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655
FXUS65 KPSR 200021
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
521 PM MST Wed Jun 19 2024

.UPDATE...
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Updated Aviation Discussion.
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&&

.SYNOPSIS...
Another day of quiet weather and near normal temperatures today
before we get our first taste of monsoonal moisture and strong high
pressure building back over the region tomorrow. An Excessive Heat
Warning will be in effect tomorrow for portions of South Central
Arizona. On Thursday afternoon, thunderstorms are expected to
develop along a line east of Phoenix extending south southeast
through central Pinal County and down to Tucson. Strong, gusty
outflow winds are anticipated with the thunderstorms that develop
tomorrow, and with this there will be the potential for blowing dust
in the eastern deserts. Daily chances for mainly mountain and some
isolated lower desert thunderstorms will exist Friday and through
the weekend.

&&

.DISCUSSION...
Troughing still dominates much of the Western US, with 500 mb
heights over the region ranging from 583-587 dam according to RAP
analysis as of late this morning. As a result, temperatures across
the lower deserts will once again hover near normal in the 100-107
degree range this afternoon. However, these quiet conditions and
near normal temperatures will be very short lived. Overnight
tonight, some large-scale flow features are expected to set up in a
way that will allow moisture to surge into southeastern AZ.

A broad area of abundant low-level moisture associated with Tropical
Cyclone Alberto moves westward into central Mexico tonight, high
pressure currently centered over the East-Central US elongates
westward, and a trough digs south along the California Coast. Strong
easterly winds along the northern periphery of the tropical system
will begin to wrap around the anticyclone to the north, move over
southeastern AZ, and are also enhanced by height falls to the west
thanks to the trough along the California coast. ECMWF EFIs
associated with the moist east southeasterly flow tomorrow morning
and Thursday night exceed 0.9, indicating a significant signal.
These strong ESE gradient winds will diminish during the day, but
are expected to pick back up Thursday night. Wind gusts in
Southern Gila County during this time are expected to peak between
40-50 mph (HREF 50th percentile gusts are at or above 40 mph
through much of tomorrow morning and Thursday night.) As a result,
a Wind Advisory has been issued for the populated high terrain
areas of Southern Gila County tomorrow morning through Friday
morning.

Tomorrow afternoon, several weather hazards are expected to impact
South-Central AZ. Localized areas of Excessive heat will develop as
ensemble mean H5 heights increase to around 591 dam along our
eastern CWA. Forecast highs range from 110 to 114 across the Phoenix
Metro Area, and an Excessive Heat Warning will be in effect for much
of Maricopa and Northern Pinal Counties from 10 AM - 8 PM MST. In
addition to the excessive heat, thunderstorms will begin to initiate
during the afternoon along a line connecting Tucson, through central
Pinal County, to just east of Scottsdale. This is because a pseudo
dryline sets up here, as strong, moisture-laden southeast flow
(as discussed previously) meets dry, southwest flow at the mid
levels overhead. A strong gradient in SBCAPE is apparent early
tomorrow afternoon, with HREF mean values peaking in excess of
1000 J/kg to the east of the aforementioned line, and dropping off
to near zero as you travel west towards Buckeye. The main threats
associated with these storms will be gusty outflow winds and the
potential for blowing dust, with HREF probabilities of 35+ mph
winds between 50- 70% and probabilities for 58+ mph winds around
10-30% centered over eastern Pinal County. Steering flow will be
south southwesterly tomorrow, so storms will move away from the
Phoenix Metro Area after forming, and the associated outflows will
not be quite as strong as they would be otherwise. Rainfall
accumulations will be light, if any, over the lower deserts,
however, some very localized areas (particularly over the high
terrain to the east) will have the potential to see up to 0.5".

