Severe Thunderstorm Warning 151, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
156 PM MST Sat Jul 20 2024

.SYNOPSIS...Temperatures will remain 4 to 7 degrees above normal
through the weekend, with abundant moisture in place through
Monday. This will result in daily chances for showers and
thunderstorms with the main threats being strong gusty outflow
winds and heavy rainfall leading to flash flooding. After Monday
expect normal afternoon and evening Monsoon thunderstorm activity.

&&

.DISCUSSION...Radar and satellite shows storms already forming
over the higher terrain. Expect this activity to continue through
the afternoon with hi-res guidance showing storms moving into the
lower elevations along outflows. Coverage of storms will increase
as the day goes with storms impacting Tucson as early as 4 PM.
Aloft winds are northeasterly around 10 kts with GOES 16
precipitable water values ranging from 1.2" along the AZ/NM border
and 1.75" in western Pima County. Mesoanalysis shows 1000-1500
J/kg of MLCAPE, which will be available into the evening hours.
Wet microbursts will be possible thanks to DCAPE values around
1000 J/kg, leading to strong to severe winds. The HREF has most of
southeast 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. 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. WPC has areas east of Tucson in a
slight risk for excessive rainfall.

Sunday looks to have a similar set up, although mid-level winds
look to be more northerly and slightly stronger around 10-15 kts.
Moisture amounts look to be roughly the same as today with
perhaps a small increase along the AZ/NM border. Hi-res guidance
is hinting at a rim shot type of day with storms forming along the
rim early afternoon then pushing southward into Pinal and Pima
Counties, then down to the international border. The main caveat
to this scenario will be if mid-level winds will be strong enough
to sustain storms traveling from the rim down into the lower
deserts. However, any storms that do form will again be capable of
strong to severe wind gusts as well as heavy rainfall. HREF
probabilities for one inch or greater range from 20-50 percent
across much of the CWA. Also, areas that receive rainfall today
will be at greater risk of flash flooding Sunday, which could
expand the threat. It may be necessary to consider putting out a
Flash Flood Watch based on what happens today. WPC also has most
of the CWA in a slight risk for excessive rainfall Sunday, with
the area expanded further west compared to today.

The upper level ridge stays centered over southern Nevada until
at least the middle of the week. However starting Monday areas
east of Tucson begin to dry out a little bit. PWAT values drop to
around normal or a little below normal. This will diminish the
larger areal threat for flash flooding, but daily isolated to
scattered showers and thunderstorms are still expected. Threats
will be more localized compared to this weekend.

Late next week ensembles show the high moving south thanks to a
shortwave trough moving through northern California. This doesn't
seem to affect moisture in southeast Arizona, but with the high
being closer overhead this could lead to some down days for the
monsoon by next weekend.

&&

.AVIATION...Valid through 22/00Z.
SCT-BKN clouds at 8k-12k ft AGL this afternoon and evening. SCT
TSRA/SHRA through 21/05Z with decreasing coverage thereafter.
MVFR conditions in TSRA with wind gusts in excess of 40 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 is in place this weekend leading
to a threat of heavy rainfall and flash flooding and strong winds.
After the weekend moisture decreases a little, although daily
thunderstorms are still expected with heavy rainfall and strong
winds being more localized. 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.
&&

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