553
FXUS65 KPSR 281138
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
438 AM MST Mon Sep 28 2026

.UPDATE...Updated Aviation Discussion.

&&

.KEY MESSAGES...

- Bands of mostly light rain will be present over the region for
  much of today with more isolated moderate to heavy showers and
  thunderstorms possible by mid to late afternoon.

- A Flood Watch has been issued for portions of the region with a
  focus on tonight through much of Tuesday for higher terrain
  areas of central and eastern Arizona. Moderate to heavy rainfall
  could also impact portions of the Arizona lower deserts, but it
  is expected to be more localized.

- Below normal temperatures are expected through the middle part
  of the week before warming to slightly above normal by the
  weekend as much drier conditions eventually spread over the
  region.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...
Flood Watches go in effect for much of the region today and
tonight with mid and upper level moisture already streaming into
the region from the southwest followed by an additional influx of
lower level moisture starting later this morning. Forecast
moisture levels are quite good for today and tonight, but not
quite full tropical moisture levels. Still, forecast PWATs reach
between 200-250% of normal with the bulk of the moisture held
within 725-350mb layer (8000-25000 ft AGL). We have had other
events in recently memory with better moisture this time of year,
but this one will involve plenty of moisture for the incoming
strong Pacific trough to work with.

Current objective analysis shows the Pacific trough now digging
southeastward into western portions of the region with a closed
low moving into central California. The trough continues to be
supported by a strong polar jet and now has become linked up with
a modest subtropical jet situated over the southern tier states.
Satellite imagery continues to show a stream of mid and upper
level remnant moisture into the Southwestern U.S. from what used
to be TC Odalys. The very light shower activity on Sunday has
picked up in intensity since late evening, partially due to the
improving moisture but mainly due to the increasing forced ascent
from the incoming Pacific trough. Light shower activity has also
begun to blossom over south-central into eastern Arizona since
midnight, much farther east than the main band of rain. Hi-res
CAMs seem to be handling the rain band across southeast California
and southwest Arizona, but not the latest development to the
east. Our expectations are for this banding light rain to continue
through at least this afternoon with a few embedded moderate
showers or thunderstorms at times. Rainfall amounts through this
afternoon in this jet forced ascent and upslope region of the
storm system should mostly remain below 0.5".

By this afternoon, we will begin to see some lower level moisture
advection into Arizona from the weakening TC Polo, but the
NHC track of Polo remains well to our south and southeast. Once
the better lower level moisture enters the picture this afternoon,
some areas across southern and central Arizona may start to see
better rainfall rates, but the lack of decent instability will
largely keep thunderstorm potential fairly low. CAMs generally
show a more solid band of showers and embedded thunderstorms
forming somewhere from around Phoenix and/or south and east of
Phoenix by late afternoon before moving to the east through the
evening hours. Rainfall amounts with this band could reach
0.25-0.5".

For the western lower deserts, the current rain band should move
out of the area later this afternoon leaving a break in the rain
and potentially some breaks in the clouds leading to CAPE building
to around 500 J/kg. This increased instability should be available
once the strong forcing associated with the low center moves into
southern California later this evening. CAMs show scattered storms
developing by early evening just west of the Colorado River and
becoming more of a broken line of showers and storms as it moves
into western Arizona by around midnight tonight. This broken line
of storms should present the heaviest rainfall potential for
tonight across western Arizona before gradually pushing eastward
through central Arizona Tuesday morning. Rainfall rates upwards of
an inch per hour seem likely with this convection as well as
threat of some strong winds. Guidance since yesterday has been
trending toward a heavy rain threat for La Paz County, so a we
have added that area to the Flood Watch (in effect during the
evening and early overnight hours. Localized rainfall amounts of
1-2" are possible within the Watch area.

As the Pacific low continues to slowly track east southeastward
through our region on Tuesday, we expect the broken line of
showers and thunderstorms to move through south-central Arizona
during the morning hours and mostly during the afternoon across
eastern Arizona. As the line moves east on Tuesday, it will
encounter even better moisture but instability will be marginal
at best (250-500 J/kg). Tuesday morning through the afternoon will
present the best time frame for moderate to heavy rainfall across
south-central and eastern Arizona with rainfall rates increasing
to over 0.5" in some locations. This will add to the rainfall
totals with average rainfall amounts between 0.5-1.0" across much
of the Arizona lower deserts to 1-2" over the higher terrain north
and east of Phoenix. The latest NBM (maybe too optimistic) shows
48 hour QPF probabilities of 2" or greater at around 20% in the
Phoenix area to around 40% over higher terrain areas. WPC`s
updated Excessive Rainfall Outlooks for today and Tuesday covers
most of the Arizona lower deserts in a Slight Risk (15%
probability within 25 miles of a point) and a Moderate Risk (40%
probability) over the higher terrain.

