103
FXUS65 KPSR 172145
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
245 PM MST Wed Sep 17 2025

.KEY MESSAGES...

- Rain chances will increase through Thursday with the best chances
for showers and thunderstorms across southeast California and
southwest Arizona.

- A Flood Watch has been issued from early Thursday morning through
late Thursday night for southeastern California, due to the
potential for locally heavy rainfall leading to flash flooding
concerns.

- Lingering showers and thunderstorms chances will be possible
Friday with the best chances across south-central and eastern
Arizona, particularly over the Arizona high terrain.

- Temperatures will fall below normal tomorrow before quickly rising
over the weekend into early next week.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...
The main forecast concern over the next couple of days continues to
be the potential for thunderstorms to produce locally heavy rainfall
leading to localized flash flooding concerns. This is a high impact
low confidence forecast as there are a lot of different variables
that will affect the overall outcome. With an abundance of moisture
in place and some good mid-level forcing, the ingredients are there
to produce isolated areas of very high rainfall amounts.

Early this afternoon objective analysis reveals a closed low
pressure system off the south-central coast of California, the
remnants of TC Mario well off the central coast of the the Baja, and
a subtropical high over southwest CONUS. The combination of the
closed low and the remnants of TC Mario have brought an abundance of
moisture into our region. The SPC mesoanalysis shows PWATs of 1.0-
1.5" across south-central and southeastern AZ and Riverside County
in southern CA and PWATs of 1.5-1.8" across southwestern AZ and
Imperial County in California. PWATs will likely rise another few
tenths, pushing PWATs to 1.5-1.8" across much of the CWA, over the
next 24 hours as the remnants of TC Mario continue to push northward.

With the increase in moisture, some isolated showers and storms have
already formed early this afternoon along prominent terrain
features. Due to the lack of forcing, the terrain features are
needed to help with the lift and formation of any activity today.
Also due to the lack of forcing today, storms will quickly fall
apart as they move away from terrain features. The SPC mesoanalysis
shows a mostly uncapped environment with CAPE values around 1000-
2000 J/kg across SE CA and SW AZ, along with DCAPE values of 1200-
1500 J/kg. So, some strong storms and a few severe storms are
possible. Corfidi vectors do support training storms today, and with
the high PWATs, some flash flooding is possible.

Heading into the overnight hours, models show the remnants of TC
Mario bringing a wave of vorticity up through the region. This will
likely spark some isolated to scattered showers and storms across
the area during the overnight and early morning hours tomorrow. This
activity and its associated cloud cover will play a major role in
how the rest of Thursday plays out. If the cloud cover hangs on and
the atmosphere can`t destabilize again, the threat for showers and
storms, particularly strong storms, will decrease for tomorrow
afternoon and evening.

Depending on how the overnight and early morning hours play out, the
biggest concern will be Thursday afternoon through the overnight
hours into Friday morning. The remnants of TC Mario will swing
another arm of vorticity through the region, mainly western portions
of the CWA, during the afternoon and evening hours. If the cloud
cover clears out enough from the morning activity, CAMs show MUCAPE
values of 1500-2000 J/kg. This instability in combination with the
vorticity and abnormally high PWATs (150-200% of normal) make for an
environment favorable for heavy rainfall. Storm motion will be
around 20 kt, which is not slow enough to be favorable for flash
flooding. However, the corfidi vectors will once again be
favorable for training storms. And with the high PWATs, if you get
training storms, flash flooding would be likely. Soundings also
show an environment that could capitalize on warm rain processes.
HREF members highlight SE CA as the biggest threat, with less
activity across AZ, particularly south-central and eastern parts
of the state. The HRRR, is the outlier and shows rather robust
activity across south-central and southwestern AZ compared to the
other members. So, the potential is still there for heavy rainfall
and flooding across our AZ locations, particularly SW AZ.
However, better confidence exist across SE CA. The WPC has
upgraded to a Slight Risk for their Day 2 Excessive Rainfall
Outlook across SE CA and far SW AZ. A Flood Watch was issued for
SE CA (excluding the lower CO River Valley) from 5am PDT Thursday
through 2am PDT. Rainfall totals: 0.00-0.50", isolated 1-2"+. Some
people will not see any rainfall tomorrow, this is not a
widespread event.

