928
FXUS65 KPSR 280556
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1056 PM MST Sun Sep 27 2026

.UPDATE...Updated 06Z Aviation Discussion.

&&

.KEY MESSAGES...

- Bands of light rain will be present over the region through the
  remainder of today into Monday morning before more scattered
  showers and thunderstorms become possible starting Monday
  afternoon.

- A Flood Watch has been issued for Monday night and much of
  Tuesday for higher terrain areas of central and eastern Arizona.
  Moderate to heavy rainfall may also impact portions of the
  Arizona lower deserts, but it is expected to be more localized.

- Near to below normal temperatures are expected through the
  middle part of the week before warming to slightly above normal
  by next weekend.

&&

.SHORT TERM /Today through Tuesday/...
The Desert Southwest finds itself in the middle of a number of
weather regimes with sub- tropical high pressure to our southeast,
an approaching Pacific low to the northwest, and Tropical
Cyclones Odalys and Polo still lingering over the Eastern Pacific.
Due to the position and interactions of the former two systems,
enhanced southwesterly flow aloft is currently pulling in sheared
moisture away from the center of what remains of Odalys toward our
forecast area resulting in considerable cloud cover. Though the
aforementioned cold-core is still several hundred miles away, its
associated ascent appears to have a far enough reach to lift the
tropical moisture and set off some light rain showers over the
southwestern CONUS. Bands of rain are likely to be observed
through tonight as the current setup will not change much over the
next through tonight allowing for more and more moisture to get
pumped in. The lack of large scale ascent will limit coverage so
not everyone will see showers, and even those who do, it should
not amount to much as a decently dry-sub cloud layer is in place.
A few hundredths to maybe around a tenth of an inch at most is
anticipated.

As we transition into Monday, the Pacific trough will begin to dive
to the southeast while Hurricane Polo inches closer to the Baja
Peninsula, increasing regional ascent and enhancing moisture
profiles even further across the region. This combination will
help to produce some heavier bands of rain compared to this
afternoon, and if enough surface heating can take place through
the day, isolated to scattered thunderstorms will also be
possible. The best chances for convective activity will be
focused around the Lower Colorado River Valley as model guidance
points towards enough clearing taking place to where marginal
atmospheric destabilization can take place. The current HREF run
indicates SBCAPE values reaching 100-500 J/kg across our western
most counties, not all that impressive, but enough to potentially
spark some strong to even severe storms given the dynamics in
place. Any storms that can form will not be the typical pop-up
monsoon storms. With 0-6 km shear, it isn`t out of the question
that could become sustained supercells capable of producing
strong/damaging winds even some small hail.

Elsewhere across South-Central Arizona, continued abundant cloud
cover will limit destabilization, but a few rumbles of thunder
cannot be completely ruled out due to the more vigorous ascent as
low a draws closer along with orographic enhancement. Rain totals
will vary greatly from locations to location. Most areas can
expect anywhere from a few hundredths up to half an inch, with
locally higher amounts in the high terrain north and east of
Phoenix along with areas of the western deserts. Isolated
instances of flash flooding cannot be completely ruled out for
those western areas if strong storms were to be realized.

Tuesday is shaping up to be the most interesting day of the next
three, and how things evolve will depend mainly on the locations of
the cold-core. Models are still not in full agreement regarding
where the center of circulation will be come Tuesday morning, but
its eventual location will determine where more rounds of showers
and thunderstorms develop. Regardless, the interaction between
the lingering tropical moisture and the anomalously strong low
will at the very least maintain ongoing activity and/or produce
more during the pre-dawn hours. Most, if not all, the rainfall by
this point will be focused from the Colorado River on eastward.

