881
FXUS65 KPSR 142356
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
456 PM MST Sat Sep 14 2024

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
After a quiet day today with temperatures near normal, unsettled
weather conditions are expected for Sunday and Monday as good
chances for showers and thunderstorms cover much of southern and
central Arizona. Thunderstorms that develop may produce strong
winds along with locally heavy rainfall that could lead to some
localized flooding. Dry and mild conditions are then expected
starting next Tuesday with temperatures running close to 10
degrees below normal through around next Friday.

&&

.DISCUSSION...
The weekend is off to a pleasant start with mostly clear skies
showing up on visible satellite imagery this afternoon across much
of the region with some CU development across portions of Arizona.
Temperatures today will top out around near normal as lower
desert highs warm to around 98-103 degrees. Going forward through
the rest of the weekend and into the beginning of next week,
unsettled weather will prevail, bringing increasing rain chances
along with cooler temperatures.

Visible/IR satellite imagery showed Tropical Storm Ileana situated
near the northern part of Sinaloa in northwest Mexico as it makes
its way into the Gulf of California. The latest NHC forecast
shows Ileana quickly weakening into a tropical depression going
forward through the day today. While the tropical system itself
won`t make it`s way all the way up the Gulf of California and into
the southwest CONUS, it will help in advecting moisture into
Arizona, which has already begun to occur. This increasing
moisture is evident on this morning`s 12Z PHX sounding where the
moisture increase is most notable at around 700 mb, helping to
boost the RAOB PWAT to 1.13". At the surface, dew points across
south- central Arizona have risen to around 45-55 degrees and to
around 60-65 degrees across Yuma County as of early this
afternoon. Moisture advection into the state will continue through
the weekend with PWATs expected to climb to upwards of 1.5" by
Sunday across central and southern Arizona, though some of the
latest ensemble runs have been backing off on the amount of
moisture that will be advected into Arizona.

A weak shortwave trough seen on WV imagery near the coast of
southern California this afternoon will push inland across
southern California and then across northwest Arizona going
forward through the rest of the day today and into the first part
of Sunday. By this point, adequate moisture and instability is
expected to be in place across south-central Arizona Sunday
morning, which combined with some forcing from this shortwave may
spark some elevated isolated to scattered showers and perhaps an
isolated thunderstorm or two. Moisture and instability will peak
over the area going into Sunday afternoon with CAPE values
climbing upwards of 1000-1500 J/kg, while DCAPE values will climb
to around 1500 J/kg. 0-6 km Bulk shear is also expected to climb
upwards of 25-35 kts. These should be able to support some strong
to severe thunderstorms capable of producing damaging winds and
perhaps some marginally severe hail Sunday afternoon. Strong
outflows may generate areas of blowing dust, leading to reduced
visibilities. Hi-res guidance suggest isolated to scattered
showers and thunderstorms will continue throughout the evening and
overnight hours Sunday into Monday.

Heading into Monday, large scale forcing will increase over the
region as a potent Pacific trough dives down along the West Coast
before shifting into the Great Basin through the day Monday. This
will help to produce additional shower and thunderstorm development
on Monday, but will also help to pull drier air northeastward into
Arizona throughout the day. PWATs less than an inch across southeast
California and southwest Arizona will shift eastward across south-
central Arizona by Monday evening, resulting in rain chances quickly
dropping off. Exact storm total rainfall amounts are uncertain as
global ensembles continue to back off on the amount of moisture that
will be advected into the region and thus leading to lower forecast
rainfall amounts. The 12Z HREF keeps the heaviest rainfall across
southeast Arizona with lesser amounts across south-central Arizona.
The quick storm motion will limit the amount of time any thunderstorms
producing heavy rainfall has over a given area and thus limit
heavier rainfall amounts. However, any areas that see training
thunderstorms could see locally heavy amounts that could lead to
some localized flooding. Across south-central Arizona, the
greatest rainfall totals are expected to favor orographic/upslope
areas north and east of Phoenix while amounts quickly drop off
west of Phoenix.

Aside from the rain chances, a cold front associated with the
aforementioned potent trough will push into southern California
Monday afternoon/evening and will help to increase winds across
southeast California, especially Imperial County. Winds across
western portions of Imperial County could exceed 40 mph and thus a
Wind Advisory may be needed as we get closer.

Temperatures will trend downward for the rest of this weekend and
through the first part of next week. NBM temperatures have trended
upwards a bit for Sunday and Monday and now keep highs at or just
above the century mark, though if we end up seeing more thicker
cloud cover or greater shower/thunderstorm coverage on either of
these days, then we may be able to keep highs under 100 degrees. By
Tuesday, confidence is high that temperatures will stay below 100
degrees with temperatures across the lower deserts forecast to top
out in the upper 80s to low 90s. Persistent negative height
anomalies over the Desert Southwest will act to keep temperatures
well below normal through much of the week next week with continued
lower desert highs in the upper 80s to low 90s. By next weekend,
global ensembles indicate that heights will rebound as a trough
ejects out of the region while ridging builds closer to the West
Coast.

&&

.AVIATION...Updated at 2356Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Primary aviation impact over the next 24 hours will be the
potential for VCTS/TS impacts Sunday afternoon. Until then, light
W/SW winds, with light variability, will prevail at all terminals
through midnight tonight, with a late E shift anticipated early
Sunday morning. There is moderate confidence in a few rain showers
developing and moving northward for a few hours Sunday morning,
mainly between 11-17Z. Showers that do develop will be capable of
causing brief sudden wind shifts. A period of S crosswinds is
expected by midday and may continue through 22-23Z before shifting
more SW. Convection is expected to begin around 20-21Z Sunday and
spread into the Phoenix area mainly between 22-02Z. Any storm will
be capable of locally strong wind gusts, with 30% chance for >30
kt gusts. Lowest cloud bases will be around 7-8K ft AGL, with SCT
to BKN coverage through most of Sunday.

Southeast California/Southwest Arizona including KIPL and KBLH:
No weather concerns will exist through Sunday under mostly clear
skies. Winds will favor a S/SE component at KIPL for the next few
hours, followed by a W/SW shift for a few hours tonight. Winds
will again favor S/SE at KIPL Sunday following some morning light
variability. South winds will be favored at KBLH through the TAF
period with breezy afternoon conditions, as gusts reach up to
20-25 kts.


&&

.FIRE WEATHER...
A more unsettled weather pattern is expected through Monday as
moisture increases today ahead of an approaching weather system
expected to impact the region on Sunday and Monday. Expect overall
light winds today before winds increase on Sunday predominately
out of the south. Humidities will be more elevated this weekend
with MinRHs mainly between 20-30%. Sunday into early Monday
should bring periods of showers and thunderstorms across south-
central Arizona with CWR between 30-60%. Drier air will begin to
push eastward through the area later on Monday, but chances for
rain will linger from Phoenix and areas east of Phoenix through
the afternoon. Dry air will fully push through the area on Tuesday
as below normal temperatures take hold across the region. Dry
conditions with temperatures running 5-10 degrees below normal are
likely to persist well into the latter half of next week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Smith/Kuhlman
AVIATION...Benedict
FIRE WEATHER...Kuhlman