707
FXUS65 KPSR 130533
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1033 PM MST Fri Jul 12 2024

.UPDATE...Updated 06z Aviation Discussion.

&&

.SYNOPSIS...
A strong ridge of high pressure will continue to result in
excessive heat through Saturday. This ridge will finally migrate
over the 4-Corners region this weekend, resulting in a noticeable
increase in moisture levels and a slow cooling trend. Isolated
showers and storms will be possible across mainly Arizona high
terrain through Saturday with the best chances for storms
reaching the lower deserts on Sunday. Drier conditions are
expected to gradually spread over the area during the first half
of next week.

&&

.DISCUSSION...
This afternoon and evening is not looking nearly as active with
showers and thunderstorms as yesterday was. Mid level water vapor
satellite imagery shows some dry mid level air moving into AZ from
NM. This and the still relatively limited surface moisture is
resulting in limited instability this afternoon, especially across
Southeast AZ where surface dew points are in the 40s and MLCAPE
values of only 100 J/kg or so. Aside from isolated storms in
south-central Pima county, most thunderstorm activity this
afternoon and evening is expected across northern AZ, as supported
by the latest HREF. The latest HREF also supports similar
conditions Saturday, with thunderstorm activity focused mainly
over northern AZ high terrain and isolated activity near the US-MX
border. In addition, steering flow has become more easterly today
and will shift more southeasterly Saturday. With most of the
activity over northern AZ and very little in southeast AZ, this
is not a favorable steering flow to get many storms moving toward
the South-Central AZ lower deserts. There are also no notable
synoptic shortwaves or vorticity maximums in modeling to aid
forcing over the region. All this said, there are still low
chances of some spotty high-based ACCAS convection to develop
over lower desert locations during the overnight and morning
hours the next few days, especially with any mid-level gravity
waves percolating above the stable PBL. Moisture and instability
will be highest, although mostly capped, so cannot rule out some
isolated showers and thunderstorms over the western deserts,
especially within JTNP and over any locally higher terrain
features like the Kofas.

Despite this "negative" thunderstorm outlook for the next couple
of days, PBL moisture will continue to rise with nightly gulf
surges and fluxes from Sonora thunderstorm complexes. PWAT values
are forecast to rise up to 1.5-1.7" by Sunday and surface instability
will increase as a result. Surface dew points will also to surge
to around 70-75 degrees in Yuma and El Centro to around 60
degrees in the Phoenix area this weekend. Most of the GEFS
membership has MLCAPE values up around 1000-1500 J/kg Sunday
afternoon, but with still some modest CIN in place, while the EC
ENS is slightly drier with notably lower MLCAPE. Sunday continues
to look like the best shot and showers and storms in the lower
deserts, with the higher instability. It still may come down to
forcing and being able to overcome the cap. Latest NBM PoPs are up
to 20-30% in South-Central AZ lower deserts Sunday and 50-60% in
southern Gila County. While PoPs may not be anything prolific for
the lower deserts it is important to remember that mesoscale
influences, unresolvable by models this far out, can lead to
things becoming more active. So, given the potential for high
instability, any deep storm outflows or colliding outflows will
likely be enough for lower desert convective development. Any
strong thunderstorms Sunday will likely have the potential to
produce strong to possibly severe wind gusts and locally heavy
rainfall.

With the increase in low level moisture the next few days, the
forecast high temperatures finally come down, bringing an end to
the long stretch of excessive heat. The Excessive Heat Warning,
however, was extended another day, through Saturday. Although, the
forecast highs Saturday are expected to be a few degrees cooler
than previous days and HeatRisk falls to Moderate, the higher dew
points will make thing more muggy and morning low temperatures
may be the warmest they will be during the stretch of heat. The
forecast low for Phoenix Saturday morning is 94F. Heat indices in
Southwest AZ and Southeast CA will push 114-118F Saturday
afternoon. Starting Monday, guidance is still showing some drier
air working from west to east through southern California into at
least western portions of Arizona. There should still be enough
moisture to work with across the eastern half of Arizona for
another round of shower and thunderstorm activity on Monday, but
instability is likely to be lower and thus the convective
potential should be noticeably lower than Sunday. The drier air
should then continue to spread eastward Tuesday into Wednesday
mainly limiting any convection to the eastern Arizona high
terrain. Temperatures should bottom out on around Monday, but
highs are still forecast to be 2-4 degrees above normal. As the
drier air works through the region during the middle part of next
week, temperatures should slowly rise with highs likely topping
110 degrees over much of the lower deserts by next Wednesday or
Thursday.

&&

.AVIATION...Updated at 0530Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The main aviation weather concern through the TAF period will be
the potential for temporary gustiness associated with a
northeasterly thunderstorm outflow, which would reach KDVT/KSDL
around 06Z or just after. This outflow will likely not cause a
significant wind shift, as flow at the terminals is already
favoring light NNE. Confidence in this same outflow reaching
KPHX/KIWA is very low. A consequence of this outflow passing over
the north valley may be some light shower development, though it
will be scattered, and therefore not included in the TAF. Aside
from any outflow impacts, winds will mostly be light (aob 8 kt
sustained) overnight and settle out of the east eventually before
shifting back to the west tomorrow afternoon. Gusts into the upper
teens are expected to materialize by late tomorrow afternoon. Slantwise
visibility reductions will continue to be a concern through
tomorrow morning, particularly for KPHX/KIWA due to smoke moving
over the area from fires in Pinal County.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation weather concerns are expected through the next
24 hours. Winds will continue to favor SE at KIPL, while a
southerly direction is expected at KBLH. Gusts upwards of 20 kts
will be possible at times at both sites. A shortwave disturbance
is expected to move over the western deserts by tomorrow morning,
which could induce some nocturnal convection. If this materializes
(10-20% chance), the impacts would be confined to KIPL. The most
likely outcome would be light showers and a period of gusty winds,
but the direction is likely to remain SE`rly. Visibilities could
temporarily be reduced to MVFR if light rain passes over the
terminal. Otherwise, FEW-SCT mid to high clouds will continue
throughout the period.

&&

.FIRE WEATHER...
Very hot temperatures will be seen again today with thunderstorm
chances steadily increasing over the eastern districts through the
weekend as moisture improves. Temperatures will slowly retreat
over the weekend while humidity levels increase. Storms will
initially produce only isolated wetting rain chances in eastern
districts with areal coverage likely becoming far more widespread
by Sunday. Minimum humidity values will fall to around 10-15%
today, 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 12   116 (2020)  119 (1939)  118 (1964)

&&

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

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

&&

$$

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