216
FXUS65 KPSR 120609
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1109 PM MST Thu Jul 11 2024

.UPDATE...Updated 06z Aviation Discussion.

&&

.SYNOPSIS...
A strong ridge of high pressure located over the Great Basin will
continue to result in excessive heat through Friday. This ridge will
finally migrate over the 4-Corners region this weekend, resulting in
a cooling trend and a noticeable increase in moisture levels.
Isolated showers and storms will be possible across southcentral
Arizona beginning today and persisting through early next week. The
best chances for storms reaching the lower deserts will be Saturday
and Sunday. Drier conditions are expected to arrive by the middle of
next week.

&&

.DISCUSSION...
The current upper level pattern remains unchanged from what has been
in place over the past several days. However, we will finally begin
to see a pattern shift this weekend which will lead to cooler
temperatures and an increase in monsoonal thunderstorm activity
across Arizona.

500 mb hghts this afternoon will range from 595-597 dam resulting
in temperatures reaching 111F-119F across the lower deserts.
Highs in the Phoenix Metro are expected to top out around
112F-116F. The forecast high of 116F is expected to fall just
short of the record 118F set in 1958. A mid-lvl shortwave trough
is expected to dive southward out of NM into SE AZ this afternoon
which will foster enough instability and lift for a cluster of
thunderstorms over the White Mountains and the Rim Country.
Northeasterly steering flow will bring storms off of the higher
terrain towards the lower deserts this evening. However, there is
still a lot of MLCIN to overcome and it is unlikely that storms
will survive into the deserts of southcentral AZ. The bigger issue
will be outflow generated from decaying thunderstorms which will
pose a wind risk. Currently the SPC is highlighting a marginal
risk for severe winds (>50 kts) across Maricopa County and a
higher (Slight) risk for Pinal County this afternoon and evening.
Latest HREF shows around a 70-90% chance of winds exceeding 30 kts
and 30% chance of winds exceeding 50 kts in N Pinal County this
evening. Therefore, a Blowing Dust Advisory has been issued for
the dust prone areas along I-10 and I-8 in Pinal County. It is
possible that westward moving outflow boundaries could make it all
the way to the Colorado River valley late tonight.

On Friday, the 500 mb high centered over southern NV will begin to
migrate slowly ewd. This will result in a slight decrease in hghts
aloft and cause the upper level flow to switch from northeasterly
to easterly. Thus, we should start to see an uptick in moisture
beginning late Friday into early Saturday with PWAT values
increasing from 1.00" to near 1.50". Despite this pattern shift,
Friday will still remain very hot across the lower deserts with
record breaking heat still possible in Phoenix. The current record
high on Friday is 116F at Phoenix Sky Harbor and the latest NBM
forecasted high is 117F. There could be another round of
convection late Friday afternoon through the early evening. The
main threat again will be strong outflow from any storm due to a
dry subcloud environment and high DCAPE values around 1500-2000 J/kg.

Heading into this weekend we will start to see a more favorable
monsoon setup as the center of the subtropical high repositions over
the 4-Corners area. Dewpoints will increase into the upper 50s to
lower 60s on Saturday which will yield CAPE values rising above 500
J/kg. This moist and unstable environment will be prime for more
widespread, robust convection across the lower deserts. Currently
PoPs range from 20-25% across the central deserts to around 55%
in Gila County on Saturday afternoon/evening. Temperatures will
begin to cool by a few degrees on Saturday due to decreasing hghts
aloft, however it will still feel very hot due to higher heat
index values, especially across southeast CA and southwest AZ.
Sunday still looks to include the best rain chances across the
entire region as a potential stronger embedded shrtwv trough in
easterly flow passes overhead. The arrival of this wave along with
increasing sfc moisture and instability should result in stronger
outflow potential and longer-lived storms. Rain chances Sunday
afternoon and evening are around 30-40% for the Phoenix Metro and
up to 60% for eastern Gila County. It will be noticeably cooler on
Sunday due to the increase in moisture and cloud cover with highs
remaining at or below 110F across the lower deserts.

Drier air is expected to gradually filter into the region from
west to east early next week, likely limiting convection to the
higher terrain of southcentral and eastern Arizona by Tuesday.
Temperatures are expected to remain near normal through the middle
of next week as the upper lvl high remains well north of the
area.

.AVIATION...Updated at 0610Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The primary aviation weather concern through the TAF period
continues to be strong winds associated with thunderstorm outflows
from earlier this evening and additional outflows expected
tomorrow evening. Winds have established a southerly component
across the TAF sites as storms brushed the Phoenix Metro to the
east/south this evening, and these strong outflow winds should
subside over the next few hours and settle out of the east
southeast. Light east winds and some periods of variability will
carry us into early tomorrow afternoon before west winds pick up
across the terminals between 21-23Z. Tomorrow evening, Hi-Res
guidance indicates the potential for storms to push into the
Phoenix Metro area (10-25% chance), but uncertainty is high in the
exact locations that will see VCTS/TSRA. The most likely impact
will be an outflow initially pushing through most of the Greater
Phoenix Area from the northeast. Following the initial push from
the northeast, winds will likely be erratic and confidence in
exact directions is very low, especially if further storm
development occurs over the lower deserts.

Southeast California/Southwest Arizona including KIPL and KBLH:
The potential for increasing wind speeds overnight will be the
main weather issue through Friday evening with occasional SCT
mid/high level cloud decks. S/SE winds will be preferred through
the entire TAF period at both terminals. Onset of stronger winds
tonight looks to be around or just prior to midnight. A long-
lived outflow might make its way to BLH tomorrow evening,
depending on the evolution of convective activity over South-
Central AZ and over the high terrain north of Maricopa and La Paz
Counties. Confidence is too low to mention a wind shift at BLH in
the TAF at this time.

&&

.FIRE WEATHER...
Very hot temperatures will persist through Friday with thunderstorm
chances steadily increasing over the eastern districts. Moisture
will continue to increase over most districts through the weekend
allowing temperatures to retreat while humidity levels and rainfall
chances increase markedly. Storms will initially produce only
isolated wetting rain chances in eastern districts with areal
coverage likely becoming far more widespread by the weekend.
Minimum humidity values will fall to around 10-15% through
Friday, however by Sunday, values should struggle to fall below
20%. Some slight drying may wash over the districts by the middle
of next week when minimum values fall closer to a 10-20% range.
Occasional afternoon upslope gusts near 20 mph will be common the
next few days, and the increase in thunderstorm activity will
produce more frequent strong outflow winds across a larger
expanse.

&&

.CLIMATE...

Record Highs

           Phoenix      Yuma      El Centro
          ---------    -------   -----------
July 11   118 (1958)  118 (1958)  117 (1975)
July 12   116 (2020)  119 (1939)  118 (1964)

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Friday for AZZ530>555-559.

CA...Excessive Heat Warning until 8 PM PDT Friday for CAZ560>570.

&&

$$

DISCUSSION...Salerno
AVIATION...Whittock
FIRE WEATHER...Salerno/Kuhlman
CLIMATE...Berislavich/Kuhlman