462
FXUS65 KPSR 212010
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
110 PM MST Wed Oct 21 2026

.KEY MESSAGES...

- A strong low pressure system will support multiple rounds of
showers and thunderstorms, some of which may be strong to severe
through Thursday afternoon across much of central and eastern
Arizona.

- Moderate to heavy rainfall with totals over one inch may result
in areas of flash flooding across south-central Arizona with a focus
on upslope, terrain areas.

- Drier air filtering into the region by the end of the week will
lead to more benign weather conditions during the weekend and
through at least early next week.

- Near normal temperatures will cool to below normal levels
by the latter half of the week and continuing through early next
week.

&&

.SHORT TERM /Today through Thursday/...
A high impact weather event continues to unfold across much of
central and eastern Arizona, as an anomalously deep low pressure
system along the southern California coast sends multiple shortwaves
and favorable jet forced ascent into the region intercepting a very
moist airmass. Objective analysis indicates a plume of total column
PWATS in excess of 1.25" focused into central Arizona with low level
mixing ratios 8-10 g/kg, and combined with steepening midlevel lapse
rates (~7 C/km in the H7-H5 layer) will result in MLCape values
nearing or exceeding 1000 J/kg by this afternoon. Juxtaposed with
these thermodynamic profiles, deep layer 0-6km shear will easily
exceed 50kt supporting supercellular structure/splitting cells in
more robust thunderstorm updrafts. Special 18Z KPSR sounding
highlights an even higher instability potential should sufficient
heating continue supporting a severe potential with large hail and
damaging the primary threats. Interestingly, however, the latest
HREF and REFS output is less enthusiastic regarding the spatial
coverage of storms across lower elevations until later this evening,
and it remains to be seen if the ingest of updated thermodynamic
profiles changes short term model output. Nevertheless, there is
still modest support for terrain encouraged ascent spurring rapid
updraft development around the Phoenix metro with better confidence
of thunderstorm clusters to the north and east of the city through
the evening.

Confidence in thunderstorm development and coverage across south-
central Arizona grows markedly overnight through Thursday morning as
the aforementioned California low edges inland placing the eastern
CWA within a right rear jet entrance region providing a prolonged
period of upper divergence and ascent. The nocturnal tendency will
be for increasing H9-H8 southerly winds, and within a unidirectional
wind profile, reduce Corfidi vectors encouraging training echoes.
Moist orographic upslope will also aid in persistent ascent over the
same areas, and with PWATS solidly holding or exceeding 1.50"
overnight, the flash flood threat will be heightened heading into
Thursday morning. A notable handful of high resolution models
support a conceptual idea and historical precedent in similar
scenarios of storms aligned through northern Maricopa County through
the morning hours before shifting into Gila County during the
afternoon. The magnitude of elevated instability combined with
abundant moisture profiles will yield a favorable environment for
rainfall rates in excess of 1-2"/hr.

As the upper trough lifts inland towards the Great Basin Thursday,
supporting jet energy will exit the region while low level winds
veer helping erode boundary layer moisture, driving a frontal
boundary and lower theta-e airmass into lower elevations and
eliminating instability. However, moist upslope will persist across
the eastern CWA with lingering instability juxtaposed with strong
deep layer shear supporting the continued threat for severe weather.
In fact, numerous HREF and REFS members depict very impressive 2-5km
updraft helicity signatures with notable 0-1km shear indicative of
rotating updrafts in the stronger storms through the afternoon.
Subjectively, the synoptic setup and model output looks more
impressive for portions of southern Gila County tomorrow than much
of the forecast area currently. Thus, will amplify the severe
threat in the eastern CWA during the afternoon before activity
rapidly exits the area in the evening.

&&

.LONG TERM /Friday through Tuesday/...
With strong dry air advection in place behind the departing trough,
weather conditions will turn much quieter beginning Friday. A piece
of energy left behind from departing trough is currently being
depicted by the model guidance to separate itself from the main jet
energy and briefly morph into a cut-off low over AZ during the
weekend. This feature for the most part should be inconsequential,
although enough moisture could be present for some showers to
materialize over the AZ high terrain on Saturday but for the most
part should remain on the light side given the overall lack of
instability that will be present. The lower deserts should remain
rain-free as there will be too much dry air in place. Under the
presence of the low, temperatures will be several degrees below
normal during the weekend with afternoon highs struggling to reach
80 degrees across the lower deserts. As the upper-level low exits
the region and weak ridging builds overhead early next week,
temperatures will rebound to more seasonal levels.

&&

.AVIATION...Updated at 1756Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Rounds of isolated to scattered strong thunderstorms with heavy
rain leading to sub-VFR VIS, brief strong and erratic wind gusts,
isolated large hail, and low CIGs will be possible today through
at least Thursday morning. Confidence in timing out best chances
RA/TS is moderate, as isolated RA/TS may develop at any time.
Uncertainty remains in how much SHRA/TSRA may develop near the
terminals this afternoon and have highlighted this potential with
PROB30s. Storms that do develop may become severe. There may be a
lull in activity this evening before SHRA/TSRA increases heading
into the overnight hours and continuing through part of Thursday
morning. A downtrend in activity is expected by Thursday evening.
Lowest CIGs today are expected around 5-7K ft AGL and then may dip
to 3-5K ft Thursday morning with showers and storms. Winds are
expected to favor an E-SE component throughout much of the TAF
period. However, depending on SHRA/TSRA development, wind
directions may become erratic at times with an increase in winds
from the S-SW possible.


Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation weather concerns are anticipated through the
next 24 hours. Winds will favor a southerly component at the
terminals, with gust around 20-25 kts this afternoon at KBLH. KBLH
will maintain winds out of the S through much of the period while
directions will switch around to the W this evening at KBLH.
Periods of variability are expected. FEW-SCT clouds, mostly aoa
6-7 kft, will be common through this evening

&&

.FIRE WEATHER...
A low pressure system will bring very active weather conditions
across much of central and eastern AZ through Thursday as multiple
rounds of showers and thunderstorms are expected. Significant
rainfall activity is likely in some areas, particularly across the
higher terrain areas of the eastern districts where rainfall amounts
in excess of 1-2" are possible. MinRHs through Thursday will range
between 30-40% across the western districts and between 40-70%
across the eastern districts along with good to excellent overnight
recoveries. Some gusty winds with peak gusts upwards of 25-30 mph
are likely this afternoon and once again on Thursday across the
western districts. Much quieter conditions are expected starting
Friday along with a decrease in humidity values. Near normal
temperatures today will cool into a below normal category starting
Thursday and lasting through the weekend.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through Thursday evening for AZZ545-547-552-556>558-
     560>563.

     Flood Watch through Thursday afternoon for AZZ534-537-538-
     540>544-546-548>551-553>555.

CA...None.

&&

$$

SHORT TERM...18
LONG TERM...Lojero
AVIATION...Smith/Benedict
FIRE WEATHER...Lojero

NWS PSR Office Area Forecast Discussion