Flash Flood in Arizona, September 7, 2016

Area Forecast Discussion
National Weather Service Tucson AZ
904 AM MST WED SEP 7 2016

.SYNOPSIS...Tropical Storm Newton will move through the area today,
weakening as it moves north but not before bringing widespread
rainfall and breezy winds to the area. The showers will diminish
rapidly this evening as storm moves off to the northeast with
lingering moisture maintaining a few thunderstorms Thursday and
again later in the weekend.

&&

.DISCUSSION...
Tropical Storm Newton continues to quickly move through Mexico this
morning with a 15Z position about 70 miles south of Nogales, AZ.
Stratiform rain continues to overspread southeast Arizona although a
few lightning strikes were noted across far western Pima Co. Thus
far precip totals have been on the order of a half inch to an inch
in the lower elevations with higher amounts (2+ inches) in the
higher terrain of Cochise and Santa Cruz Co's, although some higher
amounts have been observed in Nogales, Mex. At this point it's
questionable if it will retain Tropical Storm strength as it crosses
into the Arizona. Will need to hunt around for strong enough winds
once it enters the state before determining it's status as the first
Tropical Storm in the state since Nora in 1997.

The only area of concern so far is elevated flow along Nogales Wash
as well as washes in the Huachuca Mtns. Flooding elsewhere has been
minor and relatively insignificant (mostly poor drainage areas). In
terms of rain rates, they have steadily increased and are generally
on the order of 0.25-0.50 inches per hour. Normally this wouldn't be
anything to be too concerned about but radar indicates there is
still quite a bit of precip south of the border heading our way.
Efficient warm rain processes will dominate in a tropical
environment and as such, there will be a continued threat of
flooding across the area. Latest few runs of the HRRR suggest the
stratiform rain will migrate north of the border and through Tucson
shortly before noon and given current rain rates, it's conceivable
that we could see an additional 0.5-1.0 inches of rain area-wide
through noon. Looking ahead into the afternoon hours, things get a
bit more tricky. There is quite an extensive cloud shield extending
through Sonora into the Gulf of California and it could very well
stay overcast all day. However if a few breaks develop (as the HRRR
and HRRRx are suggesting), convection could develop later in the
afternoon hours. I'm going to leave the Flash Flood Watch in effect
unchanged and retain elevated PoPs across the area through at least
early evening to give the stratiform rain sufficient time to exit
the area, as well as to give storms a chance to develop and
dissipate if we get clearing. Otherwise the precip portion of the
forecast looks on track.

I did make an adjustment to the winds and have replaced the High
Wind Watch with a Wind Advisory for the mountains along the US/Mex
border. While we have seen one location gusting into the upper
60+mph range, it has a favorable east exposure all other sites are
well below High Wind criteria. Winds will certainly be breezy today
and should reach their peak by early afternoon, but should remain in
the 25G35-40KT range.

&&

.AVIATION...Valid thru 08/12Z.
Scattered to numerous -SHRA with embedded isold TSRA thru 08/04z
as remains of Tropical Storm Newton impacts the area. Cloud decks
mostly 6-10k ft agl. Locally heavy rain and MVFR CIGS/VSBY is likely
with stronger cells. Conditions slowly improving from W to E aft
08/05z.

Surface winds will continue to be on the incr thru 07/23z as the
circulation reflection of Newton pushes into the area aft 07/16z.
Strongest winds are expected to occur across Santa Cruz and srn
Cochise counties. Looks for sustained winds of 20 to 30 kts with
gusts to 40 kts will primarily affect the KOLS and KFHU with less
wind at KTUS. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Showers and isolated thunderstorms will be around
today as the remnants of Tropical storm Newton pushes across the
area. Strong winds of 20 to 30 mph with gusts to 40 mph will also be
an issue this morning and afternoon, especially near the Int'l
Border. Lingering moisture will result in a slight chance of showers
and thunderstorms Friday into Sunday, with 20-ft winds returning to
their normal diurnal trends.

&&

.CLIMATE...The lowest maximum daytime temperature on record for
Tucson International Airport today, September 7, is 77 degrees set
in 1919. At midnight it was 77 degrees and dropping and provided we
don't rise above 77 the remainder of the day we will have tied the
old 97 year old record. The maximum recorded rainfall for Tucson
International Airport on Wednesday, September 7, is 1.15 inches set
in 2006.

&&

.PREV DISCUSSION...
Once Newton slips off to the northeast the shower and storm activity
will trail off quickly this evening. Without significant flow to
clear out the moisture tomorrow would expect the sun to work on the
moist air mass and trigger scattered convection Thursday around the
area. This is covered well in the current forecast. Temperatures
Thursday will rebound significantly from the near record "cool"
conditions today but will still remain a bit below normal.

Friday into Saturday a 590+ 500 mb high over the area will allow
afternoon temps to climb further to above normal levels again and
should suppress convection across the area thus I went for a mostly
dry forecast for those two days and tweaked temps up a couple of
degrees. May see another 100 in Tucson before we put that to bed.

By Sunday, an upper level trough is begining to develop along the
west coast while the high slips to the east which will allow for a
return flow of some moisture and scattered convection Sunday onward.
This will also allow afternoon temperatures to gradually trend down
once again back to seasonal norms.

&&

.TWC Watches/Warnings/Advisories...
Flash Flood Watch until 8 PM MST this evening for AZZ502>515.

Wind Advisory until 5 PM MST this afternoon for AZZ503-504-507-
508-512>515

&&

PUBLIC...Leins/Cerniglia
AVIATION....Glueck
FIRE WEATHER...Glueck

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