Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 271630
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
930 AM MST TUE SEP 27 2016

.SYNOPSIS...A low pressure system over northern Baja California will
move north into southern California Wednesday. This system will
bring showers and thunderstorms into this evening. There is a chance
of showers and thunderstorms mainly east of Tucson Wednesday, then
more moisture from tropical system Roslyn will increase rain chances
Thursday. Dry conditions will then prevail this weekend.

&&

.DISCUSSION...Scattered to numerous showers and a few thunderstorms
were occurring mainly across the central portion of this forecast
area at this time. Precip-free conditions were east of a Safford-
Bisbee line, and across western Pima County. Rainfall amounts since
early this morning have been mostly less than one quarter of an
inch, though some amounts mainly near mountains have been one third
of an inch or so.

These showers and thunderstorms were ahead of an upper low pressure
system centered this morning over the northern Gulf of California.
Have noted that the 14Z-15Z HRRR solutions and the 27/12Z Univ of AZ
WRF-NAM were fairly similar with decreasing the coverage of the
ongoing showers/thunderstorms while moving this area northward
during the next 2-3 hours. Thereafter, additional showers/tstms are
progged to develop southwest-to-south of Tucson by early this
afternoon, then northward and westward across much of the area.

Based on satellite/radar trends, especially given the clearing near
the International border south of Tucson, these solutions seem to be
plausible. Thus, scattered to numerous showers and thunderstorms
should prevail this afternoon and evening. The stronger
thunderstorms still have the potential to produce patchy blowing
dust this afternoon, as per the inherited forecast depiction for
locales northwest of Tucson. The coverage of showers/tstms should
decrease late this evening, and a slight chance of showers continues
late tonight into Wednesday morning.

For this update, will make some upward PoP adjustments this
afternoon, mostly across eastern sections. For the time being, only
minor maximum temp adjustments will occur in spite of the recent
rainfall.

Please refer to the additional sections for further detail.

&&

.AVIATION...Valid through 28/18Z.
Scattered to numerous -TSRA/-SHRA decreasing in coverage thru 18Z
today, but scattered to numerous -TSRA/-SHRA developing mostly SW-
west of KTUS early this afternoon and spreading northward across the
area into this evening. Brief wind gusts to 40 kts with the stronger
TSRA. Decreasing precip coverage tonight with isolated -SHRA mainly
east of KTUS Wednesday morning.

Otherwise, cloud decks generally 6k-10k ft AGL into Wednesday
morning. Surface wind ely/sely 10-15 kts with gusts to 20 kts this
evening then decreasing to less than 10 kts late tonight into
Wednesday morning. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Periods of showers and thunderstorms will occur into
this evening followed by a chance of thunderstorms mainly east of
Tucson Wednesday. Scattered showers and thunderstorms should return
for most sections Thursday, then a slight chance of thunderstorms
east of Tucson Friday. Dry conditions area-wide Saturday into early
next week. East winds will continue through Thursday, then normal
diurnal wind trends Friday through Monday.

&&

.PREV DISCUSSION /245 AM MST/...Water vapor imagery this morning
showed upper level cut-off low over central Baja/Gulf of California.
Radar showed scattered showers/thunderstorms across northern Baja
moving north northwest while across SE Arizona there were spotty
light showers. As the cut-off low lifts to the north today, showers
will be increasing in coverage as the morning progresses, especially
south of Tucson. Latest HRRR runs along with 12z UofA WRF runs of
NAM/GFS all spread showers/storms near the intnl border north and
northeast across Pima/Santa Cruz. Some of these storms will briefly
be strong with outflow winds potentially bringing areas of blowing
dust between Tucson and Phoenix. Localized mod/hvy rainers with PW
values slightly over 1". However flash flooding threat low due to
speed the storms will be moving. With that said, made some
significant upward adjustments in PoPs and QPF for central/eastern
Pima and Santa Cruz counties.

Upper low continues north into southern California overnight with
gradual decrease in areal coverage of showers across the area.

Some moisture from TS Roslyn may spread toward the area on Wednesday
and Thursday keeping a chance of showers and storms across the area
as Monsoon 2016 comes to a close.

Drier SW flow aloft will push moisture to the east on Friday but
enough will be around to support a slight chance east of Tucson.

Dry westerly flow aloft this weekend into early next week as upper
level trofs pass by to the north.

Although warmer today versus Monday, highs will remain below normal
through Thursday then hang around normal values Friday into early
next week.

&&

.CLIMATE...The rainfall total at Tucson International Airport since
midnight is 0.10". This brings the total rainfall at TIA since June
1 to 7.20". The 7.20" ties for 31st place in total monsoon season
rainfall (defined as June 15 - September 30).

&&

.TWC WATCHES/WARNINGS/ADVISORIES...NONE

&&

$$

Francis/Meyer/Glueck

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