347
FXUS65 KPSR 171710
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1010 AM MST Wed Sep 17 2025

.UPDATE...
Updated Aviation

&&

.KEY MESSAGES...

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

- The potential for locally heavy rainfall and minor flooding
will be maximized on Thursday with the best chances across southwest
Arizona.

- Lingering showers and thunderstorms chances will be possible
Friday and focused more across the Arizona high terrain.

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

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...
The main forecast concern over the next 2-3 days will be the
potential for thunderstorms producing heavy rainfall which could
create some localized flooding issues. Forecast uncertainty is
still higher than normal, but with the dramatic increase in moisture
we will see over the next 24-36 hours it will at least bring some
chances for heavy rainfall.

TC Mario has transitioned into a remnant low, but it continues to
help to advect moisture northward with some of it already reaching
far southern California and southern Arizona. PWATs across our area
are still only between 1.0-1.3" over the bulk of the lower deserts,
but they now have reached as high as 1.5-1.7" over Yuma. Richer
moisture just to our south is forecast to advect northward up the
Lower CO River Valley today and then spread outward with PWATs of
1.5-1.8" likely over much of our area by late this evening.

Rain chances will start to pick up later this afternoon, especially
across southeast California and southwest Arizona where some
scattered showers and maybe a few isolated thunderstorms are
expected. The lack of strong instability and relatively low DCAPEs
means the threat for any strong storms will be fairly low today,
while the risk of heavy rainfall also remains on the low side. A
wing of vorticity is then forecast to move from south to north
across the area Thursday morning likely sparking off some additional
showers and isolated thunderstorms. CAMs show this activity likely
being focused more across southwest Arizona.

Thursday is still looking like the most active weather day of this
week as we reach peak moisture levels in the morning and upper level
support is expected to be at its highest. However, model uncertainty
remains fairly high as CAMs have been in constant disagreement on
the amount of convection. Model runs from 18Z and 00Z yesterday had
backed off on the potential convection for Thursday and this has
filtered into the NBM PoPs lowering again, but the 06Z HRRR has just
come in looking very optimistic for storms on Thursday. The best
chances for showers and thunderstorms still looks to favor southeast
California and southwest Arizona on Thursday with PoPs of 40-60%,
while areas to the east in Arizona are mostly between 20-40%. The
06Z HRRR also shows the potential for nearly stationary or back
building storms (focused over La Paz into southern Maricopa
Counties) which means the potential for heavy rainfall and flooding
will definitely be present.

The rainfall event Thursday is likely going to fall under a scenario
where a few localized spots see heavy rainfall of 1-2", a bit larger
area sees amounts of 0.25-0.75" and about half of the area see
little to no rainfall. WPC has maintained their Marginal Risk on the
Day 2 Excessive Rainfall Outlook, but they have mentioned the
potential for upgrading to a Slight Risk for a portion of the area.

By Thursday night into Friday, guidance has been consistent in
showing considerable dry air advecting southwest to northeast across
the region. This is expected to happen as the trough to our west
lifts to the north northeast into central California and Nevada.
Given our residual good moisture within the low and mid levels on
Friday, we are still likely to see some scattered showers and maybe
some isolated thunderstorms but much of the activity is likely to be
focused over higher terrain locations.

&&

.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 (particualrly
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
slightly late week with increased cloud cover and rain chances. Rain
chances will begin today mainly over the western districts before
peaking on Thursday areawide. Chances of wetting rains on Thursday
range from 30-50% over the western districts to 20-30% over eastern
districts. MinRHs will continue to improve into Thursday mostly
staying above 25-30% into Saturday. Outside of any storms, 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...None.

&&

$$

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