Toltec, Arizona Weather Forecast Discussion

118
FXUS65 KPSR 211124
AFDPSR
Area Forecast Discussion
National Weather Service Phoenix AZ
424 AM MST Sun Sep 21 2025
.KEY MESSAGES...
- Rain chances will be on the increase today through Monday before
  decreasing heading into the middle of the week.
- Temperatures will be near to slightly below normal today and Monday
  before warming to slightly above normal levels by the middle of
  the week.
- A low pressure system will move through the region by the end of
  the week leading to cooler temperatures and possibly renewed
  rain chances.
&&
.SHORT TERM /TODAY THROUGH MONDAY/...
The latest water vapor satellite imagery as well as streamline
analysis depict an upper- level low situated off the central CA
coastline and subtropical ridging over the central Baja Peninsula.
In between these features, strong southwesterly flow is
transporting a deep plume of mid-level moisture, which is expected
to surge into the forecast area later this morning. This will
cause PWATs to rise into the 1.5-1.7" range later this afternoon
through tonight. This moisture increase in combination with
increased ascent due to positive vorticity advection downstream
from the upper-level low will cause an area of light showers to
break out first across southeast CA later this morning. The latest
HREF membership shows the more widespread shower activity the
rest of today staying across the western deserts where the best
forcing will be situated. Further east, across south-central AZ,
rain coverage is likely to be more limited given the lack of
overall forcing. However, with favorable moist upslope flow out of
the southwest, some of the HREF members do show activity
developing across the terrain escarpments and it is not
inconceivable to have some of that activity that does develop over
these terrain features to affect portions of the Phoenix area
late this afternoon through early this evening, with coverage
remaining isolated at best.
Rain chances do increase and expand eastward into central AZ by late
this evening through Monday as the upper-level forcing shifts
slightly eastward. Most of the HREF membership shows an expansive
area of showers with isolated thunderstorms stretching from
southeast CA through La Paz County east-northeastward through
Yavapai County. Across south-central AZ, not much activity is
being depicted by the HREF, however, any outflows originating from
the main rain band may instigate further shower and thunderstorm
development southeastward, affecting portions of Maricopa County,
including the Phoenix area. As the upper forcing shifts away from
the area heading into Monday afternoon and evening, rain chances
will on the decrease.
Given the increase in cloud cover and moisture, afternoon high
temperatures later today will be slightly cooler compared to
yesterday with readings around 100 degrees across the Phoenix area
to the mid to upper 90s across the western deserts. Afternoon high
temperatures on Monday are forecast to be a couple of degrees
cooler with readings topping out in the mid to upper 90s across
the lower deserts.
&&
.LONG TERM /TUESDAY THROUGH SATURDAY/...
The upper-level low is forecast to become cutoff from the main
jet stream by Tuesday and meander off the CA coast through
midweek. The positioning of the cut-off low off the CA coast will
cause the subtropical ridge to amplify across the region,
resulting in increased subsidence aloft and decreasing moisture
levels. As a result, rain chances starting on Tuesday and
continuing through early Thursday decrease substantially with NBM
PoPs at less than 10% across the lower deserts and 10-20% across
the higher terrain areas of eastern AZ where enough moisture will
be present to spark some isolated afternoon storms. With the
ridge amplifying overhead, ensembles show 500 mb heights peaking
near 590dm during the Wednesday-Thursday timeframe. In response,
temperatures will be on increase with highs above 100 degrees
starting on Tuesday, peaking on Wednesday with highs potentially
reaching 105 degrees in Phoenix, and remaining above 100 degrees
on Thursday.
The weather pattern heading into the end of the week and next
weekend becomes much more uncertain as the cut-off low is
forecast to migrate inland through central/southern CA and then
into AZ. However, model guidance continues to differ in terms of
the overall positioning and speed of the low, which will
determine how much moisture gets advected into the region from the
south and thus the rain potential. It may take another couple of
days to get a more definite picture as to what may eventually
occur as models historically have a very difficult time with cut-
off lows, with this one not being the exception. With the low
nearby, temperatures by the end of the week into next weekend are
likely to cool off with the latest NBM showing highs only in the
90s across the lower deserts.
&&
.AVIATION...Updated at 1125Z.
South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major aviation weather concerns are expected through the
forecast period. Winds will continue to follow typical diurnal
tendencies through at least this evening. Thick mid-lvl clouds
will arrive by mid morning with bases aoa 12-14 kft. There could
be a period of SHRA/VCSH during the late afternoon and evening,
but little if any impacts are anticipated at this time.
Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation weather concerns are anticipated through the TAF
period. Winds will remain westerly at KIPL and KBLH through much
if not all of the period. Thick mid-lvl cloud cover is expected
through the period with bases aoa 12 kft through this afternoon
lowering to just below 10 kft tonight. Expect periods of VCSH/SHRA
at both terminals through much of the period, but with little if
any impacts.
&&
.FIRE WEATHER...
Moisture will be on the increase today into Monday along with
increasing rain chances. Rain chances peak later this evening into
Monday morning with wetting rain chances generally between
10-30%. MinRHs will bottom out between 20-30% today and 25-40% on
Monday with good to excellent overnight recoveries at between
50-90%. Some modest drying with MinRHs dropping between 15-25% is
expected into midweek. A slow-moving weather system may impact the
region by the end of the week leading to increasing moisture once
again and potentially additional rain chances. Winds will generally
remain light with limited afternoon upslope gusts through the
next several days.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&
$$
SHORT TERM...Lojero
LONG TERM...Lojero
AVIATION...Kuhlman
FIRE WEATHER...Lojero

NWS PSR Office Area Forecast Discussion