725
FXUS65 KTWC 181638
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
938 AM MST Tue Aug 18 2026

.SYNOPSIS...
Hot conditions this week with an Extreme Heat
Warning in effect today through Friday for portions of the
Southeast Arizona. Isolated thunderstorms are expected today
along and near the International border in south-central and
southwest Pima county. An uptick in thunderstorm strength and
coverage is anticipated the second half of the week and into
the upcoming weekend.

&&

.UPDATE...
Residual surface moisture in the region could help moderate
high temperatures this afternoon. However, the moisture combined
with above normal temperatures will make it feel like we are a
bag of steamed vegetables the next several days. Thus, we will
be continuing the Extreme Heat Warning which is valid through
Friday evening. There is a slight chance for showers and storms
this afternoon mainly over the White Mountains and areas west and
south of Tucson with lingering activity a possibility across
central and western Pima County into the early evening.

Most of the HREF members depict isolated showers and storms forming
in the White Mountains around 1-2 pm MST this afternoon. A few
showers and storms are then depicted to the west and south of the
Tucson Metro. HREF members tend to keep this activity confined to
the International Border and south, but the WRF-HRRR and WRF-GFS
depict isolated showers and storms later this afternoon continuing
across central and western Pima County. My confidence on this is
low, but I did increase the precipitation chances out west into
the evening to be mentionable.

As for the environment, overall not looking quite as rambunctious
as yesterday, but RAP forecast DCAPE upwards of 1500 J/kg out west
where I upped precipitation chances could support an isolated severe
wind gust on the condition that we get a stronger storm out there
during the late afternoon. The threat for heavy rain remains lower
given the drier environment as latest precipitable water estimates
are in the ballpark of 1.0-1.3 inches. Looks like we could be
getting an upcoming surge of moisture overnight though.

&&

.DISCUSSION...
The upper pattern this morning is defined by a
broad ridge of high pressure extending across the Desert
Southwest with two individual high centers noted over the
NRN Baja MX/SW AZ and the other over central TX. In between
these two features is an inverted trough with an axis extending
from the bootheel of New Mexico down into southern Sonora
Mexico. The latest GOES precipitable water imagery still
indicated a rather dry airmass across Southeast Arizona this
morning, with PWATs generally between 1.10-1.25 inches and
deeper moisture (PWATs 1.50 inches) residing around 50 miles
south of the International border in Sonora Mexico. An outflow
from convection in Sonora last night will push some of this
moisture to our south northward this morning, seeping into the
lower deserts west of Tucson including the Tohono O`odham Nation
through the morning. Today, the northern periphery of the
inverted trough will finally track west along the International
Border today. This will shove the upper high center currently in
NRN Baja northward into SRN CA, with the upper high centers
becoming more consolidated over AZ/NM. This will focus the best
deformation aloft/lift today across south-central and southwest
Pima county with more stable conditions over the majority of SE
AZ today. The moisture moving into the aforementioned area from
the south this morning will be quite shallow, but it will help
assist heavier rainfall amounts with the storms that do develop.
The northern periphery of the inverted trough will also provide
0-6km bulk shear between 20-25 kts, which should help support
stronger storms that can produce gusty outflows and small hail.
So, despite the fact it will be a down/low-grade Monsoon day
across the majority of Southeast Arizona, some stronger storms
will develop this afternoon into early this evening along the
International Border in south-central and southwest Pima county.

Expect deeper moisture will surge up the gulf into the lower
deserts of SW AZ tonight and Wednesday once that inverted
trough pushes west of the Baja Peninsula and higher SFC
pressure in the southern Gulf will assist winds pushing up the
gulf toward the thermal low in Arizona. This should be a deep
push of moisture into the region with PWATs between 1.50 and
2.00 inches expected across SW AZ by 19/12Z Wednesday morning.
The combination of this moisture and the upper high moving into
a more favorable location across NW Arizona by 20/00Z Wednesday
afternoon will result in another uptick in thunderstorm strength
and coverage across Southeast Arizona Wednesday. This is
especially the case Thursday as the upper high moves toward the
Four Corners resulting in strong deformation/lift aloft.

otherwise, temperatures remain hot this week with highs 5 to
near 10 degrees above normal. These above normal highs along
with warm overnight lows will produce areas of Major HeatRisk,
with the Extreme Heat Warning in effect through Friday.

&&

.AVIATION...Valid through 19/18Z.
FEW-SCT 12-15k ft AGL through 18/20Z, then SCT 8-12k ft AGL
SCT- BKN 12-15k ft AGL with a slight chance of -SHRA/-TSRA
through 19/03Z. Gusty and erratic winds possible near TSRA with
gusts to 35-40 kts. Winds outside of thunderstorms mainly light
and terrain driven. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...
Hot conditions this week with temperatures 5 to 7 degrees above
normal across Southeast Arizona. An Extreme Heat Warning is in
effect today through Friday for portions of the area. Isolated
thunderstorms are expected today along and near the International
border in south-central and southwest Pima county. An uptick in
thunderstorm strength and coverage is anticipated the second
half of the week and into the upcoming weekend. Winds mainly
light and terrain driven. Strong, gusty and erratic winds will
accompany thunderstorms. Minimum relative humidities 15-25
percent in lower elevation areas this week with 25-35 percent in
higher terrain.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Extreme Heat Warning until 8 PM MST Friday for AZZ501-502-504>506-509.

&&

$$

Update...DVS

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