Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
150 PM MST Thu Sep 9 2021

&&

.SYNOPSIS...
Strong high pressure aloft will persist across the Southwestern U.S.
through the weekend providing above normal temperatures for much
of the region. Dry conditions will also mostly prevail, however
there will be a slight chance for afternoon isolated thunderstorms
across southeast California through Friday. After high
temperatures up to around 110 degrees for a good portion of the
lower deserts today, high temperatures through early next week
will mostly range from 105 to 109 degrees across the lower
deserts.

&&

.DISCUSSION...
Late season heat continues to be the main concern over the next
several days as the unseasonably strong upper level ridge
continues to dominate the Southwestern U.S. The center of the
ridge is now positioned more over the Four Corners area with 500mb
heights topping out around 596/597dm, at or near record climo for
the date. 500mb heights across our area have been fairly stable
the last couple days ranging from 592-594dm, or closer to 95% of
climo.

Enhanced boundary layer moisture resulting in surface dew
points mostly in the 60-65 degree range has kept recent daytime
highs from nearing daily records, but things are definitely
different today. Drier air mixing down to the sfc has resulting in
more efficient surface heating today. The high at Sky Harbor has
already reached 110 degrees, which ties the record for the date. The
latest NBM shows highs today ranging from 107-110 degrees across the
south-central Arizona deserts to 108-112 degrees across much of
southwest Arizona and southeast California. These very warm
temperatures are resulting in area of High HeatRisk stretching from
the Phoenix area westward through La Paz and eastern Riverside
Counties. An Excessive Heat Warning remains in effect for much of
the aforementioned areas for today.

Lingering moisture across southeast California today along with
modest CAPE and lowering CIN are allowing the development of a few
thunderstorms across parts of the region, especially over/near
Joshua Tree NP at this hour. Latest high-res CAMS model output
indicates that the activity will likely be short-lived,
dissipating/moving n-ward out of the region by early this evening as
a PV anomaly rotating northward around the main upper-level high
center moves out of the region. Chances across southeast California
may linger into Friday, but decreasing moisture and lower
instability should limit coverage even further than today.

The eastward progression of the upper level ridge should push the
high center into New Mexico starting tonight while also lowering
heights slightly over southern California and much of Arizona.
With surface dew points expected to remain stable through
Saturday, the lowering heights aloft will give some slight cooling
dropping highs back to between 105-108 degrees across the lower
deserts for Friday and Saturday. Starting Sunday, the upper level
ridge is expected to begin to flatten out, but also expand
marginally higher heights back to the west over our area.
Additionally on Sunday, more dry air is expected to move into the
area pushing afternoon surface dew points down well into the 40s.
As a result, forecast highs on Sunday kick back upward to between
108-111 degrees for much of the lower deserts. Very similar
conditions are likely on Monday, but a likely incoming upper level
trough through the Northwestern U.S. may further flatten the
ridge and begin depressing heights over our region. Forecast highs
on Monday are at or maybe a degree cooler than Sunday, so it now
seems very likely that a good portion of the lower deserts will need
to be covered by additional Excessive Heat products for Sunday and
Monday. Upper level troughing should eventually win out across much
of the Western U.S. by the middle of next week providing a gradual
cooling trend for much of next week. Dry conditions should continue
within this pattern with a dry west southwesterly flow very likely
persisting over our region through the majority of next week.

&&

.AVIATION...Updated at 1710Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Skies will remain mostly clear through the TAF period with just a
few mid and high level clouds developing. Winds near the surface
will likely stay east or southeast throughout the entire day.
There is still a small chance of gulf surge late tonight, but
confidence in this is low so it is not included in the TAF for
now. In general, wind speeds will likely remain below 10 kts. Any
thunderstorms will be well removed from the TAF sites and will not
result in impacts.

Southeast California/Southwest Arizona including KIPL and KBLH:
Thunderstorm activity will be most prevalent over the Southern
California mountains and Baja spine during the early afternoon and
evening hours. Chances for storms drifting into the lower
elevations between Joshua Tree NP/Blythe/El Centro are low, at or
below 15%, so chances and confidence for any storm to occur over
either terminal is way too low to mention in the TAFs. Chances for
outflows from distant storms is also low at 25% or less.
Otherwise, southeasterly winds will predominate most of the time
at KIPL at speeds of 7-11 kts with periods of 20-25 kt gusts
possible during late evening gulf surges. Winds may turn to the
west for a few hours this evening before southeast flow resumes.
At KBLH, winds will generally vary between the SSE and SSW at 8-12
kts with occasional gusts upwards of 20-25 kts. A few hours of
light and variable wind are expected Friday morning.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Strong high pressure will dominate over the Southwestern U.S.
through the weekend and likely into early next week providing dry
conditions and above normal temperatures. High temperatures are
expected to run several degrees above normal through much of the
period with some lower desert areas likely approaching 110 degrees
on Sunday and Monday. Min RH levels will range from 15-25% on
Saturday with readings dropping mostly between 10-15% starting
Sunday, while max RHs generally range from 30-50%. Winds will
overall remain light through the period while following typical
diurnal patterns.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ530-
     533-537-538-540>544-546-548>551.

CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ561-
     569-570.

&&

DISCUSSION...Percha/Kuhlman
AVIATION...Hodges/AD
FIRE WEATHER...Kuhlman

Special Weather Statement
National Weather Service Phoenix AZ
121 PM PDT Thu Sep 9 2021

CAZ560-561-564-568-092045-
Chuckwalla Mountains CA-Chiriaco Summit CA-Joshua Tree NP West CA-
Joshua Tree NP East CA-
121 PM PDT Thu Sep 9 2021

...A strong thunderstorm will impact portions of Riverside County
through 145 PM PDT...

At 121 PM PDT, Doppler radar was tracking a strong thunderstorm near
Chiriaco Summit, moving northwest at 20 mph.

HAZARD...Wind gusts up to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Chiriaco Summit and Cottonwood Visitor Center.

This includes CA Interstate 10 between mile markers 85 and 100.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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 11547 3369 11548 3369 11545 3368 11544
      3359 11562 3373 11587 3387 11572 3387 11558
TIME...MOT...LOC 2021Z 137DEG 16KT 3371 11561

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

INS

Special Weather Statement
National Weather Service Phoenix AZ
158 PM PDT Thu Sep 9 2021

CAZ560-561-092130-
Joshua Tree NP West CA-Joshua Tree NP East CA-
158 PM PDT Thu Sep 9 2021

...A strong thunderstorm will impact portions of San Bernardino and
Riverside Counties through 230 PM PDT...

At 157 PM PDT, Doppler radar was tracking a strong thunderstorm near
Cottonwood Visitor Center, or 11 miles northwest of Chiriaco Summit,
moving northwest at 20 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.

This storm will remain over mainly rural areas of San Bernardino and
Riverside Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

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.

Persons in campgrounds should consider seeking sturdy shelter until
this storm passes.

&&

LAT...LON 3403 11600 3404 11583 3387 11571 3373 11586
      3389 11609 3404 11609 3405 11601
TIME...MOT...LOC 2057Z 138DEG 16KT 3382 11580

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

INS

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