Flash Flood in Arizona, August 10, 2016

Area Forecast Discussion
National Weather Service Tucson AZ
230 AM MST WED AUG 10 2016

.SYNOPSIS...Expect numerous showers and thunderstorms off and on
through Thursday. Heavy rainfall and flash flooding will occur with
the stronger thunderstorms. A drying trend will then prevail Friday
into early next week.

&&

.DISCUSSION...At this point no denying that the flash flood watch
should have begun about 12 hours earlier than the initial effective
time, which was 5 PM MST yesterday. Storms moved in from northeast
Sonora on the tail end of my shift yesterday morning and spread into
parts of Santa Cruz and Cochise counties, producing copious amounts
of rain in a short period of time, especially in the Sierra Vista,
Hereford and Sonoita areas that resulted in a couple of flash flood
warnings and urban and small stream flood advisories before my shift
ended. However, the day shift did amend the watch to begin at around
10 AM MST, based on the satellite and radar trends at that time.

And now for the current situation. Water vapor imagery shows a ridge
planted over eastern Chihuahua, Mexico with a trough axis extending
from the Pacific Northwest into central California and the eastern
Pacific waters. Plenty of mid to upper level dry air ahead of this
trough covers much of the eastern Pacific, southern California,
northern Baja and a significant part of the southwestern half of
Arizona. Current surface observations show dewpoint temperatures
generally in the 60s for most of my forecast area. In addition, the
latest GPS Precipitable water values from the Univ of AZ show Tucson
at a tad over 1.5 inches, with similar readings elsewhere.
Hermosillo, Mexico shows a value of just under 2.25 inches. Also,
the latest CIRA LPW Totals as of 06Z show values of 1.1-1.4 inches
over southeast Arizona, while values of 1.7-2.0 inches just on the
other side of the international border and values exceeding 2.5
inches reside over the north central Gulf of California and south of
Hermosillo (MMHO), Mexico. Based on these observations, including
yesterday afternoons KTWC sounding the bulk of the moisture is in
the lower to mid levels with drier air above 500MB. Models continue
to indicate that the very moist air to our south will make its way
northward today, with readings in the 1.75 to near 2 inch range over
a good part of my forecast area.

The most difficult part of the forecast for today is determining if
a flash flood watch is indeed needed for all of my forecast area.
Initial impressions would suggest that removing western and central
parts of Pima county, which include the Tohono O'Odham Nation may
not be a bad idea. However, models are not much help this morning,
with depictions of moisture and QPF fields from various models in
AWIPS2 definitely over playing the situation since they did not
initialize well and all currently show widespread showers over much
of northern Sonora and extreme southern Arizona, yet radar is only
showing a couple of isolated cells straddling the Graham and Cochise
county line, with showers and storms also near Silver City, NM,
while northern Mexico is void of echoes at this time.

The Univ of AZ WRF/NAM & GFS models seem to be doing a fairly decent
job at what is currently ongoing, which is not much. The WRF/NAM
shows isolated showers/storms developing around sunrise over mainly
Pinal county, with activity then increasing during the late morning,
mostly from around Sells and eastward to the New Mexico border, but
mostly isolated to scattered in nature. The WRF/GFS shows convection
sprouting up over most of the forecast area, but with more of an
emphasis along the border with Mexico, especially during the
afternoon hours. However, it also shows convection mostly isolated
to scattered. That said, the MOS POP numbers from all the models
depict fairly high numbers, generally scattered to numerous for
today. Based on the drier mid to upper level air having moved in
from the west (although mid to lower levels will continue to be
quite moist), I did decrease POPs a bit across the board mainly for
this morning, but still in the numerous to categorical range for
this afternoon. Think we will still be plenty moist and unstable
enough to generate quite a bit of shower and thunderstorm activity,
and it appears that there is a disturbance riding along the
dry/moist air interface over northern Sonora that could help to
enhance activity today. So for now, will keep the flash flood watch
as is but the day shift may be able to modify it to account for the
satellite/radar trends during their shift. I will also be very
curious to see what our morning sounding reveals in a couple of
hours.

Once the Pacific trough slides by to our north and east Friday, we
will see a considerable downswing in thunderstorm activity through
the remainder of the forecast period and mainly along the border
with Mexico and New Mexico, mostly during each afternoon and
evening.

For Tucson, high temperatures will generally range from 5 to 7 degs
below normal today and Thursday, then hover within a degree or two
either side of normal each day through early next week. Low temps
will be near normal Thursday through Saturday mornings, then around
2 to 4 degs above normal for Sunday through Tuesday.

&&

.AVIATION...Valid through 11/12Z.
FEW-SCT clouds around 10k ft AGL into late morning with increasing
clouds and lowering CIGS through the day. SHRA/TSRA will occur again
this afternoon, with very heavy rain/brief MVFR conditions possible
with the stronger TSRA. Sfc winds generally 10 kts or less outside
of possible wind gusts to 40 kts associated with TSRA. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Scattered to numerous showers and thunderstorms will
occur through Thursday, with the potential for very heavy rain. Much
higher relative humidities and cooler daytime temperatures are also
expected through Thursday. Thereafter, warmer temperatures and a
drying trend will occur into early next week, with the best rain
chances mainly from Tucson east to the NM border. Outside of
thunderstorm wind gusts, 20-ft winds will generally remain less than
15 mph.

&&

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

&&

PUBLIC...Mollere
AVIATION/FIRE WEATHER...French

Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson