Flash Flooding in Arizona, August 14, 2021

Area Forecast Discussion
National Weather Service Tucson AZ
1035 AM MST Sat Aug 14 2021

.SYNOPSIS...A disturbance moving in from the east this weekend
will continue scattered showers and thunderstorms for much of
southeast Arizona. Some storms will generate strong winds and very
heavy rain which will result in continued flooding concerns.

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.UPDATE... As mentioned in the previous discussion, the large area
of light to moderate precipitation over Pima and southern Pinal
counties this morning has diminished over the last couple of hours
with focus now turning to the next area of activity. The next round
is already beginning to develop in a band across Graham and Greenlee
counties as well as over the higher elevations of Cochise County.
Breaks in the cloud cover are already resulting in more cu
development and light shower activity beginning to initiate.  Santa
Cruz, Cochise, and southern Pima counties are expected to see the
highest coverage of storms this afternoon along with the highest
threat of flash flooding as storm motions slow with the overhead
passage of a mid-level wave. The third spot to keep an eye on will
be storms as the develop and move off the rim later in the day which
would impact northern Greenlee, Graham, and Pinal counties into
tonight. Outflow from this activity could even trigger some storms
across far western Pima County late tonight where lingering
instability will be present. Rim activity will have a slightly lower
flash flood threat with storms still moving along at about 10-15 mph
to the west to southwest. With that being said, any training storms
or even a quick shot of heavy rain could produce flooding issues in
this area. There is plenty of moisture in place as the 12z sounding
supported with a PWAT of 1.95 inches which is near record for this
date. This combined with the previous widespread 24 hr totals of
0.75-1.5 inches and locally up to 2-3 inches will warrant the Flash
Flood Watch to remain in effect today and into tomorrow. The severe
threat with storms today looks to be low but an isolated storm
producing strong winds and hail is not out of the question.

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.DISCUSSION...An area of scattered showers/embedded lighting was
moving across the area this morning being aided by multiple
outflow boundaries moving across the area in addition to the
approaching inverted trof to our east. Some of the showers have
been efficient rainers with rates of over 1" in less than 1 hour.
Should see a winddown in this activity after 8 am with activity
picking up again this afternoon mainly east and south of Tucson.
Main threat from storms today will be heavy rain. This threat
continues tonight into Sunday afternon. Flash flood watch remains
in effect through Sunday afternoon for all of the forecast area.

Monday: Down day for most of the area into the afternoon except
for the areas along the Intl border from Nogales east and up
across the White mtns. Models indicating that a disturbance will
be move down the eastern flank of the upper ridge with center
near Las Vegas. This may bring the potential for a rim shot to
develop late Monday afternoon into the evening hours.

Tuesday and Wednesday: Scattered afternoon and evening showers
thunderstorms. The inverted trof that moved across the area this
weekend will be offshore of Baja California on Tuesday before
lifting across the area as an open wave on Wednesday.

Thursday and Friday: Drier southwest flow aloft will bring a brief
shutdown in monsoonal activity.

&&

.AVIATION...Valid through 15/12Z.
Scattered SHRA, isolated TSRA this morning will decrease in areal
coverage by late morning then pick up later this afternoon into
this evening. Gusty winds up to 40 kts with stronger afternoon and
evening thunderstorms, otherwise expect sfc winds 10 kts or less.
Clouds will be SCT-BKN 7k-10k ft MSL through the period with
occasional MVFR conditions in SHRA/TSRA. Aviation discussion not
updated for TAF amendments.

&&

.FIRE WEATHER...Ample moisture this weekend along with periodic
mid level disturbances will result in scattered to numerous
showers and thunderstorms across southeast Arizona. Storm
coverage then reduces starting Monday to isolated to scattered.
Given the higher levels of moisture and cloud coverage,
temperatures will be cooler this weekend into next. Strong,
erratic winds and locally heavy rainfall will be possible with any
stronger storms that develop. 20-foot winds will generally be
less than 15 mph when not influenced by thunderstorms.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
 Flash Flood Watch through Sunday afternoon for AZZ501>515.

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