713
FXUS65 KTWC 180832
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
132 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.

&&

.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 19/12Z. 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.

&&

$$

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