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970
FXUS65 KPSR 202343
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
443 PM MST Thu Jun 20 2024

.UPDATE...
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Updated Aviation section.
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&&

.SYNOPSIS...
As moisture begins to surge westward into Arizona today,
thunderstorm chances will increase this afternoon with activity
focused mainly east of Phoenix. Strong gusty winds and blowing dust
will be the main hazards with the activity this afternoon. Very hot
temperatures are expected this afternoon and again on Friday with
highs topping out around 110 degrees across the lower deserts. As
abundant moisture remains in place through the weekend, daily
chances for thunderstorms will exist with the highest chances
across the higher terrain areas.

&&

.DISCUSSION...
Latest water vapor satellite imagery as well as streamline analysis
shows a broad upper-level trough over CA with an upper-level ridge
centered over the eastern CONUS retrograding westward. Trapped
underneath the upper-level ridge is Tropical Depression Alberto,
which is moving westward across Mexico. The retrograding ridge along
with influence from Alberto is enhancing southeasterly flow across
most of eastern AZ with strong moisture advection ongoing. The
southeasterly flow will enhance winds across southern Gila County
today and into early Friday, with peak gusts ranging between 40-50
mph. As a result, a wind advisory remains in effect through early
Friday. With the strong moisture advection, enough moisture in
the low to mid-levels will allow for afternoon convection to
develop with the latest hi-res guidance showing the activity
across the eastern half of Pinal, far eastern Maricopa and through
southern Gila Counties where CAPE values as high as 1000 J/Kg is
expected. Forecast model soundings show DCAPE values as high as
2000 J/Kg and thus strong downbursts winds will be a major threat
with these thunderstorms with HREF showing between a 10-30%
chance of 50+ kt winds just east and southeast of Phoenix. Strong
outflow winds, in excess of 40-50 mph, will likely surge westward
into central and western Pinal County and into the Phoenix area,
with the potential for dense blowing dust to materialize. As a
result, a Blowing Dust Advisory remains in effect for late this
afternoon into early this evening for Pinal County. Aside from
the thunderstorm potential, the other main hazard for today will
be the excessive heat. Building 500 mb heights from the
retrograding ridge will allow temperatures to increase,
particularly across the south-central AZ lower deserts, where
highs are likely to top out between 110-114 degrees.

The strong southeasterly flow will continue through Friday and thus
additional moisture will surge further westward with ensemble
mean PWATs ranging between 1.2-1.4" over much of south-central AZ.
Thus, afternoon convective activity is expected once again,
mainly over the higher terrain areas to the east of Phoenix.
However, outflow winds emanating from the convective activity will
likely impact the lower deserts. With 500 mb height fields rising
above 590dm for much of the area and not much in the way of cloud
cover expected through mid-afternoon, high temperatures across
most of the lower desert communities will likely range between
110-115 degrees, with areas of major HeatRisk once again
materializing, particularly across south-central AZ. As a result,
the Excessive Heat Warning which was originally in effect through
this evening, has been extended through Friday evening.

Heading into the weekend, the model guidance is showing additional
surges of moisture moving into central and southern AZ and as far
west as the southeast California lower deserts with PWATs
increasing to between 1.6-1.8". However, at the same time that the
moisture is on the increase, the upper-level ridge is expected to
move directly overhead, which will increase the subsidence aloft.
Therefore, the overall convective coverage will likely be limited
to the higher terrain areas of AZ. The one caveat is that guidance
is indicating that an inverted trough moving westward through
northern Mexico will reach southern AZ on Sunday, which could
potentially provide greater convective coverage. This is reflected
in the latest NBM PoPs, which shows the greatest areal coverage
for Sunday. High temperatures over the weekend, with the
increasing moisture and potential cloud cover will be slightly
cooler with overnight lows expected to more elevated. In fact,
overnight and early low temperatures are forecast to remain near
to just above 90 degrees across central Phoenix. Thus, even though
the overall HeatRisk is expected to drop into the moderate
category, the necessary heat precautions should still be
performed, especially that there will not be as much relief during
the overnight hours.

Ensembles are strongly favoring the subtropical ridge to strengthen
even further heading into the first half of next week with 500 mb
height fields climbing to between 594-597dm. The high will likely
be centered between eastern AZ and western NM with subsidence
aloft increasing even further. This will cause some moisture
erosion and thus convective potential will decrease as a result
starting on Tuesday. With the strong ridge positioned directly
overhead, extreme heat will be a concern with temperatures heading
into the middle of next week rising to near 115 degrees with the
areal coverage of major HeatRisk becoming more widespread. By the
latter half of the week, there are indications from the ensembles
that the ridge will gradually weaken as a trough moves through
the Pacific Northwest and thus a slight cooling trend would
ensue.

&&

.AVIATION...
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Updated at 2342Z.

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

Thunderstorms have been occurring this afternoon in a north-south
oriented corridor over Gila and the eastern Pinal Counties.
Outflow from some of those storms has entered far southeast
portions of the Greater Phoenix area. That outflow is encountering
southwesterly winds and that will help weaken it. But, the end
result is expected to be just enough breakthrough to cause a wind
shift - albeit with a lot of variability in direction. Subsequent
outflow from near Apache and Roosevelt Lakes along with the larger
scale easterly pressure gradient will favor easterly directions
later this evening and especially overnight. Lingering dust
originating from New Mexico along with local dust will cause
reduced visibilities but generally AOA 6SM. But, slantwise
visibilities will be reduced. There is potential for renewed
Ely/SEly breeziness before sunrise but anticipate it will be after
15Z or so before gusts above 20kt are seen.

During the day Friday, expect directions to slowly trend southerly
then southwesterly by late afternoon (less so for KIWA). Another
round of storms is anticipated over portions of southeast and
east-central AZ in the afternoon. Confidence in impacts at the TAF
sites too low at this time to reflect in the TAFs.


Southeast California/Southwest Arizona including KIPL and KBLH:

Dry southwesterly flow aloft will begin weakening during the day
Friday as an upper trough weakens and high pressure centered to
the east begins to expand westward. Surface winds will generally
follow familiar warm season patterns with westerly gustiness in
the evening at KIPL and southerly gustiness at KBLH in the
afternoon.
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&&

.FIRE WEATHER...
Moisture will continue to increase across the region into the
upcoming weekend bringing daily chances for afternoon
thunderstorms, with the best chances expected across the higher
terrain areas of south-central AZ. MinRH values for today will
range around 25% across the far eastern districts to between
10-15% across the western districts. Similar MinRH values will be
seen for Friday before increasing to between 20-35% areawide for
the upcoming weekend. Gusty easterly winds upwards of 35-45 mph
can be expected at times across southern Gila County through early
Friday morning with some breezy easterly winds extending through
the south-central AZ lower deserts. High pressure is expected to
strengthen over the region by early next week leading to even
hotter temperatures along with slightly drier conditions.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Friday for AZZ534-537>555-
     559.

     Blowing Dust Advisory until 8 PM MST this evening for AZZ553.

     Wind Advisory until 5 AM MST Friday for AZZ560-562.

CA...None.
&&

$$

DISCUSSION...Lojero
AVIATION...AJ
FIRE WEATHER...Lojero/Kuhlman