389
FXUS65 KPSR 141712
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1012 AM 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. Dry and mild conditions are then expected
starting next Tuesday with temperatures running close to 10
degrees below normal through around next Friday.

&&

.DISCUSSION...
High pressure has mostly vacated the region with the center of the
high roughly 1000 miles west of Baja and a weak shortwave trough
now approaching from the west. To the south, over the southern
part of the Gulf of California, Tropical Cyclone Ileana continues
to spin, but it is forecast to gradually weaken over the next
couple of days before dissipating along the west coast of Mexico.
Despite TC Ileana not having any direct impacts on our area this
weekend, it will at least help to advect moisture northward into
southern and central Arizona. Lower level moist southerly flow
started across our area earlier on Friday with the moist flow
now extending upward into the mid levels. This moisture advection
is forecast to continue through Sunday with PWATs increasing up to
around 1.5" and low level mixing ratios increasing to around 12
g/kg. The moisture advection today will largely go unnoticed as
skies remain clear to mostly clear and afternoon highs reach to
around normal.

Starting Sunday morning, the atmosphere over south-central Arizona
should be moist enough to promote some modest elevated instability
and possibly some scattered showers and a thunderstorm or two.
Going into Sunday afternoon, we should see our peak moisture and
instability over the area with a largely uncapped sounding profile
supporting 1000-1300 J/kg of CAPE. 0-6km Bulk Shear is also
forecast to be between 25-35 kts, while DCAPEs are expected to be
between 1000-1400 J/kg. This should to support a few strong to
severe thunderstorms Sunday afternoon across south-central
Arizona with severe winds and marginally severe hail possible. The
severe threat should dissipate quickly during the evening hours,
but the threat for isolated to scattered showers and thunderstorms
are likely to persist through the overnight hours Sunday night
into Monday.

By Monday morning, the large scale forcing will quickly increase
across the region as a strong Pacific trough tracks southeast
through northern California into Nevada. This system should help
to drive additional shower and thunderstorm development on Monday,
but it will also help to pull drier air northeastward into
Arizona throughout the day. The drier air will first surge in
aloft and it is likely to hamper or even completely cut off
convection west of Phoenix by noon Monday. However, there may be a
short period during the early to mid afternoon hours that could
bring some additional strong to severe storms focused over Pinal
and Gila Counties to as far west as Phoenix. By Monday evening,
the dry air is likely to push through all of the Arizona lower
deserts ending any rain chances with only lingering chances over
the higher terrain through mid to late evening Monday.

Forecasting rainfall amounts for this upcoming weather event is
actually fairly difficult as the showers and thunderstorms are
expected to be fairly scattered in coverage. This is mostly due to
the lack of large scale forcing with the Pacific trough mostly
bypassing our region to the north. Given the ample moisture
forecast to spread into south-central and eastern Arizona through
Sunday, any stronger storms could drop decent rainfall amounts of
0.5" or more. The fast ~25 kt storm motion will largely prevent
any one storm from causing any flooding issues, but any location
that sees a series of storms over a period of any hour or two
could see rainfall amounts of over an inch and may result in some
localized flash flooding. The latest forecast average rainfall
amounts still shows amounts ranging from 0.1-0.2" over western
portions of Maricopa County, 0.25-0.5" in the Phoenix area, to
upwards of an inch over portions of the higher terrain north and
east of Phoenix.

Near normal temperatures are expected through the weekend before
highs finally drop into the 90s starting Monday. Forecast highs
then drop into the upper 80s for Tuesday across portions of the
western deserts to the lower 90s over the south-central Arizona
lower deserts. Well below normal temperatures should then persist
through the rest of the work week as a second strong but dry
Pacific trough passes across northern portions of the region
Wednesday into Thursday. Forecast highs mainly stay in the lower
90s through Friday with overnight lows mostly in the mid 50s to
lower 60s across the western deserts to the 60s across much of
the south-central lower deserts. Model guidance eventually
signals a moderation of temperatures closer to normal for next
weekend as a ridge builds off the West Coast displacing the trough
to the northeast.

&&

.AVIATION...Updated at 1715Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Monitoring an increase in mid clouds and shower/thunderstorm
potential late in this forecast period. Intially, no concerns
across the terminals. Overall light and variable winds could
transition to westerly winds for a period tonight before the
diurnal east shift late tonight. By Sunday morning, mid level
clouds will be on the increase and we will likely see some
scattered showers begin around sunrise and persist through the
morning. Then, for the extended KPHX TAF, there is an increasing
thunderstorm threat around 21z Sunday so have made note of VCTS.


Southeast California/Southwest Arizona including KIPL and KBLH:
No weather concerns will exist through Sunday morning under mostly
clear skies. Winds will favor a SE component at KIPL, making a
shift to the southwest tonight. South winds will be favored at
KBLH.

&&

.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...Kuhlman
AVIATION...Frieders
FIRE WEATHER...Kuhlman