Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
458 PM MST Tue Oct 5 2021

.UPDATE...
Updated Aviation discussion.

&&

.SYNOPSIS...
A low pressure system will continue to move through the Southwest
today. Periods of showers and thunderstorms will continue this
morning, then gradually end from west to east this afternoon and
evening. Severe thunderstorms are expected to develop this
afternoon, capable of producing strong, damaging winds, large
hail, and perhaps an isolated tornado. Some blowing dust will
also be possible. Dry weather returns tomorrow and Thursday with
near normal temperatures. A low end rain chance is possible with
another passing system Friday and Saturday with slightly cooler
temperatures. A quiet weekend with typical temperatures is expect,
with the potential for a dry but notably cooler and winder
weather system next week.

&&

DISCUSSION...
A compact upper level low is moving east near the
California-Mexico border. East of this low, warm and moist air
continues to stream northward into the forecast area, leading to
scattered showers and thunderstorms. So far, the relatively dry
and cool boundary layer has only allowed for elevated convection.
Rainfall and wind have both been limited, but we have received
several reports of small hail. As we progress into the afternoon,
several factors will come together for a potentially significant
severe weather event by Arizona standards.

Visible satellite imagery continues to show mostly clear
conditions south of Phoenix, with a mixture of clouds and sun over
the metro area. Overall, this is allowing for good low level
heating. The 18z PSR sounding and ACARS data confirm this to be
the case, with a gradually warming boundary layer. Model
forecasts show that this should allow for surface temperatures to
warm enough to overcome any convective inhibition by this
afternoon. As convection becomes increasingly tied to the surface
layer, it will dip into the strong low level directional shear
present today. Meanwhile, as the upper level low advances towards
the area (along with a corresponding jet streak) upper level wind
speeds will increase along with diffluence aloft. This will result
in effective bulk shear values of 40-50 kts continuing into the
early evening. In addition to the favorable wind shear, upper
level temperatures will cool significantly with 500 mb
temperatures dropping to -15 to -16C or so. Very steep lapse rates
into the mid levels will allow for a favorable hail growth zone
to develop. CAPE values of 1500-2000 J are anticipated by the mid
afternoon. Lastly, PWAT values will remain in the 1-1.2" range,
which is easily sufficient for convective development.

Taken together, the environment will be well primed for supercells
to develop this afternoon with large hail (potentially up to 2"
or so) and high wind gusts (potentially up to 65 kts or so). The
strong low level shear may also allow for an isolated tornado or
two. the most favored location for tornadic development is likely
north of the Phoenix metro. The SPC has issued a severe
thunderstorm watch through 03Z this evening to account for these
threats.

The risk for flash flooding appears to be much less than the risk
for severe weather due to fast storm motions. Even so, as stronger
thunderstorms develop this afternoon, the potential for locally
heavy rain will increase. Accordingly, an isolated flash flood
warning cannot be ruled out today, especially over the burn scars.

The HREF has handled the strongly forced convective pattern quite
well to this point. It indicates that scattered showers and
storms will continue throughout the day, but that the strong to
severe storms are likely to develop roughly along a line from west
of Phoenix up through the Prescott area around 1-4 pm this
afternoon. These storms will then likely traverse through the area
between 3-7 pm. The threat of precipitation in the PSR forecast
area will quickly wind down after that. As always, the exact
locations which will see severe weather is never known for sure
ahead of time and conditions can change quickly. Today would be a
good day to check back frequently for forecast updates.

After today's active weather, the next few days will be quiet with
highs near 90 under partly sunny skies. Global ensembles indicate
that another system will graze the area Friday night into
Saturday. The chance of precipitation is quite low, with NBM
showing less than a 10% chance of measurable precipitation.
Temperatures will likely cool off though, with highs in the mid
80s over the weekend. An even stronger storm system (also likely
dry) will affect the area next week. Details are limited at this
range, but there is excellent agreement that temperatures will
cool dramatically. The NBM only shows a 55% chance of exceeding
72F next Wednesday in Phoenix. Should this occur, morning lows
would fall into the 40s and 50s for the first time since last
spring.

&&

.AVIATION...Updated at 2357Z.

