Areal Flood Warning 9, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
304 PM MST Sat Jul 24 2021

.SYNOPSIS...
An approaching weather system will continue an active monsoon
pattern this weekend. Showers and storms that develop will be
capable of producing heavy rainfall which combined with already
saturated ground will result in a high flash flood threat. A
decrease in storm activity is expected Monday through midweek.
Below normal temperatures will continue through early next week.

&&

.DISCUSSION...The low near the AZ/NM border will continue to
dominate our weather with elevated chances of showers and
thunderstorms into Sunday. With the ground now saturated, water
basins will react very quickly with any additional rainfall. Our
Flash Flood Watch remains in effect through Sunday evening.

The low is starting to fill a bit with the consolidated center
likely to lose some cohesion over the next 12 hours. Short range
hi res deterministics and ensembles are in pretty good agreement
with some elongation of the low as it pushes through Arizona,
reaching the southwest corner of the state by late tomorrow
afternoon. Threat for areas of showers and thunderstorms will
remain high until then, with subsidence behind the low tending to
shut things down Sunday evening.

The remnants of the inverted trough should make southern
California Monday, with our thoroughly-worked-over atmosphere
taking a breather to start the week. With precipitable water
values falling back below 1.4 inches (probably down to about 1.25
or so), we should have a couple of afternoons with below average
storm coverage.

Ensemble means are suggesting the high will remain a little
further north than usual, so the easterlies will have plenty of
room to remain impactful for our area. Ensembles bring another
inverted trough into the weather picture the second half of next
week, with a solid impulse pushing across north central Mexico.
That will set us up for a strong surge by Thursday afternoon or
Thursday night, with a subsequent upturn in thunderstorms (which
will probably continue a trend that starts as early as Wednesday
afternoon). If the high shifts far enough eastward we could draw
the remnants of that wave up from the south and into Arizona
Friday or Saturday. That would make things interesting.

&&

.AVIATION...Valid through 26/00Z.
SCT-BKN clouds at 7k-11k ft MSL thru the period, along with periods
of SHRA/TSRA. Expect MVFR CIGS/VSBYS and mountain obscurations
around TSRA along with gusty outflow winds, potentially in excess of
40 kts with the strongest storms. Otherwise, SFC wind generally less
than 12 kts. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Periods of heavy rainfall are expected through
Sunday, with scattered to numerous showers and thunderstorms.
Flooding issues are expected to continue, due to part to saturated
soils. Showers and storms will be possible through the day and
also at night. Gusty winds in excess of 40 mph and numerous
lightning strikes may accompany stronger storms. Daytime
temperatures will remain below normal. 20-foot winds will generally
be less than 15 mph when not influenced by thunderstorms. Shower and
thunderstorm chances decrease slightly Monday through the middle of
next week.

&&

.HYDROLOGY...Given the chance for heavy rainfall and already
saturated soils the potential for flows in rivers and washes is
high.

A couple of potential trouble areas to make note of...Finger Rock
and Sabino Canyon. Significant sediment and silt have moved around
the Finger Rock area with Thursday night's activity, causing some
localized damage. Any additional high rainfall rates in this area
could cause more trouble. Sabino Canyon also is an area to keep
an eye on, where high rainfall rates could cause issues quickly.

Most river forecasts across southeast Arizona show elevated stream
flow over the weekend. With high enough rainfall amounts other
locations could reach flood stage and possibly out of bank flow.
Stream flow may also take longer to recede if basin receives multiple
days of rainfall. Potentially leading to elevated stream flow into
early next week.

River observations and forecasts can be found at the following
sites:
https://water.weather.gov/ahps2/index.php?wfo=twc
http://www.cbrfc.noaa.gov

&&

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

&&

Public...Meyer
Aviation...Zell
Fire Weather....Zell

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