevelt, Gold Camp,
Roosevelt Dam, Lost Dutchman State Park, Goldfield, Top-Of-The-World,
Florence Junction, Queen Valley, Canyon Lake, Superstition Mountains,
Tonto National Monument, Kings Ranch, Apache Lake, Boyce Thompson
Arboretum, Inspiration, Roosevelt Estates and Tortilla Flat.

This includes the following highways...
US Highway 60 between mile markers 198 and 241.
AZ Route 79 between mile markers 143 and 150.
AZ Route 177 between mile markers 162 and 167.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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.

If on or near Lake Roosevelt, get out of the water and move indoors
or inside a vehicle. Remember, lightning can strike out to 10 miles
from the parent thunderstorm. If you can hear thunder, you are close
enough to be struck by lightning. Move to safe shelter now! Do not
be caught on the water in a thunderstorm.

&&

LAT...LON 3347 11083 3323 11102 3308 11130 3330 11151
      3343 11153 3354 11149 3374 11109 3366 11094
TIME...MOT...LOC 2318Z 324DEG 14KT 3361 11104 3341 11130

Iniguez