794 
FXUS65 KPSR 181808
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1108 AM MST Thu Sep 18 2025

.UPDATE...Updated Aviation Discussion.

&&

.KEY MESSAGES...

- A Flood Watch has been issued from early this morning through
  this evening for southeastern California and southwest Arizona,
  due to the potential for locally heavy rainfall leading to flash
  flooding.

- Lingering shower and thunderstorm chances will be possible
  Friday, primarily focused across the Arizona high terrain.

- Near normal temperatures over the next couple of days should
  warm to above normal this weekend into early next week with
  Moderate Heat Risk returning across the lower deserts.
  
&&

.SHORT TERM /TODAY THROUGH FRIDAY/... 
Deep tropical moisture has advected northward into southeast
California and southwest Arizona over the past 12-18 hours with
PWATs at around 2" in Yuma and 1.6-1.9" for the rest of the
aforementioned areas. Good moisture also extends eastward through
much of south-central Arizona with PWATs between 1.3-1.6". The
remnants of TC Mario have also merged with a weak upper level low
off the coast of southern California providing ample dynamics for
showers and a few embedded thunderstorms across southern 
California early this morning. 

The forecast for the rest of today into tonight remains
challenging due to a number of factors. The moisture has basically
reached its peak across the region and drier air above 15-20K feet 
is already starting to push north northeastward across northern 
Baja. The drier air should reduce the cloud cover to a degree 
later this morning allowing for better destabilization across 
southeast California and southwest Arizona. Assuming this occurs, 
it should give rise to scattered thunderstorm development as early
as mid to late morning, but more so during the afternoon with 
PWATs still hovering somewhere between 1.7-1.9" across the 
aforementioned areas. Hi-res guidance is still quite unsure on the
areal coverage for thunderstorms today, but it has at least been 
consistent in showing the potential for heavy rainfall. It seems 
quite likely a portion of this area will see anywhere from 1-2" of
rainfall today with some low-end potential for very localized 
amounts exceeding 2". The best potential for heavy rainfall and 
any flooding impacts are likely to occur across southeast 
California into La Paz County. We have expanded the areal 
coverage of the Flood Watch to include all of La Paz County and 
much of Yuma County, although Hi-res CAMs have not specifically 
shown much activity for Yuma County. 

Looking farther to the east across south-central and eastern
Arizona, guidance continues to struggle with the rainfall
potential for today. Some model runs have indicated heavy rainfall
potential into western and southern Maricopa County, but they have
been quite inconsistent and have somewhat backed off on this
threat over the past 24 hours. Given moisture will still be quite
high into south-central Arizona, we can't rule out some 
thunderstorms producing heavy rainfall but the chances are much 
lower compared to out west. It would not be surprising to see at 
least one or two good thunderstorms later today that drop 1" 
rainfall amounts outside of the Flood Watch area, but given the 
low probability we do not see a need to expand the Watch into 
Maricopa County. Rainfall amounts across south-central Arizona may
only average around 0.1" with a good portion of the area barely 
seeing any rainfall. 

The drier air aloft will spread across the rest of the region by 
tonight with any shower and thunderstorm activity diminishing
considerably by around midnight. Forecast PWATs are expected to
drop to between 1.2-1.5" already by Friday afternoon with the best
rain chances shifting over the Arizona high terrain. Some
locations to the north and east of Phoenix may see an additional
0.25" of rain on Friday, but most locations will barely see any
measurable rainfall. The 06Z HRRR is quite optimistic for showers
and thunderstorms between Phoenix and Flagstaff Friday
afternoon/early evening, but that seems to be an outlier at this
point.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/... 
Model uncertainty will continue over the weekend into early next
week as models are having a difficult time resolving what will
happen with a cut-off low that is likely to form off the 
California coast Friday into Saturday. Yesterday's model runs had 
mostly dismissed this cut-off low as it called for it stay well 
to our west, but the latest trends indicate it may get quite close
to southern California Sunday into Monday. We are still expecting
our region to largely be under the influence of the subtropical 
ridge this weekend with overall dry conditions, but if the low 
does creep close enough to our area we may see some moisture 
return Sunday into Monday. The NBM shows 10-20% rain chances for 
later Sunday into Monday, but this seems overly optimistic given 
the amount of moisture that is likely to be available. We tend to 
believe the ridge will win out keeping the cut-off low far enough 
to the west resulting in the bulk of our area staying dry through 
at least early next week. 

Temperatures will be trending upward this weekend into early next
week as the ridge builds in from the south, but with the model 
trends of keeping the cut-off low closer forecast highs are 
noticeably lower than what we saw 24 hours ago. Highs are
currently expected top out at around 100 degrees this weekend 
before pushing closer to 105 degrees by around next Tuesday. The 
upper end of guidance does still show the potential for record 
highs for Phoenix next Tuesday and Wednesday, but that will only 
happen if the cut-off low stays far enough to our west.

&&

.AVIATION...Updated at 1808Z. 

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The primary weather concern during the TAF period will be the
potential for thunderstorms across the Phoenix area, though
forecast confidence is low. Morning shower activity will continue
to lift north of the terminals over the next hour or so with
clearing conditions. Confidence detioriates later this afternoon
through this evening on how thunderstorm activity will evolve.
Latest hi-res guidance continue to show large discrepancies in 
the overall coverage of thunderstorms this evening, yielding low 
forecast confidence. Have maintained the PROB30 in the TAFs for 
now when thunderstorm chances will be the highest. Any 
thunderstorms that develop will be capable of producing gusty 
erratic outflows, localized blowing dust, and locally heavy 
rainfall. Guidance then suggest the potential for additional 
shower/storm activity overnight tonight. SCT-BKN cloud bases will 
mostly remain aoa 10 kft throughout the period, though could 
briefly drop lower in thunderstorms. Outside of any thunderstorm 
impacts, winds will generally follow diurnal trends with speeds 
mostly below 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:
The primary weather concern during the TAF period will be
increasing chances for thunderstorms going into this afternoon.
Hi-res guidance show thunderstorms developing across SE CA this
afternoon that will be capable of producing gusty erratic outflow
winds, localized blowing dust, and locally heavy rainfall. Chances
for outflow wind gusts in excess of 30 kts climb to 50% during
this time. The primary window for these thunderstorms looks to be
between 21Z-02Z. SCT-BKN clouds will mostly remain aoa 8-10 kft
through this evening, briefly dropping lower in thunderstorm
activity, before clearing this evening. Outside of thunderstorm
impacts, winds will favor the W-NW at KIPL and fluctuating between
the S to NNW at KBLH with speeds mostly below 10 kts.

&&

.FIRE WEATHER...
Abundant moisture with a weather system residing across western
portions of the region today will help to bring fairly widespread
showers and thunderstorms. Best chances will be across southeast 
California and southwest Arizona where a Flood Watch will be in 
effect for much of today. Rain chances will continue into Friday, 
but mainly across the Arizona higher terrain. Humidities will be 
elevated over the next couple of days with MinRHs mostly between 
30-40% before gradually lowering closer to 20% by Sunday. Outside 
of any storms, winds will remain light over the next few days with
directions mostly following diurnal trends. High pressure should 
settle back into the region over the weekend and into early next
week resulting in a warming trend and overall dry conditions.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through late tonight for AZZ530>533-536.

CA...Flood Watch through late tonight for CAZ560>570.

&&

$$

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