224
FXUS65 KTWC 071019
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
319 AM MST Tue Oct 7 2025

.SYNOPSIS...

Moisture moves into the region mid-week bringing increasing cloud
cover and isolated to scattered shower and thunderstorm chances
Wednesday through Friday. Then more widespread chances for showers
arrive this weekend. Temperatures transition from above normal
through mid-week to below normal by the end of the weekend.

&&

.DISCUSSION...

The upper level pattern over the eastern Pacific to western CONUS
will become increasingly amplified over the coming several days,
with a closed Pacific trough off the northwest coast and a
building ridge centered over the high plains. Meanwhile further
south, Hurricane Priscilla will be making its way north-northwest
from off the western coasts of central Mexico and then the Baja
peninsula.

Starting Wednesday, the westward shift and amplification of the
ridge will turn lower level winds to the southeast, allowing
tropical moisture associated with Priscilla to move into the
desert southwest. Impressive precipitable water values to the
tune of 1.25" to over 1.75" are expected to overspread southern
Arizona through the end of the week. Shower chances begin to
increase Wednesday near and east of Tucson-Nogales with the
initial introduction of this tropical moisture. However coverage
remains uncertain and potentially initially limited as instability
should be difficult to come by. Surface dew points are forecast
to be on the lower end compared to the overall moisture push (only
rising into the mid 50s by Thursday) and cloud cover will likely
keep CAPE low. This mitigating factor along with the initial lack
of synoptic support keeps shower chances low on Wednesday, with
just isolated 10-20 percent chances east of Tucson. If the
clouds hold off a bit Wednesday and allow instability to develop,
there could be an isolated heavy rain threat. Shower chances
increase and become more widespread Thursday and Friday as
tropical moisture fully sets in, with the same uncertainty in rain
rates as instability may again be difficult to realize.

This weekend the upper low is expected to open and move inland
through the Great Basin. At this point Priscilla should have made
it northward to just off the west coast of central Baja,
potentially beginning a northeastward turn. Even without this
turn, the associated moisture along with the antecedent moisture
pool sitting ahead of the incoming trough will lead to widespread
rain chances across southeastern Arizona this weekend. Where the
 questions still lie is in the depth of the trough (with stronger
forcing likely kept to the north), the subsequent heavy rainfall
potential, and of course overall amounts. Less confidence exists
in the heavy rainfall threat with the overlap of synoptic forcing
and potential Priscilla impact currently favoring areas west/north
of southeastern Arizona, but there remains plenty of time for
details to shift. To highlight the range in possibilities the
higher end event could feature rainfall totals in the 2 to over 3
inch range, while currently a middle- ground forecast puts totals
in the 0.5 to 1.5 inch range. Overall the point remains that a
wet weekend is likely with a low but non-zero chance of
problematic rains.

Temperatures this week will be heavily dependent on cloud and
shower coverage, and should see a trend downwards as moisture
moves in. The forecast transitions from above normal highs through
mid-week, to below normal temperatures by the end of the weekend.
That transition period, particularly Friday-Saturday, may see
current forecast highs a bit too warm with the possibility of
widespread cloud cover.

The active pattern should continue through early next week as
global models continue to show a secondary wave reinforcing the
trough over the western United States, bringing precipitation
chances through Tuesday.

&&

.AVIATION...Valid through 08/12Z.

Light winds expected to become southerly after 07/16Z, then
southwesterly after 07/21Z. Winds should mainly remain under 12
kts, with some afternoon gusts to 20 kts. Winds at KSAD will
become southeasterly 10-15 kts with gusts to 20 kts after 08/06Z,
further strengthening Wednesday. FEW-SCT clouds at 15k-20k feet
mainly from KOLS-KSAD-KDUG, with FEW at KTUS. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...

Minimum relative humidities in the 10-20 percent range today, then
quickly rising Wednesday onwards as moisture moves in. Winds light
and mainly under 12 mph today, with some afternoon gustiness.
Winds become southeasterly Wednesday through Friday. Strongest
southeasterly winds likely in the Gila River Valley where 15-20
mph winds and gusts to 30 mph are forecast. Isolated to scattered
shower chances from Wednesday through Friday, then increasing
chances for wetting rain arrives this weekend.

&&

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

$$

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