606
FXUS65 KTWC 201556
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
856 AM MST Sun Sep 20 2026

.SYNOPSIS...A modest increase in moisture today will bring some
afternoon clouds and a very slight chance of a weak shower.
Moisture will further increase this coming week bringing chances
for showers and thunderstorms.

&&

.UPDATE...The upper pattern this morning is defined a trough of
low pressure across the Intermountain West/Northern Plains with
a low center over Wyoming, a second trough west of California
centered near 35N/129W and a dampening ridge of high pressure
over the SERN states with a high center over ERN TX/WRN LA. In
the lower-to-middle levels of the atmosphere (850 to 700mb),
there is an weak trough moving into southwest Arizona from the
northern Baja Mexico.

The latest GOES precipitable water imagery indicated that the weak
lower-level feature has imported some moisture into Southeast
Arizona from the SW. Precipitable water values between 0.80 to
1.00 inches extend across Pima/SE Pinal and Santa Cruz counties,
with lower values between 0.65 and 0.80 inches east of Tucson.
As this feature ejects across southern Arizona today, we should
have enough lower-level moisture convergence to support a weak
shower to develop across south-central Arizona. I still wouldn`t
expect anything strong given the shortwave ridging occuring in
the middle to upper levels, but since the sub-cloud layer is
dry, we may see some gusty/breezy winds to 35 mph accompany the
light shower downdrafts. Nothing too impactful.

&&

.PREV DISCUSSION (issued at 338 AM MST Sun Sep 20 2026)...

The upper levels this morning features mostly westerly flow
over southeastern Arizona, while a Pacific low approaches the
California coast. Ahead of this low, low amplitude ridging will
move over southeastern Arizona today, with a modest increase in
precipitable water from the south. This shortwave ridging will
suppress overall precipitation chances today, but an isolated
shower/thunderstorm can`t be ruled out with the moisture
increase.

While the initial Pacific low will cross central Canada into the
Intermountain West Monday and Tuesday, broad troughing will
linger centered over southern California and northern Baja.
Upstream of this troughing, moisture will surge northward,
especially over New Mexico into eastern Arizona. Daily shower
and thunderstorm chances increase to the 30-50 percent range
from Tuesday through much of the week across Graham, Greenlee,
and Cochise counties with 10-30 percent chances into Pinal,
Santa Cruz, and eastern Pima counties.

By Friday and this weekend, guidance weakens the influence of
the upper level trough and precipitation chances come down a
bit. Chances don`t completely fall off however with the
potential for some tropical influence from the south. There is
plenty of uncertainty in this scenario at this moment and the
potential for tropical development off the western Mexico coast
will need to be monitored.

&&

.AVIATION...Valid through 21/12Z. Mainly SKC through 20/18Z,
then FEW-SCT clouds 8k-11k feet with a 5-15% chance for -SHRA
through 21/02Z. Surface winds light and under 12 kts through the
forecast period. Surface wind direction west to northwest in
the afternoon, otherwise terrain driven. Aviation discussion not
updated for TAF amendments.

&&

.FIRE WEATHER...A modest increase in moisture today will bring
minimum relative humidity values up to 20-30 percent in the
lower elevations and above 30 percent in the mountains. An
isolated shower can`t be ruled out this afternoon with the
potential for a gusty downburst wind. Moisture values will
increase 10-20+ percent, especially in Graham, Greenlee, and
Cochise counties, from Tuesday through the end of the week.
Chances for showers and thunderstorms will also arrive daily
with this moisture increase. Gusty and erratic winds will
accompany any thunderstorms.

&&

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

&&

$$

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