128
FXUS65 KTWC 031025
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
325 AM MST Wed Sep 3 2025

.SYNOPSIS...

Scattered thunderstorms are expected to develop this afternoon and
evening across portions of Santa Cruz and Pima county, potentially
moving into Pinal 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, especially through
Santa Cruz and Pima counties. Increasing moisture Thursday through
Saturday will bring daily chances for showers and thunderstorms as
well as cooler temperatures. Dry conditions arrive next week as
well as a return to near normal temperatures.

&&

.DISCUSSION...

A corridor of 1000-1500 J/kg of MUCAPE will be found again across
Pima, Santa Cruz, and Pinal counties this afternoon...with a
slight eastward expansion from yesterday into eastern Pima and
eastern Pinal. The Tucson airport is reporting a dew point 6
degrees higher than this time last night (3 AM MST), a sign of
Tuesday`s convective outflows that pushed moisture eastward.
Scattered thunderstorms should develop on the western periphery of
this instability field, gradually spreading westward with some
organization of outflows. Greatest initial coverage should be
across Santa Cruz into far south central Pima, with additional
development atop high elevation locations including the
Catalinas/Rincons of Pima county. With healthy instability and
precipitable water values in the 1.3-1.75 inch range heavy rain
and a flash flooding risk will accompany any thunderstorm this
afternoon and evening. The 00Z HREF local probability matched mean
product highlights the potential for isolated 1.5 inch or greater
totals, with highest probabilities from western Santa Cruz county
through southern and western Pima...which is also the coverage of
WPC`s Day 1 slight risk of excessive rainfall.

Thursday through the remainder of the week sees an increasingly
complicated forecast, heavily influenced by the northward track of
Hurricane Lorena. There remains significant differences in which
path the storm takes by the time it`s off the western Baja coast
on Thursday. The current NHC forecast takes Lorena northeastward
into northern Sonora on Saturday, however the forecast discussion
points to the uncertainty in the track as model cores such as the
ECMWF/Canadian take it west into the Pacific, compared to the
eastward track favored by the GFS ensembles. Additionally, those
watching the GFS QPF should note the highest magnitude of the 00Z
deterministic QPF output lies well above the mean of its ensemble,
underlining the high uncertainty of the situation. Lorena aside,
there is greater confidence in persistent southeasterly flow
providing an uptick in atmospheric moisture Thursday, leading to
more widespread shower and thunderstorm chances. The quality of
instability however remains a question mark as this moisture push
may arrive with thicker cloud cover, which would reduce the heavy
rain threat. Friday has similar questions, with even more
uncertainty given the dependency of how Thursday will play out. A
Day 2 slight risk for excessive rainfall has been introduced by
WPC for Thursday given the moisture increase, but details need to
be worked out still.

A transition to westerly flow will set in Sunday into early
next week, pushing moisture out of the region and cutting off
precipitation chances for the time being. The temperature trend
over the next several days will likely see highs drop well below
normal by the end of the week, then rise back up to around normal
next week.

&&

.AVIATION...Valid through 04/12Z.

Scattered thunderstorms developing near and west of KTUS-KOLS
after 03/20Z. Gusty winds up to 45 kts possible with any
thunderstorm along with heavy rain. Otherwise light winds
generally under 12 kts. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...

Driest minimum relative humidities today will be in the Gila
River Valley with 15-18 percent values this afternoon. Otherwise
humidities generally above 20-30 percent across southeastern
Arizona. Winds today will be light and terrain driven. Scattered
thunderstorms with chances for wetting rain possible for Santa
Cruz, Pima, and Pinal counties, as well as the White Mountains.
Moisture will increase further over the coming days bringing daily
widespread chances for showers and thunderstorms through
Saturday.

&&

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

$$

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