239
FXUS65 KPSR 282321
AFDPSR

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

.UPDATE...
Updated Aviation

&&

.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 early
evening.

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

- Below normal temperatures are expected through 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/...
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 ECMWF 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 2320Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Weather issues through Tuesday will include periods of SHRA/TSRA,
lower CIGs, and erratic wind directions. Confidence is good that
banded -SHRA will gradually clear the PHX metro by mid evening with
light E/NE winds becoming more variable and cloud decks flirting
with 060 cigs. A break in -SHRA and lower cloud decks is likely late
evening with moderate confidence of additional SHRA and lowering
flight categories moving into the region overnight. Periods of MVFR
vsby are almost certain in moderate SHRA with cigs falling below 6K
ft AGL and winds becoming N/NW. Brief instances of IFR vsby/MVFR
cigs is possible around sunrise, however confidence is too low to
include in this TAF package. Winds should turn E/NE as SHRA moves
east of terminals mid morning, however guidance suggests directions
quickly shifting to W/SW early Tuesday afternoon as 040-050 BKN
decks develop with afternoon heating.


Southeast California/Southwest Arizona including KIPL and KBLH:
The primary weather concern will be the potential for SHRA/TSRA at
KBLH tonight, then gusty NW winds Tuesday. Confidence is good that
S/SE winds will prevail this evening before veering to W/NW by late
evening. While confidence is rather low, scattered SHRA/TSRA may
develop near KBLH around midnight before moving eastward overnight.
Confidence is excellent that winds will become N/NW Tuesday with
gusts 20-25kt developing in the afternoon.

&&

.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...18
FIRE WEATHER...Kuhlman