Severe Thunderstorm Warning 109, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
110 PM MST Sat Sep 27 2025

.SYNOPSIS...Drier air has moved in across the area which will
limit the chance of showers and thunderstorms this weekend. High
temperatures will be below normal this weekend then warm next
week under dry conditions.

&&

.DISCUSSION...The upper pattern today is still dominated by a
strong upper-low positioned across southern California/extreme
southwest Arizona. A substantial dry slot had rotated along the
southern periphery of this feature into Southeast Arizona. This
has resulted in the latest GOES precipitable water imagery
depicting low PWAT values between 0.65 and 0.75 inches southeast
and east of Tucson, 1.00 to 1.10 inches near the Tucson
Metro...and deeper moisture to around 1.25 inches across western
and central Pima and southeast Pinal counties. Although the upper
pattern will result in a decent diffluent pattern aloft over SE
Pinal/Graham and Greenlee counties this afternoon/evening, the
drier-air will limit the overall coverage of thunderstorm
activity. We have seen isolated thunderstorms develop around the
Tucson Metro Area early this afternoon. The warm subsident layer
seen on the 27/18Z KTWC sounding between 600-500 mb looks like it
is choking off deep updrafts with these storms, limiting their
ability to produce heavy rainfall. In any event, expect isolated
to scattered thunderstorms this afternoon/evening.

The upper-low finally begins to eject northeast into northern Arizona
Sunday, and into Colorado Monday as a more established trough
develops along the West Coast. As this happens, we see a shortwave
ridge build into the Desert Southwest the next several days, with
additional ridge amplification anticipated by Wednesday as the
overall pattern across the CONUS becomes amplified. The main
impact with this pattern shift for Southeast Arizona will be the
general drying trend associated with the westerly flow. In fact,
look for the surface dewpoints in the valleys this afternoon (mid
50s) to dip into the upper 40s Sunday, then down into the lower
40s by the second half of the upcoming week. With the drier
airmass, it will become difficult for storms to develop across our
neck of the woods past Sunday. We will tho be able to warm up
with the building ridge and the drier airmass. Expect above normal
the second half of the upcoming work week.

&&

.AVIATION...Valid through 28/00Z. SCT-BKN 6k-10k ft AGL with
ISOLD-SCT -SHRA/-TSRA through 28/03Z...then SKC-SCT 8k-10k ft AGL
overnight and Sunday. ISOLD -SHRA/-TSRA is possible east of Tucson
Sunday. SFC winds SLY to SWLY at 10-15 kts with gusts up to 25
kts thru 28/03z, then SELY less than 10 kts overnight. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Isolated showers and thunderstorms this weekend.
Generally winds will be south to southwest through next week with
speeds up to 15 mph and gusts up to 20-25 mph. Minimum RHs
between 25-35 percent Sunday will lower through the week as a
drier airmass becomes more established across Southeast Arizona,
lowering to 15-25 percent the second half of the upcoming work
week. Temperatures will be slightly below normal through the
weekend, warming to above normal by midweek.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

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