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306
FXUS65 KPSR 201005
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
305 AM MST Thu Jun 20 2024

.SYNOPSIS...
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Today will see a big pattern change as moisture begins to surge
into Arizona from the southeast bringing our first chances for
monsoon thunderstorms generally east of Phoenix. However, gusty
winds and blowing dust may impact areas across Pinal County into
Phoenix late this afternoon and evening. Temperatures will also
increase today into Friday with highs topping 110 degrees across
the south-central lower deserts starting today and over the
western deserts starting Friday. Daily chances for mainly mountain
and some isolated lower desert thunderstorms will exist Friday
and through the weekend.
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&&

.DISCUSSION...
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We will mark a fairly early start to the monsoon later today with
moisture already beginning to surge into southeast Arizona since
midnight. Low level moist flow will continue to strengthen through
the rest of this morning and spread west northwestward into
south-central Arizona. Windy conditions are expected to develop
into southern Gila County after sunrise with advisory level wind
gusts up to 45 mph possible from mid morning through mid
afternoon. This moist flow will quickly add enough moisture to the
low to mid levels to allow for some afternoon convection to
develop across the eastern half of Pinal, far eastern Maricopa
into southern Gila County as an area of MUCAPE of around 1000
J/kg develops by early afternoon. Building heights over the region
from the east today will also allow temperatures to heat up,
especially over the south-central lower deserts where sunny skies
are still expected through mid afternoon. An Excessive Heat
Warning is in effect for late this morning through this evening
across Maricopa and Pinal Counties.

Aside from the excessive heat, the main forecast concern for today
will be the potential for strong thunderstorm outflow winds. The
ingredients for an impactful wind and blowing dust event across
portions of Gila, Pinal, and Maricopa Counties will likely be
there for late this afternoon and early evening. Hi-res guidance
shows plenty of instability and good bulk shear for thunderstorm
development east of Phoenix this afternoon with forecast soundings
showing very high DCAPE extending into the lower deserts. This
will likely allow for strong convective cores and then strong
downdraft winds with the HREF showing greater than a 30%
probability of 50+ kt winds just east and southeast of Phoenix.
Storm outflow winds, in excess of 45 mph could very well surge
westward through much of central and western Pinal County and into
Phoenix. A Blowing Dust Advisory has been issued for late this
afternoon into early evening for northwest Pinal County due to the
greater than 70% probability of 30+ kt winds and 20-30%
probability of 50+ kt winds across the eastern and central part
of the county.

The strong southeasterly moist flow with 850mb winds of 20-30 kts
will continue through tonight into Friday morning, expanding
westward through all of Maricopa County and as far west as La Paz
County. This will push PWATs to around 1.2-1.4" over much of the
area by Friday afternoon with low level mixing ratios of around
9-10 g/kg. This is a bit less moisture than what we saw with
yesterday`s model runs and hi-res guidance reflects this in
overall weak to at most modest convective development Friday
afternoon over south-central Arizona, focused again mostly east
of Phoenix. Additional surges of moisture are likely to occur
Friday night and through the weekend with ensemble mean PWATs
increasing to between 1.6-1.8" over much of southern Arizona and
as far west as the southeast California lower deserts. Despite the
increasing moisture over the weekend, the convective potential
will likely be hindered by the sub-tropical high center developing
over far eastern Arizona into New Mexico. This will increase
subsidence aloft as indicated by the capping inversions shown in
forecast soundings across the lower deserts. Higher terrain areas
over the weekend will continue to be the more favored locations
for convective development across Arizona with PoPs as high as
50-60% east of Phoenix. The south-central Arizona lower deserts
will still 20-40% chances for showers and thunderstorms this
weekend as any decent colliding outflows should be enough for
additional development. Given the expected moisture levels this
weekend, but rather lackluster mid-level lapse rates instability
should be fairly marginal overall. Strong gusty winds, patchy
blowing dust, and localized brief heavy rainfall will all be
possible this weekend, but likely limited in coverage.

