Flash Flooding in Arizona, August 2, 2016

Area Forecast Discussion
National Weather Service Tucson AZ
355 AM MST TUE AUG 2 2016

.SYNOPSIS...Expect scattered to numerous showers and thunderstorms
and below normal temperatures through Wednesday. Abundant moisture
in combination with slow moving thunderstorms will be capable of
producing flash flooding. A gradual reduction in showers and
thunderstorms and a warming trend will then start Thursday and
continue into early next week.

&&

.DISCUSSION...Scattered showers and a few thunderstorms were
occurring across portions of southern Pinal County, Graham County,
and northern Cochise County north-to-east of Tucson at this time.
Although most rainfall amounts during the past couple of hours have
generally been less than 0.25", a gauge at Cluff Ranch Reservoir
west of Safford recorded 1.02" since midnight. Other isolated
showers were developing during the past hour near the International
border adjacent Central Pima County and Santa Cruz County south-to-
southwest of Tucson.

The synoptic-scale pattern early this morning depicting a broad
upper ridge over much of the western/central CONUS, with 500 mb
highs centered over eastern Texas and southern Nevada. A fast belt
of westerlies was located over the Pacific Northwest/Northern
Rockies, and a 564 DM low was centered just northwest of the
Washington coast. Closer to southeast Arizona, a weak cyclonic
circulation was evident via water vapor satellite imagery over
northern Sonora Mexico, and a deformation axis between the
aforementioned high pressure centers was over this forecast area.

The various NWP models yielded some differences regarding the timing
and areal extent of rainfall across southeast Arizona during the
next few days. However, forecast confidence was sufficiently high
enough to issue a Flash Flood Watch from 10 AM MST this morning
through late tonight for the zones east of Tucson. Given abundant
moisture in place with precip water values progged generally to
range across the area from 1.50" - 1.90", synoptic-scale ascent as
per the deformation axis aloft and an approaching inverted trough,
and weak storm motions, conditions appear to be favorable for flash
flooding to occur later today into tonight, especially across
eastern sections. However, am somewhat concerned that these Flash
Flood Watch issuances historically have not necessarily yielded the
desired results.

At any rate, the official forecast depicts scattered to numerous
showers and thunderstorms from Tucson eastward into tonight. The
gridded data weather fields depict heavy rainfall this afternoon and
tonight in the Watch area. Slightly decreased PoP chances prevail
across western sections, but remain in the chance-category. Somewhat
lesser precip numbers prevail Wednesday, though essentially the same
scenario of scattered to numerous showers and thunderstorms
particularly east of Tucson, and a chance of showers and
thunderstorms across western sections.

The 02/00Z GFS/ECMWF/CMC/UKMET remained consistent versus their
respective previous solutions in suggesting that a drying trend will
begin at least across western sections Thursday, followed by a
gradual daily reduction in showers/tstms by this weekend. This
scenario seems plausible given the similarities between the
GFS/ECMWF with depicting increased southwesterly flow aloft in
response to an upper trough approaching the California coast by
Thursday. This upper trough is then progged to remain generally
quasi-stationary near the west coast through the weekend. The main
impact of the drying will be across western sections, with precip-
free conditions forecast to occur as early as Friday across western
Pima County. Although the deeper moisture is progged to be shunted
east of the area, moisture should remain adequate to support at
least a slight chance of showers/tstms from Tucson eastward to the
New Mexico border, mainly during the afternoon/evening hours.

High temps today and Wed will average about 5-10 degs below normal.
A warming trend will then begin on Thursday and continue into this
weekend, with daytime temps Sat-Mon to average a few degs above
normal for most locales.

&&

.AVIATION....Valid through 03/12Z.
Sct-bkn clouds around 8-12k ft AGL with Isold-Sct SHRA/TSRA becoming
sct-nmrs SHRA/TSRA this afternoon and evening. Brief MVFR conditions
around stronger TSRA along with wind gusts to 45 kts. Otherwise, sfc
winds generally 10 kts or less thru the period. Aviation discussion
not updated for TAF amendments.

&&

.FIRE WEATHER...Scattered to numerous afternoon and evening showers
and thunderstorms are expected each day through much of week, with
occasional nighttime activity. Locally heavy rain will occur with
the strongest storms as they will be efficient rain producers today
and Wednesday where local flooding is possible mainly zones 152, 153
and eastern 151. Winds will generally be less than 15 mph except
near thunderstorms where gusts to 50 mph is possible. A slow and
weak drying trend will start late in the week.

&&

TWC Watches/Warnings/Advisories...
    Flash Flood Watch from 10 AM MST this morning through late
      tonight for AZZ507>514.

&&

DISCUSSION...Francis
AVIATION/FIRE WEATHER...Cerniglia

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