420
FXUS65 KTWC 201908
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
1208 PM MST Sun Sep 20 2026

.SYNOPSIS...A modest increase in moisture will bring partly
cloudy skies with a slight chance of a weak shower this
afternoon and early this evening. Moisture will continue to
increase this coming week bringing daily chances for showers
and thunderstorms.

&&

.DISCUSSION...The upper pattern today 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.

Monday, both of the upper troughs continue to track east with
one centered over the upper Midwest and the other along the
central California coastline. An amplified shortwave ridge
building between these two features into north-central Mexico
will result in a southerly flow across Sonora/Chihuahua MX. This
will draw deeper moisture starting Monday. The bulk of the
moisture will advect into New Mexico and far eastern portions of
Arizona Tuesday and Wednesday. The best deformation aloft/lift
between the upper trough to our west and the subtropical ridge
to our east will occur over New Mexico. However, there appears
to be a westward expansion of this area of lift into our neck of
the woods. This should increase the potential for heavy
rainfall across Southeast Arizona from Tucson east on both
Tuesday and Wednesday. In fact, the latest NBM suggests there is
a 10 percent chance of rainfall totals greater than 1.25 inches
along the Arizona/New Mexico border between 12Z Tuesday and 12Z
Wednesday.

The NHC still identifies an area 350 miles south of Manzanillo
Mexico (Invest 99E) with conditions conducive for it to become a
tropical depression or storm within the next day or so. There
is still a ton of uncertainty with the forecast track and
strength of this feature. In a general sense, it will work its
way northwest to a location west of the tip of the Baja between
200-240 hours. My intuition tells me that we will see a broad
ridge develop over northwest Mexico that will block the deeper
moisture and force it to track farther west where moisture
eventually shears out to the NE across the Desert Southwest. We
will see...there is a lot of time to monitor its development.

&&

.AVIATION...Valid through 22/00Z. SCT-BKN clouds 8k-11k feet
with a 5-15% chance for -SHRA through 21/02Z, then SCT-BKN high
clouds AOA 20k ft AGL overnight through 21/18Z, then SCT-BKN
clouds 8k-11k feet with a slight chance for -SHRA/-TSRA.
Although localized wind gusts to 30 kts will be possible near
storms today, SFC winds will be 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 to 35 mph. Moisture values
will increase 10-20+ percent, especially in Graham, Greenlee,
and Cochise counties, 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.

&&

$$

Visit us on Facebook, X, YouTube, and at weather.gov/Tucson

NWS Tucson (TWC) Office