As for Friday, guidance continues to show dry air quickly advecting
into our area with PWATs falling to 1.1-1.5" across the CWA by
Friday night. Isolated to scattered showers and storms will still be
possible across the region. CAMs show a wave of activity moving
through mainly during the morning and afternoon hours and drying out
quickly during the evening. This looks to be the best timeframe for
activity across south-central AZ, including Phoenix, and the higher
terrain in eastern AZ. However, PoPs only max out around 30-40%
across the lower deserts.

Temperature wise with the abundance of cloud cover rain chances,
temperatures will fall below normal with highs in the mid-to-upper
90s across the lower deserts. Temperatures could be lower depending
on the timing of the rainfall and associated cloud cover.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As we continue to dry out Saturday into Sunday, rain chances will
essentially come to an end with very minimal 10% chances for a
shower or weak storm over higher terrain locations. Forecast PWATs
are shown to drop to between 1.2-1.4" by Saturday afternoon and
closer to 1" for much of the area on Sunday. Our region will also
see the subtropical ridge centered to our south shift more over
our region with H5 heights rising to between 590-592dm again. The
near normal temperatures that are expected on Thursday and Friday
will quickly trend higher beginning this weekend with highs likely
reaching 100 degrees Saturday and 101-104 degrees Sunday. Moderate
HeatRisk is also expected to make a return this weekend across the
lower deserts.

Model guidance is starting to point toward even hotter temperatures
during the first half of next week as a potential second Pacific
trough is likely to stay far enough to our west to allow the
subtropical ridge to fully move over our region by Monday or
Tuesday. If this happens, we very well could see H5 heights increase
to around 594dm, or nearing climatological records for the period.
The latest NBM forecast highs show readings topping out near 105
degrees by Monday before peaking between 105-108 degrees on Tuesday
and/or Wednesday of next week (right near daily records). We can`t
even rule out the potential for reaching 110 degrees in a few spots.

&&

.AVIATION...Updated at 1710Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The potential for SHRA/TSRA Thursday will be the greatest forecast
challenge under thickening mid/high level cigs. Confidence is good
that light east winds will become W/SW early afternoon with only
modest gustiness. However, an increasing HZ layer and potential
gusts/minor lofted dust from storms well to the SW could affect
slantwise vsby mid/late evening. Timing of the overnight nocturnal
easterly wind shift is more uncertain, and it would not be out of
the question for some elevated SHRA to sweep across the PHX airspace
Thursday morning with a thick midlevel cloud deck (at this time,
chances are too low to include in this TAF iteration). Any
further TSRA impacts will likely be timed into Thursday
early evening, and future TAF packages may need a Prob30 mention
during this time frame.

Southeast California/Southwest Arizona including KIPL and KBLH:
Fairly low overall forecast confidence this TAF package due to a
marked increase in moisture and heightened SHRA/TSRA chances. While
precise wind directions at any hour may be dictated by proximity to
SHRA/TSRA influence, a general southerly flow will be preferred
through the period. Later this afternoon, there is a 10-20% chance
of TSRA moving into the vicinity of the aerodromes (particularly
KIPL), but not enough evidence exists to mention in this TAF
package. Better chances of SHRA/TSRA should arrive Thursday morning
and have introduced VCSH mid/late morning. Larger impacts to
operations may occur into the afternoon hours and stronger
indications may be needed in future TAF updates.

&&

.FIRE WEATHER...
Above average temperatures will continue today before cooling off
a few degrees below normal on Thursday and Friday with increased
cloud cover and rain chances. Isolated showers and storms have
already popped up over the western districts this afternoon
Chances will peak areawide tomorrow, with the best chances across
SE CA. Chances of wetting rains on Thursday range from 60-70%
over the western districts to 20-30% over eastern districts.
MinRHs will continue to improve mostly staying above 25-30% into
Saturday. Outside of any storms, winds will remain light over the
next few days with directions mostly following diurnal trends.
High pressure will settle back into the region over the weekend
and through much of next week resulting in a warming and drying
trend.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Flood Watch from late tonight through late Thursday night for
     CAZ560>568-570.

&&

$$

SHORT TERM...Berislavich
LONG TERM...Kuhlman
AVIATION...18
FIRE WEATHER...Berislavich/Kuhlman