Special Weather Statement, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
307 AM MST Fri Jul 23 2021

.SYNOPSIS...An approaching weather system from the east will
continue to bring periods of widespread showers and thunderstorms
to the area through Saturday night. Heavy rainfall combined with
a saturated ground may result in areas of flash flooding and
flooding during this period. Improving weather can be anticipated
late Sunday, although elevated flows in area washes and streams
will still be possible. Thereafter, expect fewer storms to start
the week before increasing again by Thursday.

&&

.DISCUSSION...Our main weather driver will be an unusually strong
and coherent mid level low embedded in the easterlies under high
pressure to our north. As this system pushes slowly across New
Mexico and into our area over the next 36 hours, we will see
periods of widespread shower and thunderstorm activity. With
precipitable water values maxing out as high as 2 inches tonight
into early Saturday combined with large scale synoptic forcing,
flash flooding and flooding will continue to be a threat through
at least Saturday night.

Two large scale convective complexes in the upper level diffluent
field ahead of the low fed much of our weather over the past 8
hours, with one over Tucson and another over southern portions of
Flagstaff's area to our north. A remnant MCV drifting slowly to
the southwest will continue to focus moderate to locally heavy
shower activity this morning, with heaviest additional rainfall
from western Tucson to Silver Bell and Three Points. Broad outflow
from the complex north of our area will reinforce this shower
activity by 8 AM for Tucson Metro. From mid morning to early
afternoon, the strongest storms will tend to fire around the
periphery of this feature south and west of Tucson.

Partial clearing east and northeast of us this afternoon, so
expect organized convection near and north of the Mogollon Rim and
west central New Mexico as the low pushes closer. This will
likely lead to another overnight heavy rainfall event that would
tentatively include Graham and Pinal counties before midnight,
spreading southward into Pima and Cochise counties before 3 AM.

Ensembles are in good agreement with the main low shifting
overhead by Saturday night with subsidence and partial drying
behind the low as it slowly shifts west of us by late Sunday and
the heavy rain threat wanes.

A worked-over atmosphere, weak storm level flow and mid level
warming / subsidence behind the low should lead to below average
thunderstorm coverage Monday and Tuesday. Ensemble means suggest
our typical flow pattern resumes by Thursday for average coverage
typical for late July and early August.

&&

AVIATION...Valid through 24/12Z.
SCT-BKN clouds at 8k-12k ft MSL and scattered to numerous SHRA/TSRA
through the period. Expect MVFR CIGS/VSBYS and mountain obscurations
around TSRA along with gusty outflow winds, potentially in excess of
45 kts with the strongest storms. Otherwise, SFC wind generally less
than 12 kts following diurnal trends. Aviation discussion not
updated for TAF amendments.

&&

.FIRE WEATHER...Periods of heavy rainfall are expected into the
weekend, with scattered to numerous showers and thunderstorms.
Flooding issues are expected to continue, due to part to saturated
soils. While in this pattern, showers and storms will be possible
throughout both the daytime and nighttime. 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.
A bit of a reprieve from showers and storms is possible 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.
The latest river forecasts from the Colorado Basin River Forecast
Center show Sabino Creek, Pantano Wash near Vail, and Santa Cruz
River at Trico Road rising near or to flood stage on Saturday.
However, 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

&&

.CLIMATE...Daily rainfall records are likely to be broken across
southeast Arizona through Saturday. Interesting high temperature
note. There is a 20% chance that the high temperature in Tucson
on Saturday may may not get out of the 70s. Since 1895, there have
only been 10 days in July in which the high temperature was in
the 70s with the last occurrence being July 22, 1998 when the high
was 76. Interesting.

&&

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

&&

Public...Meyer
Aviation...Carpenter
Fire Weather....Carpenter

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Special Weather Statement
National Weather Service Tucson AZ
647 AM MST Fri Jul 23 2021

AZZ502-231430-
Tohono O'odham Nation including Sells-
647 AM MST Fri Jul 23 2021

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL PIMA COUNTY UNTIL 730 AM
MST...

At 646 AM MST, Doppler radar was tracking a strong thunderstorm near
Sells, moving southeast at 40 mph.

Dime size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Sells, Ali Chukson, Gu Oidak, Fresnal, No:ligk, Cababi, San Luis -
Sells District, Artesia, Rincon and Ali Molina.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with this storm, and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3188 11207 3210 11207 3215 11193 3188 11167
TIME...MOT...LOC 1346Z 332DEG 35KT 3200 11193

Singer