The low center is forecast to move right through central Arizona
Tuesday afternoon and evening which may also bring a period of
wrap-around showers and isolated thunderstorms a few hours after
the main rain band moves through. The latest HRRR is now showing
this potential focusing in on northern portions of La Paz County,
Maricopa County, and through Gila County. If we manage to get any
stronger storms with this part of the system, some small hail
should be likely. Chances for showers and isolated storms will
quickly diminish later Tuesday evening with some lingering
chances persisting over eastern Arizona Tuesday night.

The temperatures forecast is mostly an afterthought for the first
part of this week, but it does bring big changes beginning today.
The latest NBM forecast highs show readings only reaching into
the lower 80s today in the Phoenix area to the mid to upper 80s
across the western lower deserts. Tuesday is little different with
highs potentially struggling to top 80 degrees in the Phoenix
area.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
The low is forecast to move into New Mexico by Wednesday morning
and well into the Plains by Thursday with dry air advection and
subsidence increasing across our area. Generally clear skies are
expected for the lower deserts on Wednesday and eventually over
eastern Arizona by later the afternoon. Fairly strong ridging is
then forecast to move into the region on Thursday with H5 heights
quickly rising to between 586-590dm. Despite the quick boost in
heights aloft, temperatures will be somewhat slow to recover,
especially over the eastern half of Arizona. Highs should stay
below 90 degrees in the Phoenix area through Thursday with the
lower deserts warming back into the mid to upper 90s. As the
boundary layer gradually dries out going into the weekend, we are
likely to return to our typically lower humidity levels while
temperatures warm to slightly above normal.

&&

.AVIATION...Updated at 1140Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

The main aviation concern will be periods of SHRA and potentially
some isolated/embedded TS (up to 20-30% chances). Confidence is
good that a fairly prolonged period of SHRA will prevail beginning
mid to late morning and lasting through the afternoon. A period
of dry conditions is then expected overnight before the next
round of activity early Tuesday morning. Tomorrow`s activity
should mainly be RA, but a few embedded TS cannot be completely
ruled out. Wind directions will favor N/NW through the early
evening hours before shifting E`rly tomorrow evening. Wind speeds
will generally remain AOB 10 kts, but a period of elevated winds
and occasional wind gusts up around 20 kt are possible with the
SHRA. VFR CIGs will prevail through the TAF period, with bases
between 6-10K ft AGL. During shower activity, particularly early
Tuesday morning FEW-SCT lower decks (as low as 3K ft AGL) will be
possible.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation concern will be lingering SHRA for the next few
hours followed by another round early this evening with decent
chances (25-50%) of TS near KBLH. CIGs should remain AOA 8K ft AGL
through the period, but FEW-SCT decks as low as 5-7 kft are
possible with any RA. If a storm directly impacts KBLH, VIS will
likely drop into an MVFR or lower category temporarily. CIGs with
these storms could also go down to 4-6 kft with FEW-SCT clouds as
low as 3 kft. Wind directions will continue to vary between the
SE/S/SW at both TAF sites. Wind speeds will generally be around
7-12 kt, but some occasional gusts of 15-25 kt are possible with
showers/nearby showers and storms.

&&

.FIRE WEATHER...
Elevated moisture levels and periods of showers and thunderstorms
are expected through Tuesday. The peak time frame for rainfall
should be today over the western districts and tonight through
Tuesday afternoon for the eastern districts. Chances for wetting
rain between 40-80% should greatly limit any threat of dry
lightning starts. Outside of enhanced gusts associated with
stronger storms and some marginal northwesterly breeziness for the
western districts Tuesday, winds will follow light and diurnal
trends otherwise. Drier conditions and near to slightly above
normal temperatures are forecast for later this week and through
next weekend.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...     Flood Watch from late tonight through Tuesday evening for AZZ534-
     537-541-542-545-547-549-552-555>558-560>563.

     Flood Watch from this evening through late tonight for AZZ533.

CA...None.
&&

$$

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