246
FXUS65 KTWC 180802
AFDTWC
Area Forecast Discussion
National Weather Service Tucson AZ
102 AM MST Mon Aug 18 2025
.SYNOPSIS....Isolated to scattered thunderstorms this week across
Southeast Arizona, with day to day variability in intensity and
chances. The big story for Southeast Arizona will be the extreme
heat returning the second half of the week. An Extreme Heat Watch
is in effect Wednesday morning through Friday evening.
&&
.DISCUSSION...The upper pattern this morning is defined by a
ridge of high pressure extending from southern New Mexico and
northern Chihuahua Mexico toward the northeast into the Midwestern
states. At 1 am MST, radar indicated scattered moderate-intensity
showers with embedded thunderstorms persisting along the NW
periphery of the 300 mb high center extending from northeast
Sonora Mexico into Southwest New Mexico. We have actually seen a
few light showers develop early this morning across portions of
Graham and Greenlee counties in the mildly diffluent southerly
flow aloft. The upper high is expected to slowly move into
southwest/west-central New Mexico by this afternoon. As this
happens, we will see a slight warming aloft across the eastern
portions of the CWA today which should inhibit deep convection.
Granted, we will still have enough moisture and instability around
from strong surface heating to result in isolated to scattered
thunderstorms across Southeast Arizona, there is no large scale
forcing mechanism at play.
The upper high is still progged to shift into the Four Corners
region Tuesday afternoon, where it will reside and strengthen the
second half of next week. Although this location is usually
active for our neck of the woods, it all depends on the moisture
that is available. The 18/00Z deterministic GFS still indicated a
plume of higher precipitable water values (1.50+ inches) moving up
the Gulf of California into the lower deserts of SW Arizona this
week. Unfortunately, we also see a general drying across the
eastern half of our forecast area, with PW values below an inch
for the majority of the week. As mentioned in previous forecast
discussions, this will still likely setup a pattern where we have
isolated storms east of Tucson capable of producing strong and
gusty thunderstorm outflows but little rainfall, then moving west
to southwest across the CWA, increasing in coverage and becoming
capable of producing heavy rainfall the farther west the storms
make it.
In any event, the bigger story may end up being the increasing
afternoon high temperatures warming back up to 4-6 degrees above
normal Wednesday through Friday, increasing HeatRisk to Major by
Wednesday and locally Extreme Thursday. There is an Extreme Heat
Watch in effect Wednesday through Friday for the majority of the
lower elevations of Southeast Arizona.
&&
.AVIATION...Valid through 19/12Z. SCT-BKN clouds 10k to 14k ft
AGL through 18/17Z with a few light -SHRA possible thru daybreak
near KSAD and KDUG. SCT-BKN low clouds 7k to 10k ft AGL will
develop around 18/19Z and persist thru 19/05Z. ISOLD-SCT -SHRA/-TSRA
will be possible mainly east and southeast of KTUS in Cochise,
Graham and Greenlee counties. SFC winds will be light an variable
this morning becoming north-northwest less than 12 kts after
18/20Z. Aviation discussion not updated for TAF amendments.
&&
.FIRE WEATHER...Expect daily chances for thunderstorms and wetting
rain showers into next week. Min RHs will generally be 15-20
percent in the lower elevations and 20-30 in the mountains. Winds
will generally be less than 15 mph with typical afternoon gusts
up to 20 mph into next week. Temperatures will be trending above
normal the second half of this week, with extreme heat expected in
the lower elevations of Southeast Arizona.
&&
.TWC WATCHES/WARNINGS/ADVISORIES...
Extreme Heat Watch from Wednesday morning through Friday evening
for AZZ501>507-509.
&&
$$
Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson
NWS Tucson (TWC) Office
Tucson, AZ 