As the sub-tropical ridge continues to strengthen over the region
Friday into the weekend, very hot temperatures are expected over
the majority of the area. Lower desert high temperatures of
110-113 degrees are forecast for Friday before dropping a couple
degrees this weekend due to the increased moisture and cloud
cover. HeatRisk on Friday does show some localized areas falling
into the Major category, especially west of Phoenix before
temperatures drop beginning Saturday pushing the entire area back
into Moderate HeatRisk. Even with mostly falling into Moderate
HeatRisk through this weekend, the heat is still expected to be
dangerous to those most vulnerable. Overnight lows will also not
see much in the way of recovery due to the increased humidities
and cloud cover. Forecast lows in the mid 80s to around 90 degrees
are expected in the Phoenix area starting tonight into next week
with low to mid 80s lows spreading westward through southeast
California starting Saturday night.

Model ensembles are still favoring the sub-tropical ridge
strengthening further during the first half of next week with H5
heights potentially rising to between 592-596dm over much of the
region. The high center is likely to remain somewhere across
eastern Arizona and western New Mexico during this time prompting
fairly weak flow over the area and keeping the moisture mostly in
place. The convective potential will however likely go down early
next week due to stronger subsidence aloft with any lower desert
areas seeing chances falling to around 10% by Tuesday. The main
forecast concern for next week will be the building heat due to
the stronger ridge and expected lower monsoon activity. Forecast
NBM highs rise to 108-112 degrees next Monday and closer to 115
degrees by the middle of next week.
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&&

.AVIATION...Updated at 0540Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds will favor diurnal tendencies through the overnight period
before S to SE winds increase Thursday morning starting around
15-16Z with gusts to around 20 kts. Winds are expected to then
veer toward the southwest in the afternoon with sustained speeds
around 10-12 kts and perhaps a few gusts into the teens.
Uncertainty increases going into the late afternoon and early
evening timeframe tomorrow as thunderstorms are expected to
develop to the east of the terminals. As of now, thunderstorm
chances will be greatest east of the metro area tomorrow with the
greater threat for the terminals being gusty outflow winds moving
through Phoenix. The latest HREF indicates around a 40-70% chance
for outflow winds in excess of 30 kts and about a 10-20% chance
in excess of 50 kts. Timing and magnitude of outflow winds remain
quite uncertain, but the most favored direction is expected to be
from the east. SKC conditions will persist through much of the
period before FEW-SCT decks around 12 kft develop tomorrow
afternoon into the evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation weather concerns are expected through the next
24 hours. Winds will predominantly favor the west at KIPL with
extended periods of light and variable winds expected tomorrow
morning and afternoon. At KBLH, winds will favor the S to SW with
some afternoon breeziness. SKC conditions will persist.

&&

.FIRE WEATHER...
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Moisture will surge into eastern districts starting this morning
allowing minimum RH to only fall around 25% with fair to good
overnight recovery of 40-75%. The moisture surge will be
accompanied by unusually strong easterly winds mid morning
through mid afternoon with gusts of 35-45 mph possible across
southern Gila Co. and again overnight tonight through Friday
morning with breezy easterly winds extending through the south-
central Arizona lower deserts. With the moisture increase,
scattered afternoon thunderstorms will be possible starting today
over higher terrain areas of eastern districts and eventually
extending into portions of the lower deserts Friday into the
weekend. Western districts should see better moisture over the
weekend, but CWR will remain below 5%. High pressure is then
expected to strengthen over the region by early next week leading
to even hotter temperatures along with slightly drier conditions.
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&&

.PSR WATCHES/WARNINGS/ADVISORIES...
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AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST this
     evening for AZZ534-537>555-559.

     Blowing Dust Advisory from 3 PM this afternoon to 8 PM MST this
     evening for AZZ553.

     Wind Advisory from 9 AM this morning to 5 AM MST Friday for
     AZZ560-562.

CA...None.
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&&

$$

DISCUSSION...Kuhlman
AVIATION...Smith
FIRE WEATHER...Kuhlman