396
FXUS65 KTWC 201554
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
854 AM MST Sat Jul 20 2024

.SYNOPSIS...Temperatures will remain 4 to 7 degrees above normal
through the weekend, with increasing moisture levels across the
region into early next week. This will result in increasing 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...Lingering showers and cloud cover remain from last
night`s activity from Tucson westward. East of Tucson a few clouds
remain but are quickly dissipating. The upper level high is
centered over southern Nevada putting Arizona under northeast flow
aloft. This is evident in the 20/12Z KTWC sounding which shows mid
level northeast flow around 10 kts. PWAT was measured at 1.57",
which is about 0.2" above the climatological normal. The
atmospheric profile is moist above about 600 mb with a pronounced
inverted V profile below that. Forecasted CAPE values will be
around 1500 J/kg. All the ingredients are in place for storms, but
the main uncertainty today will be how long the cloud cover
sticks around and will it affect afternoon convection.

Hi-res models show initiation between noon and 1 PM along the
higher terrain with storms then slowly expanding west from
outflows. The flow isn`t quite strong enough for a rim shot, but
models do show fairly strong outflows coming off the rim. Tucson
could see storms by late afternoon initiated off outflows moving
in from the east and south. Wet microbursts will be possible with
storms capable of strong to severe winds. The HREF has most of
Arizona within the 70 percent chance contour for winds in excess
of 30kts this afternoon and evening. A couple HREF members show a
fairly strong outflow moving through Pinal County, which could
also lead to blowing dust issues. Heavy rainfall and flash
flooding will also be another threat. Locally heavy rainfall of
1-2 inches will be possible with the HREF highlighting the sky
islands, Santa Cruz County, and also parts of the Tohono O`odham
later this evening. The flash flooding threat will also be aided
by slower moving storms. Please see the previous discussion for
longer term details.

&&

.PREV DISCUSSION... /issued 450 AM MST Sat Jul 20 2024/

PWATs across southeast Arizona have been increasing notably
through the week with just over 1.5 inches recorded overhead per
yesterday afternoon`s rawinsonde compared to 1.15 inches reported
in Wednesday afternoon`s sounding. WPC has included south central
CO, southeast AZ, and most of NM in a slight risk for excessive
rainfall today, i.e., at least a 15 percent chance of exceeding
flash flood guidance within 25 miles of a point. The 20/00Z run of
the HREF depicts a widespread swath of 30 percent of exceeding an
inch of rainfall in a given 3-hour period this afternoon through
midnight. Tomorrow, the HREF shows even better chances (50-70
percent) for exceeding an inch of rain in a given 3-hour period
and even a slight chance (10 percent) of exceeding 3 inches of
rain in a 3-hour period over portions of Graham/Greenlee counties.

Analyzing the wind potential, the HREF shows a widespread 70 percent
chance of outflow winds in excess of 30 kts beginning over northern
Graham/Greenlee counties associated with convection beginning as
early as 2 pm MST. There is even a 90 percent along the AZ/NM border
between 4-6 pm. There is also an area of higher probabilities to the
south of Tucson over Santa Cruz county. The wind potential appears
to spread west/southwest through midnight following the propagation
of storms off of the White Mountains/higher terrain. There will also
be decent outflows to consider propagating north/northwest from
convection in northern Sonora and Santa Cruz county. HREF shows a
widespread 70 percent chance of outflow winds in excess of 30 kts in
this area as well. Outflow mergers from these two complexes will
need to be considered over the Tucson Metro which may locally
increase the thunderstorm potential between 4-7 pm.

Based on the above analysis, today`s forecast focus is geared toward
the potential impacts from scattered storms across southeast Arizona
this afternoon and evening. The primary hazards will be isolated
strong to severe storms capable of producing gusty outflow winds
greater than 35 mph and locally heavy rainfall. Impacts resulting
from the locally stronger cells may include isolated severe wind
gusts, localized reduced visibility in areas of blowing dust, and
isolated to scattered instances of flash flooding.

Despite increased confidence in seeing heavy rain out of individual
thunderstorms today, there is simply not enough confidence in the
spatial coverage of thunderstorms to warrant a flash flood watch.
That being said, if today ends up being a washout, PWATs increase
again, and HREF provides increasing confidence in widespread flash
flood potential, then antecedent conditions may warrant a headline
for tomorrow afternoon and evening. Similar story on the possibility
for strong/severe outflow winds resulting in areas of blowing dust.
The 20/12Z run of the HREF should add confidence in specific areas
to include in a Blowing Dust Advisory if necessary, but for now the
confidence isn`t quite there for widespread areas of blowing dust as
HREF didn`t highlight any areas with potential for 50 kt winds.

Looking ahead, the upper air pattern remains favorable for daily
chances for Monsoonal thunderstorms through the middle of next week
with up and down days to be highlighted in future forecast
packages.

&&

.AVIATION...Valid through 21/12Z.
FEW-SCT clouds at 8k-12k ft AGL with gradually diminishing coverage
through mid morning. SCT-BKN clouds at 8k-12k ft AGL developing
after 20/18Z. SCT TSRA/SHRA between 20/19Z and 21/07Z with
decreasing coverage thereafter. MVFR conditions in TSRA, with
mountain obscurations, vsby reductions and wind gusts to 45+ knots.
Outside of TSRA, SFC wind at most locations 12 knots or less
favoring 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 July moisture levels expected into at least early next
week. 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
20-25 mph and light winds overnight. Wind direction during the
afternoon hours will primarily be west to northwest through the
weekend, especially from Tucson westward and in the upper Gila River
Valley.

&&

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

$$

Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson