368
FXUS65 KPSR 282119
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
219 PM MST Mon Sep 28 2026

.KEY MESSAGES...

- Bands of light to moderate rain will be present over the region
  for the remainder of today with isolated heavier shower and
  thunderstorms possible over the western deserts during the
  afternoon and early evening.

- A Flood Watch has been issued for portions of the region with a
  focus on tonight through much of Tuesday for the majority of
  south-central Arizona.

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

&&

.SHORT TERM /Today through Tuesday/...
The combination of tropical moisture from Hurricane Polo and an
approaching Pacific low pressure system will continue to promote
rounds of shower and thunderstorms through the next 24-30 hours
or so. Through the remainder of today, bands of light to sometimes
moderate rainfall will be present across portions of the region
as enhanced southwesterly flow diverges from its typical dry
status to provide the region with PWATs upwards of 175-250% of
normal. However, the best mid and upper-level forcing and
instability will be focused more over the westernmost portion of
our forecast area, so most locations will see primarily a nice
periodic light to moderate rain through at least this evening.

The one exception to the lighter rainfall will be those
aforementioned western areas (mainly locations around the Colorado
River Valley), where heavier convection may be realized. There
are signs that enough clearing will take place to allow some
decent destabilization to take place, pushing MLCAPE values
toward 500-750 J/kg. With these areas also located under the best
ascent due to their closer proximity to the deepening low, the
environment will be favorable for the development of at least
some isolated thunderstorms. Add in 0-6km shear of 50-60 kt and
the potential for strong to even severe storms capable of damaging
winds and even small hail will exist as well, though these should
be very isolated at best. This heavier convection will also
present the concern for flash flooding, as rain rates could reach
upwards of over an inch an hour. Guidance points towards the best
coverage of heavy rain being focused over La Paz County, and
because of that, this area was added to the existing Flood Watch
(in effect from this evening into the early overnight hours) last
night.

As the Pacific low continues its track southeastward into Tuesday,
the more favorable environment to see further rainfall activity
will begin to shift toward South-Central Arizona. As this
progression occurs, shower and thunderstorms will begin to blossom
over Maricopa and Pinal Counties soon after midnight. Activity
will last through the remainder of the morning and into the
afternoon, with the easternmost portions of our coverage area even
seeing the rainfall window stretch into the evening. 0-6 km shear
will continue to be quite impressive hovering around 50-60 kt,
but even with a bit of low-level warm air and moisture advection
leading to a slight increase in instability, the severe risk
should be fairly low as any storms that initiate and can be
maintained should only be isolated. Perhaps a low potential severe
threat reaches into the afternoon if daytime heating can aid in
further destabilization, but the areas that will see the best
insolation during the warmest point in the day will likely be
behind the best atmospheric support and moisture. By the this
timeframe, any severe storm chances would likely be confined from
around central Phoenix and eastward.

The main concern for Tuesday will be the potential for heavy rain
and flash flooding. The greatest flooding threat will be for
foothill and high terrain areas to the north and east of the Phoenix
metro, where rain totals may reach upward of 1-2.5". The Flood Watch
that was issued Saturday will go into effect late tonight,
lasting through the evening timeframe. As for the lower deserts
of Maricopa and Pinal Counties, the heavy rain signal has
increased. HREF members have been consistent since yesterday
indicating at least a 30-50% chance of seeing greater than 1"
within 25 miles of any point for these counties, while the ECMFW
ensemble now has around 25% of its members showing upwards of 2"
or greater at Sky Harbor Airport. Not to mention that EFI (Extreme
Forecast Index), along with model shift-of-tails, a value
indicating that anomalously high QPF values are showing up in
numerous ensembles members, have been pulled back toward lower
elevation areas. The preponderance of guidance led to an expansion
of of the Flood Watch to include all of Maricopa and Pinal
Counties.

There are some signs of lingering convective showers and
thunderstorms behind the center of the low as warp-around moisture
and insolation will help keep instability in place. Due to wake
subsidence, anything that does pop will be reliant on enhanced
terrain features providing orographic ascent. Hi-res data has been
inconsistent on where it places this convection, but isolated
cells may be seen as far west as La Paz County. By the time all
this rain is said an done, most lower desert east of the Colorado
River will anywhere from 0.5-1.50", while high terrain areas can
expected closer to 1.50"-2.50". Locally higher amounts will be
possible under heavier showers and storms. West of the Colorado
River, rain totals will vary widely from a few hundredths in the
driest spots to potentially greater than 1" if the previously
mentioned strong convection is realized.

The temperature forecast has taken a back seat to all this talk of
rain, but for the fans of non-triple digit readings, you are in for
a treat. Below-normal readings will be common across the region
through today and tomorrow, with highs this afternoon ranging in the
upper 70s to lower 90s, a good 10+ degrees below what is typical for
late September, with much of the same expected for Tuesday.

&&

.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
0808eastern 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 1830Z.

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

The main aviation weather concerns throughout the TAF period will
be the potential for periods SHRA and embedded TS activity and
lower CIGs. Currently, a band of light SHRA is currently moving
through the terminals and will persist through the early/mid
afternoon hours with SCT-BKN CIGs as low as 6 kft and brief MVFR
visibilities likely. Conditions should be mostly dry late
afternoon through late tonight although areas of VCSH cannot be
ruled out. Confidence is good that another round of heavier SHRA
with embedded TS (25-35% chance) is likely between 10z-16z,
reducing visibilities to MVFR/IFR conditions and CIGs as low as 3
kft. Wind directions through early this afternoon should be out
of the NNW-N before gradually shifting out of a NE-E component by
the mid to late afternoon hours with speeds aob 10 kts, although
locally higher gusts upwards of 20-25 kts will be possible with
the shower activity.

Southeast California/Southwest Arizona including KIPL and
KBLH:

The main aviation concern will be the potential for TS activity at
KBLH during the evening hours. In the meantime, conditions should
remain dry through much of the afternoon, before scattered storms
develop, mainly in the vicinity of KBLH, during the early evening
hours. CIGs should remain aoa 7 kft throughout the period
although could lower to 4-6 kft with any SHRA activity. Otherwise,
wind directions will vary between southeast to southwest through
this evening before becoming light and variable overnight. A shift
to the north to northwest is expected by Tuesday morning. Overall
wind speeds should range between 7-12 kts with briefly higher
gusts with the shower/storm activity.

&&

.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 midnight MST tonight through Tuesday evening
     for AZZ534-537>563.

     Flood Watch from 6 PM MST this evening through late tonight for
     AZZ533.

CA...None.
&&

$$

SHORT TERM...RW
LONG TERM...Kuhlman
AVIATION...Lojero
FIRE WEATHER...Kuhlman