Flash Flooding and Flooding in Arizona, August 16 to 17, 2021

Area Forecast Discussion
National Weather Service Tucson AZ
831 AM MST Mon Aug 16 2021

.SYNOPSIS...Showers and thunderstorms will be on the increase
today into this evening across the area. Heavy rain, flash
flooding and severe thunderstorms are the threats today. More
typical monsoon days are expected Tuesday and Wednesday with most
locations dry by Thursday into Saturday. Temperatures will remain
below normal for the next few days.

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.DISCUSSION...The ingredients are all coming together for a very
active period over the next 18 hours or so. Mostly sunny skies this
morning across most of our forecast area with continued very moist
conditions. It's quite rare that KTUS has dewpoints around 70
degrees under clear skies. The 12Z KTWC sounding came in with a PWAT
of 1.69 inches which is well above climatological normal levels for
mid August. Meanwhile, the atmosphere is already quite unstable with
MLCAPE above 1600 J/KG. In addition, the wind profile is quite
favorable with mid level steering winds from the northeast with
anvil level westerly winds.

The shortwave that we've been watching coming down the east side of
the mid/upper level subtropical high is located along northern
sections of the AZ/NM border this morning and will help give an
extra boost to the convection. HREF members are in excellent
agreement regarding the evolution of the convective cycle today.
Scattered showers and thunderstorms should develop across the higher
terrain, especially to the south and east of Tucson after 11 AM and
continue into the afternoon. Steering winds along the international
border are weaker, thus those storms to the south of Tucson will be
slower moving with an increased flash flood threat. In addition,
some of those stronger cells will be capable of producing severe
wind gusts and hail. The main push of convection will simultaneously
develop across the Mogollon Rim this afternoon and move to the S/SW
into the lower elevations of SE AZ late this afternoon through this
evening. Some of these storms coming off the Mogollon Rim will have
the potential for more widespread gusty outflow winds and localized
severe criteria winds in addition to the heavy rain and flash flood
threat. We can't rule out some blowing dust in this scenario in the
normally favored locations to the NW of Tucson. However, given the
antecedent wet conditions confidence is low. Otherwise, any
remaining convection should diminish overnight.

Current forecast is in good shape. No updates this morning.

&&

.AVIATION...Valid through 17/12Z.
A few -SHRA are psbl this am N of KSAD otrw SCT SHRA/TSRA will dvlp
this aftn aft 16/19z over the higher trrn and mv S/SW across the
area thru 17/04z. Gusty winds up to 45+ kts are psbl with these
storms, otrw expect sfc winds 10 kts or less. Clouds will be SCT-BKN
6k-11k ft MSL thru the pd with ocnl MVFR conds in SHRA/TSRA.
Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...The active monsoon continues through mid-week
as a series of weather disturbances transition across the area and
plenty of moisture remains in place. Flash flooding along with
strong to severe winds and hail are the main threats this
afternoon and tonight. Thereafter, storm coverage will become
more scattered in nature with locally heavy rain the primary
threat. By Thursday into the weekend most locales should remain
dry with isolated thunderstorms mainly limited to the higher
terrain of Graham and Greenlee County. Below normal temperatures
are expected through mid- week before a gradual warm up begins
late in the week. Min RH values remain well above critical levels.
20-foot winds will generally be less than 15 mph when not
influenced by thunderstorms.

&&

.PREV DISCUSSION...Quiet morning across SE AZ while regional radar
was showing storms across S-Cntrl and SW AZ along with isolated weak
cells across Navajo and Apache counties. Those across Navajo and
Apache counties likely associated with mid-level shortwave moving
around eastern flank of upper high level centered across central CA.
Scattered showers and thunderstorms developing over the mountains
and the rim early this afternoon with steering flow pushing the
storms mainly to the S/SW this afternoon into this evening. Main
hazards with the storms will be strong to severe wind gusts and
heavy rain with a rather moist atmosphere in place for mid-August.
SPC has most of the area under Marginal risk which may be upped to
SLGT risk in later updates to the Day 1 convective outlook. WPC has
a portion of our area under SLGT risk for excessive rainfall. Some
flash flooding is likely due to the antecedent wet conditions, thus
a Flash Flood Watch is in effect until 2 am Tuesday.

Tuesday: Steering flow remains out of the N/NE thus thunderstorms
that develop over the higher terrain will move to the S/SW. Looks
like another potential active day but that'll be dependent on how
active today/tonight is and how worked over the atmosphere is.

Wednesday: Flow aloft become southwest as a short wave moves into
the area. Thunderstorm chances mainly from Tucson east.

Thursday into Saturday: Monsoon break time as precipitable water
values fall below 1" across portions of the area during this time
period thus limiting any thunderstorm development to the very
isolated side.

Sunday: There are hints in the model world that we may see a return
of monsoonal moisture as flow aloft become S/SELY.
Confidence is low at this time.

Temperatures will remain below normal through Wednesday, then
gradual warming thereafter into the weekend.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
 Flash Flood Watch through late tonight for AZZ501>515.

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