Thunderstorm chances continue Friday and through the weekend. The
trough positioned along the California Coast will begin to recede to
north and progress eastward on Friday, with a jet streak downstream
of the base of this trough. The upper level divergence over AZ from
this jet streak should create a favorable synoptic setup for the
development of thunderstorms, particularly over the high terrain
of South-Central AZ. Thunderstorm chances range from 40-60% over
Southern Gila County on Friday, and have increased up to 20-40%
over portions of the eastern lower deserts. With LREF mean dCAPE
values still reaching up to 2000 J/kg on Friday, strong, gusty
outflow winds will likely be a concern for Friday as well. With a
south southwesterly steering flow continuing on Friday, Phoenix
will not see much activity unless decent convection over the
higher terrain of southern Maricopa and southwest Pinal County,
and/or colliding outflows from convection from the east and
southeast.

.PREVIOUS DISCUSSION...

Moisture levels should continue to improve into Saturday as
ensemble mean PWATs increase to between 1.6-1.9" over much of
southwest and south-central Arizona. However, the upper level
ridge will also be strengthening and shifting right over the
region this weekend causing increasing subsidence. Forecast
confidence this weekend is fairly low due to placement of the
ridge. We will most likely have afternoon higher terrain
convection over the weekend, but it may struggle to survive into
the lower deserts. That doesn`t mean we can`t have a very good and
fairly widespread thunderstorm day with the set-up like we are
expecting this weekend, but it would take considerable colliding
outflows. For now, the NBM shows 10-20% PoPs this weekend over La
Paz and Yuma Counties, 20-30% PoPs over Maricopa County, and
30-45% PoPs over the eastern Arizona high terrain. Any convection
in this higher PWAT environment this weekend will have the
potential of bringing localized heavy rainfall.

Hotter temperatures will also spread westward Friday into the
weekend with highs topping 110 degrees across much of the
southeast California lower deserts to near 110 degrees across
southwest and south-central Arizona. The increased moisture will
also exacerbate the heat with temperatures feeling even hotter
given the increased humidities and keeping overnight lows well
above normal. In fact, overnight lows near 90 degrees in the
Phoenix area will be possible as early as Thursday night before
lows increase areawide by Saturday night with readings mostly
staying in the low to mid 80s or higher. The temperature forecast
for this weekend is of lower confidence compared to usual due to
the potential convection and cloud cover. By early next week,
ensemble guidance favors some strengthening of the sub-tropical
high with H5 heights likely rising to between 593-596dm. Moisture
levels will likely depend on the position of the high center with
some indications of the center shifting a bit more to the south
toward Mexico which may bring some drier air in from the
southwest. For now, the NBM favors this drying as it lowers PoPs
from west to east early next week. If guidance is correct and our
heights increase next week, we are nearly certain to fall under
another round of excessive heat. NBM forecast highs currently inch
toward the 115 degree mark by next Tuesday or Wednesday, but then
show signs of backing off later next week as the ridge is likely
to start to weaken.

&&

.AVIATION...
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Updated at 0020Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds will favor diurnal tendencies through the evening and
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. 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 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. Could see a period of westerly gusts to
around 20 kts this evening at KIPL before subsiding. At KBLH,
winds will favor the S to SW with some afternoon breeziness. SKC
conditions will persist.
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&&

.FIRE WEATHER...
Near normal temperatures today 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. Widespread dry conditions will prevail today 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%. The moisture surge
will be accompanied by unusually strong easterly winds Thursday
morning through early afternoon with gusts of 35-45 mph possible.
Wind speeds will gradually weaken into the weekend with east
southeasterly winds persisting through Saturday. With the moisture
increase, scattered afternoon thunderstorms will be possible over
higher terrain areas of eastern districts and potentially into
portions of the lower deserts. Lightning with fairly low wetting
rain chances combined with erratic, gusty outflow winds may be
significant concerns for eastern districts this weekend despite
the higher RHs. Western districts should see slightly 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.

&&

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

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

CA...None.
&&

$$

DISCUSSION...Whittock
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Smith
FIRE WEATHER...Kuhlman