916
FXUS65 KTWC 171628
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
928 AM MST Wed Jul 17 2024

.SYNOPSIS...Temperatures 4 to 7 degrees above normal through the
rest of this week, with typical mid July moisture levels across the
region. This will result in the daily chances for showers and
thunderstorms. The main threats will continue to be strong gusty
outflow winds and heavy rainfall leading to localized flash
flooding.

&&

.UPDATE...One of the old-school proxies that would hint at an
active afternoon/evening here in the Desert Southwest was the
presence of ACCAS in mid-morning hours. We see that across the
south central portions of Arizona this morning, with the latest
satellite visible image indicating it extends from Santa Cruz
northward across the Tucson Metro Area and into southeast Pinal.
It usually indicates that there is quite a bit of instability in
the mid-levels. However, sometimes the old rule-of-thumb
techniques can be misleading. The upper high is centered over east
central New Mexico this morning and will drift west into west
central Arizona this afternoon. As this occurs, we will see a
warming in the mid-levels. This won`t completely erode away all
the CAPE seen on the 17/12Z KTWC upper air sounding, but the warm
air AOA 500 mb will choke off deep sustained updrafts and thus
limit the overall coverage of thunderstorm to isolated from Tucson
east today. So, the ACCAS we see this morning won`t come to
fruition with regards to an active day in Tucson.

However, this isn`t true for the entire forecast area. Today the
main focus for thunderstorm activity will be a bit farther west
today compared to yesterday because of the westward drift of the
upper high, so primarily across western Santa Cruz and south
central Pima county. A slug of dry air noted in the lower and
mid-level water vapor imagery across Chihuahua and NE Sonora
Mexico will rotate NW into south central Arizona this afternoon.
This dry air will be available to entrain into the mid-levels of
thunderstorms, thus increasing the potential for strong
downdrafts/outflows. The 17/12Z HREF is picking up on this, and
has a 50-70% chance bullseye of wind gusts in excess of 30kts
within 25 miles of a given point over W Santa Cruz and the
southern Tohono Oodham Nation. The drier air aloft will also
reduce the threat of widespread heavy rain. The HREF isn`t too
excited for heavy rain, with only a 10% chance of a 3-hour
rainfall in excess of 1 inch within 25 miles of a given point in
the same location discussed above.

So, today will be more of a low-grade Monsoon day with the best
chance for strong winds/heavy rain south of Tucson along the
International Border.

&&

.PREV DISCUSSION... /issued 250 AM MST Wed Jul 17 2024/

Residual showers and embedded thunderstorms continue early this
morning to the west of Tucson. Much of this activity has formed on
the outflow from a weakening MCS in far northern Sonora. It`s a
bit difficult to discern at this juncture but it appears there is
an MCV to the south of Nogales. CAMs have been struggling to pick
up on this activity but regardless, expect it to trend downward
through the early morning hours.

The overall larger scale pattern remains quite persistent for our
region as the mid/upper level high remains to our northeast, to the
south of the Four Corners along the AZ/NM border. That continues to
result in mid level easterly flow. Moisture levels are certainly
adequate ranging from around 1 inch PWAT near the NM border to 1.5
inch over western Pima County. These levels are close to normal for
this time of year. HREF/CAMs show scattered showers and
thunderstorms developing mainly to the south and east of Tucson this
afternoon, attempting to push west courtesy of outflows into the
lower elevations/deserts this evening but confidence is low on how
much storm activity there will be. There isn`t very much discernible
in terms of disturbances/perturbations in the flow to hang our hat
on today. Regardless, as we`ve seen over the past couple of days,
there will continue to be a threat for gusty winds and localized
flash flooding.

Over the next few days and into the weekend there will be some minor
changes to the flow pattern. The mid/upper level high to our north
will begin to shift westward and slightly strengthen as it does so.
Not much change to our easterly steering Thursday and perhaps Friday
but by this weekend and into next week, northeasterly flow will
dominate and that is a favorable direction for bringing storms off
the Mogollon Rim/White Mtns into the lower elevations. Meanwhile,
temperatures will nudge upward slightly, peaking Saturday about 5 to
8 degrees above normal. A few locales mainly to the north and west
of Tucson will be teetering close to major Heat Risk levels so this
is something we`ll be keeping an eye over the next couple of days.

&&

.AVIATION...Valid through 18/12Z.
BKN-OVC clouds 8-12k ft AGL south and west of KTUS will tend to
diminish aft 17/12Z. Elsewhere, SKC-SCT clouds this morning. SCT-BKN
clouds redevelop again aft 17/18Z, with diminishing cloud cover aft
18/05Z. SCT TSRA/SHRA thru 17/12Z west of KTUS. Otherwise, SCT
TSRA/SHRA developing again for much of the area aft 17/19Z. MVFR
conditions in TSRA, with mountain obscurations, vsby reductions and
wind gusts to 40+ knots. Outside of TSRA, SFC wind at most locations
12 knots or less favoring a SWLY thru NWLY direction during the
afternoon/early evening hours and variable in direction at other
times. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Deeper moisture remains across southeast Arizona,
with typical mid July moisture levels expected through at least the
upcoming weekend. This will result in the daily cycle of mainly
afternoon and evening showers and thunderstorms. The main threats
will be strong, gusty and erratic outflow winds and heavy rainfall
leading to localized flash flooding. Minimum relative humidity
values will be 20-30 percent in most lower elevation locations and
25-40 percent in mountain areas. Away from thunderstorms, 20-foot
winds will generally be 15 mph or less, with afternoon/early evening
gusts to around 20 mph and light winds overnight. Wind direction
during the afternoon will primarily be west to northwest into the
weekend.

&&

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

&&

$$

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