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492
FXUS65 KPSR 182045
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
145 PM MST Tue Jun 18 2024

.SYNOPSIS...
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Near normal temperatures the next couple of days will warm back into
an above normal regime during the latter half of the week as strong
high pressure builds back into the region. The warmest temperatures
should spread into the south-central Arizona lower deserts where an
Excessive Heat Watch is in effect Thursday and Friday. The evolving
pattern will also promote a moisture increase across eastern Arizona
later in the week supporting isolated mountain storms and gusty,
outflow winds descending into lower desert communities.
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&&

.DISCUSSION...
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Pronounced troughing continues over the western Conus with a
midtropospheric front actually sliding into northern Arizona as H5
heights have retreated to around 582dm resulting in temperatures
near the daily normal. However, this pattern will not be maintained
as the Conus synoptic setup undergoes a retrogressive evolution
allowing anomalously strong mid-Atlantic/Ohio Valley ridging to
build back into the SW Conus. Meanwhile, a large, unorganized
tropical gyre in the western Gulf of Mexico will also be absorbed
into this retrogressive regime traveling into central/northern
Mexico with an attendant, thick midlevel moisture plume. Eventually
degrading into an inverted easterly wave, this feature will support
a moisture surge up the Rio Grande Valley aided by an enhanced
pressure gradient on the western periphery of the incoming ridge
axis. The eastern half of the CWA will experience the largest
impacts from this evolution with 50F+ dewpoints advecting westward
Thursday morning juxtaposed with unusually strong, gusty east winds
during the morning hours.

As the H5 ridge spreads westward Thursday and coincident with the
theta-e surge, heights should increase to at least 590dm with a
tendency towards a 592-594dm range over the weekend. This will yield
another warming trend with high HeatRisk and excessive criteria
likely becoming breached on Friday around the Phoenix metro. While
NBM guidance spreads are rather narrow, greater uncertainty exists
considering the potential impacts from thunderstorm outflows and
convective consequences (i.e. overturning and residual cloud cover).
Latest NBM temperature foreasts have cooled a few degrees and
overall major HeatRisk remains rather localized and not particularly
resounding. Have kept the excessive heat watch in tact given these
uncertainties and more marginal coverage and intensity. The heat
dome will continue to impact the region through the weekend with any
higher terrain monsoon activity providing little relief over the
lower elevations. In fact, NAEFS mean output suggests ridging may
strengthen and shift into northern Mexico early next week resulting
in a return to more widespread excessive heat.

The seasons first chance of monsoon showers and thunderstorms should
affect higher terrain locations through the weekend as sfc-H7 mixing
ratios reach at least 8 g/kg (and possibly as high as 10 g/kg).
Based on the lower end of moisture intrusion, isolated to scattered
mountain convection is likely, albeit with somewhat limited rainfall
amounts over higher terrain, then decay before reaching lower
deserts. However, mixing ratios closer to 10 g/kg would support some
storms being maintained into lower elevations with a more robust
rainfall potential over the eastern CWA. While the prevailing SW
midlevel flow would typically support storms moving away from the
heart of the forecast area, DCAPE 1500-2000 J/kg combined with
prevailing east flow in the sfc-H8 layer and instability pool at
lower elevations would support storms and organized outflows pushing
into the Phoenix metro. The most likely outcome points towards very
gusty outflow winds and localized blowing dust periodically
affecting south-central Arizona Thursday through Sunday. This first
round of monsoon activity may only last into early next week as
guidance favors strengthening high pressure ridging sinking south
next week - mostly directly over southern AZ/northern Mexico cutting
off the better quality moisture source.
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&&

.AVIATION...
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Updated at 1750Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds will tend to follow their typical diurnal pattern. A period of
southerly winds, between now and approximately 21Z, is once again
expected to precede the westerly/southwesterly shift this afternoon.
Wind speeds this afternoon and early evening with generally be
around 8-12 kt, with occasional gusts up to 20 kt. Winds will then
go back easterly during the overnight hours tonight. Mostly clear
skies will continue.


Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KIPL will generally be aob 8 kt and have a westerly
component through early tomorrow morning before establishing out of
the SE. There may be a brief period of sundowner winds tonight with
gusts up to around 20 kt. At KBLH, northerly winds will subside
throughout the afternoon. Winds will then go westerly this evening
and shift south overnight. Mostly clear skies will continue.
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&&

.FIRE WEATHER...
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Near normal temperatures through midweek will warm back into an
above normal category as the first signs of monsoon flow and
moisture return affect eastern districts late this week through
early next week. Through Wednesday, widespread dry conditions will
prevail with minimum afternoon humidity levels 10-15% following poor
to fair overnight recovery of 15-45%. Moisture will surge into
eastern districts Thursday morning allowing minimum RH to only fall
around 25% with fair to good overnight recovery of 40-75%. Western
districts will likely not experience much better moisture, and the
drier regime will persist. The moisture surge will be accompanied by
unusually strong easterly winds Thursday morning (gusts 25-35 mph
during an unlikely morning time frame) with speeds gradually
weakening into the weekend.  With the moisture increase, isolated to
scattered afternoon thunderstorms are possible over higher terrain
areas of eastern districts. Lightning with low wetting rain chances
combined with erratic, gusty outflow winds are significant concerns
for eastern districts this weekend.
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&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Watch from Thursday morning through Friday
     evening for AZZ534-537>555-559.

CA...None.

&&

$$

DISCUSSION...18/Kuhlman
AVIATION...Whittock/Berislavich
FIRE WEATHER...18