Even without daytime heating, hi-res models indicated an increase in
instability ahead of sunrise, likely a result of low-level warm air
and moisture advection. The WAA alone will increase support for
further showers, but the addition of instability will enhance
convective potential as well. CAPE values once again will not be
overly impressive, hovering around 500 J/kg, but the dynamic
environment will be the main reason why some of the convection
could be strong to severe as 0-6 km shear over Southwest and
South-Central Arizona will be upwards of 60 kt. As mentioned
above, the position of the low will be important. If some degree
of daytime heating is allowed to take place ahead of the areas of
greatest ascent, then thunderstorm coverage may become more
widespread than the mid-morning timeframe as further
destabilization could occur. This increase in coverage is most
likely for areas from Phoenix and eastward, but a slower
progression of the low may allow for more storms slightly further
west. Abundant moisture and the potential for sustained convection
will also increase flooding risk Tuesday morning into the
evening. Add orographic enhancement to the mix and the foothill
and high terrain areas of Maricopa, Pinal, and Gila Counties will
see the the greatest possibility for heavy rainfall. Therefore, a
Flash Flood Watch remains in effect from late Monday night through
Tuesday evening

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Upper level ridging is forecast to quickly surge in from the west
on Wednesday leaving clearing skies and lowering surface moisture
through Thursday. Temperatures for the coming week will stay
somewhat stable across the western lower deserts, but still on
average slightly below normal today through Tuesday. Much cooler
temperatures are expected for most of Arizona with highs only in
the 80s for the Phoenix area Monday-Wednesday. Once the ridging
moves in from the west later this week, temperatures will creep
upward again with highs topping 100 degrees across the western
deserts by Thursday or Friday and potentially for the Phoenix area
by next weekend.

&&

.AVIATION...Updated at 0555Z.

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

The main aviation concern will be periods of VCSH and SHRA.
Confidence is increasing on the exact timing for -SHRA for
tomorrow. VCSH arrive around 09Z and continue through mid-morning
tomorrow. Best chances for -SHRA currently look to be from
16-23Z then VCSH will linger through 02Z. A period of dry
conditions is then expected overnight into Tuesday before the next
round of activity Tuesday morning. Tomorrow`s activity should
mainly be RA, but a few embedded TS cannot be completely ruled
out. Wind directions will mainly be out of the N/NW through the
early evening hours before going E`rly tomorrow evening. Wind
speeds will generally be aob 10 kt, but a period of elevated
winds and occasional wind gusts up around 20 kt are possible
tomorrow with the -SHRA. CIGs will remain aob 10 kft through the
period, but FEW-SCT clouds with bases as low as 5-6 kft are
possible with any showers.

Southeast California/Southwest Arizona including KIPL and KBLH:

The main aviation concern will be SHRA through mid-morning and
VCSH/VCTS/SHRA/TS tomorrow evening. VCSH will continue at both
terminals through around 14Z then become VCSH through 17/18Z. Any
rainfall will remain light and should not be heavy enough to
reduce VIS below 6 SM. CIGs should remain aoa 10 kft through the
period, but FEW-SCT clouds as low as 5-8 kft are possible with any
RA. By 02-03Z another period of showers and storms will be
possible through the end of the period. KBLH has the better chance
of seeing TS, so VCTS has been introduced to their TAF beginning
at 02Z and VCSH is in the KIPL TAF beginning at 03Z. Any storms
will have the potential to reduce VIS below 6SM. 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...
Moisture will increase across the area today and Monday with
light rain developing this evening followed by locally moderate
to heavy rainfall with thunderstorm chances later Monday into
Tuesday. The bulk of the expected rainfall will favor the eastern
districts. Min RHs today will dip to 20-30% this afternoon before
improving to 40-60% over much of the area for Monday and Tuesday.
Winds through Monday will generally follow the typical diurnal
tendencies, but with a tendency to favor the south. A front moving
through the area on Tuesday will turn winds out of the west
northwest with gusty winds of 20-30 mph possible across the
western districts. Drier conditions and near to slightly above
normal temperatures are forecast for later this week.

&&

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

CA...None.
&&

$$

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