136
FXUS65 KTWC 021709
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
1009 AM MST Tue Sep 2 2025

.SYNOPSIS...Scattered thunderstorms expected this afternoon mainly
from Santa Cruz county through central and western Pima county. A
few may develop over the White Mountains as well. Heavy rain with a
threat of flash flooding will be the main hazard with any
thunderstorm in Santa Cruz and Pima counties today. Additional
thunderstorm chances are expected Wednesday. Then Thursday into the
weekend sees increasing precipitation chances as tropical moisture
moves into southeastern Arizona. High temperatures will be near
normal through Wednesday, then will become below normal the second
half of the week.

.UPDATE...Main forecast focus for today is the potential for
scattered showers and thunderstorms this afternoon mainly across
Santa Cruz and western/central Pima counties, including Tohono
O`Odham Nation. The Weather Prediction Center (WPC) has depicted a
slight risk (15%) for excessive rainfall which may result in
isolated to widely scattered instances of flash flooding this
afternoon and evening along and near the International Border in
Pima County. Particular flood prone watersheds that may be at risk
include Lower Arivaca Creek, Topawa Wash, Lower Fresnal Wash.

High precipitable water (PW) content with values of 1.2 near Nogales
to upwards of 1.8 inches near Ajo in far western Pima county
along with modest instability (MUCAPE 500-1000 J/kg) will support
the scenario of thunderstorms with heavy rain. In Tucson, 1.3
inches of PW was recorded in the 02/00Z upper air sounding from
KTWC yesterday afternoon. This is still on par via latest GOES
estimates, with a sharp positive moisture gradient immediately to
the west of a Tucson to Nogales line. It is near and west of this
gradient that high resolution guidance depicts scattered shower
and storm development around or just after noon. With Tucson on
the eastern periphery of this gradient, there is a higher degree
of uncertainty if a few showers or storms form in the Tucson Metro
later in the afternoon.

While the primary threat will be focused on heavy rainfall rather
than wind, the secondary threat will be on dry air momentum from the
midlevel (500-600 hPa) which may help to locally enhance
thunderstorm outflows resulting in minor nuisance impacts. The
02/12Z HREF guidance depicts a 70% for winds in excess of 30 knots
(35 mph) which could cause areas of blowing dust mainly across
T.O. Nation, but confidence in any impactful wind or dust related
impacts is low.

&&

.PREV DISCUSSION... /issued 329 AM MST Tue Sep 2 2025/

This morning, the combination of a vorticity lobe over the
International Border and moisture advection within southeasterly
flow was responsible for fairly widespread cloud cover over
southern Arizona. Through the day this vorticity max should exit
to the west and allow for skies to clear, leaving a swath of
precipitable water values from around 1.3 inches near Tucson to at
least 1.7 inches westward through Ajo. The precipitable water and
instability gradient will likely be just west of Tucson this
afternoon, from Santa Cruz county northwestward through central to
western Pima. It should be along this gradient where thunderstorms
develop this afternoon, with potentially some isolated movement
eastward towards Tucson. With MUCAPE likely exceeding 1000 J/kg
within the swath of richest moisture (west of Tucson), heavy rain
with a flash flooding threat will likely be the main concern. Wind
gusts to around 40 mph may produce an isolated blowing dust
hazard.

High-resolution guidance indicates an eastward push of moisture
on Wednesday which should allow for thunderstorm chances to nudge
up in Pinal and eastern Pima, though still on the drier side in
eastern locations towards the New Mexico border. Both today and
Wednesday are highlighted by a slight risk of excessive rainfall
from WPC from western Santa Cruz through central and western Pima.

The remainder of the week continues to be highlighted by the
potential approach of Tropical Depression Twelve-E (likely to
become a tropical storm later today according to the NHC),
currently off the west coast of central Mexico. Expected to track
near southern Baja by Thursday, the model spread and subsequent
uncertainty in track following this point remains very high. Even
with this uncertainty, persistent low to mid level southeasterly
flow over the region should allow for an increase in moisture
across southeastern Arizona to end the work week. Precipitation
chances along with the possibility of an increased flash flooding
threat accompanies this moisture surge Thursday through Saturday,
with the obligatory caveats at this lead time. This moisture
increase will also work to lower temperatures for the second half
of the week with highs likely becoming at least a few degrees
below normal. Going into early next week it looks likely that
winds aloft turn west to northwesterly, shutting off moisture and
precipitation chances for the remainder of the forecast period.

&&

.AVIATION...Valid through 03/12Z.

SCT-BKN ceilings at 9k-13k feet gradually clear after 02/12Z.
Scattered thunderstorms mainly near and west of KTUS-KOLS-KAVQ
after 02/20Z and lasting through 03/04Z. Southeasterly surface
winds of 7-12 kts from KTUS-KOLS to 12-17 kts in eastern terminals
including KSAD- KDUG. Gusts to around 25 kts in eastern locations
and up to 45 kts possible with any thunderstorm. Aviation
discussion not updated for TAF Amendments.

&&

.FIRE WEATHER...

Minimum relative humidity values will be driest in the Gila River
Valley today and Wednesday, then increasing above 25-35 percent
the remainder of the week. Humidities mainly above 20-30 percent
elsewhere this week, especially Thursday through Saturday as an
increase in moisture arrives. Southeasterly winds today strongest
through the Gila and southeastern Cochise at 13 to 18 mph with
gusts to 30 mph. 10-15 mph elsewhere from Tucson to Nogales west.
Scattered thunderstorms expected to develop this afternoon from
Santa Cruz through central and western Pima. A few may also
develop over the White Mountains. Additional chances again on
Wednesday, then increasing chances across southeastern Arizona to
end the week as moisture moves in.

&&

&&

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

&&

$$

Update...DVS

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