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

Another round of thunderstorms is expected to affect the metro
area roughly between 00Z-03Z. Anticipate that western portions of
the Valley will see the brunt of storms. But, it is expected to
cause a major wind shift with a westerly component at KPHX
between 01Z- 02Z and be close enough for VCTS but otherwise not
lower ceilings below 6kft AGL. But, there is definitely a
possibility that KPHX could have TSRA+ at the airfield which would
cause even more erratic winds and a major drop in ceilings and
visibility along with hail potential. Anticipate the Valley will
mostly be clear of thunderstorms by 04Z if not 03Z. Apart from
thunderstorms, anticipate northeasterly surface winds will
transition to northerly this evening then transition to familiar
nocturnal patterns near midnight. Very benign conditions are
anticipated during the overnight hours and daytime Wednesday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Shower and storm activity has ended over southeast California and
southwest AZ. There are some lingering batches of cumulus/altocumulus
which should thin out after sunset. Surface winds at KIPL will
favor familiar diurnal patterns at KIPL. At KBLH, anticipate
northeasterly winds will transition to northerly late tonight with
a period of light and variable in the morning before light south
and southeast develops in the afternoon. Otherwise, no significant
cloudiness.

&&

.FIRE WEATHER...
Thursday through Monday:
One weather system will move through the region Thursday and
Friday, bringing low end rain chances and a slight uptick in
afternoon humidity. Slightly cooler and drier weather arrives
Saturday, but warmer and drier weather is expected Sunday and
Monday. Overnight humidity values will generally be trending down
through the entire period.

&&

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

&&

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

DISCUSSION...Hodges
AVIATION...AJ
FIRE WEATHER...Iniguez

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
450 PM MST TUE OCT 5 2021

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

0200 PM     FLASH FLOOD      6 NE APACHE JUNCTION    33.47N 111.48W
10/05/2021                   MARICOPA           AZ   PUBLIC

            ***DELAYED REPORT*** SOCIAL MEDIA VIDEO
            SHOWED SWIFTLY MOVING WATER FLOWING ACROSS
            SR 88 GOING TOWARDS CANYON LAKE. TIMING AND
            LOCATION WAS APPROXIMATED.

&&

INS

Special Weather Statement
National Weather Service Phoenix AZ
454 PM MST Tue Oct 5 2021

AZZ538-060030-
Tonopah Desert AZ-
454 PM MST Tue Oct 5 2021

...A strong thunderstorm will impact portions of Maricopa County
through 530 PM MST...

At 454 PM MST, Doppler radar was tracking a strong thunderstorm 7
miles southwest of Tonopah, moving north at 25 mph.

HAZARD...Winds in excess of 40 mph and half inch hail.

SOURCE...Radar indicated.

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

Locations impacted include...
Tonopah and Wintersburg.

This includes AZ Interstate 10 between mile markers 83 and 99.

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.

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.

A Severe Thunderstorm Watch remains in effect until 800 PM MST for
south central Arizona.

&&

LAT...LON 3333 11291 3337 11314 3371 11314 3362 11277
TIME...MOT...LOC 2354Z 185DEG 22KT 3342 11302

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH

SDB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
512 PM MST TUE OCT 5 2021

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

0304 PM     FLASH FLOOD      6 SE PUNKIN CENTER      33.78N 111.26W
10/05/2021                   GILA               AZ   DEPT OF HIGHWAYS

            ***DELAYED REPORT*** ADOT REPORTED THAT SR
            188 WAS CLOSED BETWEEN MP 251-255 DUE TO A
            ROCKSLIDE.

&&

INS

Special Weather Statement
National Weather Service Phoenix AZ
546 PM MST Tue Oct 5 2021

AZZ558-560-562-563-060130-
Southeast Gila County AZ-San Carlos AZ-Globe/Miami AZ-
Pinal/Superstition Mountains AZ-
546 PM MST Tue Oct 5 2021

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

At 546 PM MST, Doppler radar was tracking a strong thunderstorm 12
miles northeast of Globe, moving north at 25 mph.

HAZARD...Wind gusts of 50 to 55 mph and nickel 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.

Locations impacted include...
Globe, Cutter and Hilltop.

This includes the following highways...
US Highway 60 between mile markers 251 and 297.
AZ Route 70 between mile markers 253 and 261.
AZ Route 77 between mile markers 168 and 170.

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.

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.

A Severe Thunderstorm Watch remains in effect until 800 PM MST for
east central Arizona.

&&

LAT...LON 3354 11042 3331 11064 3334 11079 3366 11078
      3379 11068 3385 11056 3382 11050 3383 11039
TIME...MOT...LOC 0046Z 184DEG 23KT 3354 11066

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...55 MPH

SDB

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