Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 251625
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
925 AM MST SUN SEP 25 2016

.SYNOPSIS...A strengthening low pressure system southwest of the area
will bring scattered showers and isolated thunderstorms tonight into
Monday. Strong and gusty east winds will also develop tonight and
continue into Monday. Scattered showers and thunderstorms should
prevail Tuesday mainly from Tucson westward, then a chance of
showers and thunderstorms mainly east of Tucson Wednesday into
Thursday. Expect much cooler temperatures Monday followed by a
warming trend later this week.

&&

.DISCUSSION...Visible satellite imagery and surface observations
depict partly cloudy skies east-to-south of Tucson, and mainly clear
skies elsewhere across southeast Arizona at this time. A dry surface
regime prevailed, with dewpoints valid 16Z ranging from the mid 20s-
mid 30s. These temps were about 10-20 degs lower versus 24 hours
ago. Surface temps valid 16Z were generally within a few degs of
temps recorded this time Saturday.

25/12Z KTWC sounding total precip water value of 0.45 inch was down
0.16 inch versus 24 hours ago. 25/12Z upper air plots depicted
strong high pressure centered west of the northern California coast
near 38N/130W; a trough was over the northern CONUS plains; and a
low was analyzed near the far southeast AZ/far southwest NM border.
Moderate nly/nely flow prevailed above 600 mb, and light ely flow
was observed in the surface-600 mb layer.

The upper low southeast of Tucson is progged to move southwestward
and strengthen before becoming centered over central Baja California
Monday morning. This seems plausible given that the strongest winds
aloft this morning were on the northwest periphery of the low, as
per the nearly 100 kt/300 mb wind observed at KFGZ. Precip-free
conditions will continue across much of the area into this evening.
However, a slight chance of showers/tstms exists mainly near the
International border adjacent Santa Cruz/Cochise Counties.

Strong and gusty east to southeast winds should develop tonight and
continue into Monday. Although moisture will also increase across
the area, this may be the most significant impact from the
strengthening upper low, at least thru Monday. Wind speeds may
achieve wind advisory criteria for some locales, with the favored
areas as per the 25/12Z NAM/GFS MOS guidance to be across portions
of Cochise County northwestward to the Tucson metro area.

Meanwhile, rain chances will increase from southeast-to-northwest
tonight into Monday. The inherited official forecast of scattered
showers and a slight chance of thunderstorms Monday seems quite
reasonable at this time. High temps this afternoon will be quite
similar to temps achieved Saturday, then daytime temps Monday will
be 5-10 degs cooler versus this afternoon.

There are no updates planned at this time. Please refer to the
additional sections for further detail.

&&

.AVIATION...Valid through 26/18Z.
A low pressure system will strengthen southwest of the area into
Monday morning. This system will bring a slight chance of -TSRA/
-SHRA this afternoon and evening mainly near the International
border south-to-southeast of KTUS, and may affect KOLS/KDUG.
Scattered -SHRA with isolated -TSRA is then expected to spread
northwest across the area tonight into Monday morning.

Surface wind this afternoon will generally be ely/sely at 5-15 kts
with gusts to 20 kts, then surface wind tonight into Monday morning
ely/sely 12-25 kts with gusts to 35 kts. Clear skies northwest of
KTUS thru much of the period. A FEW-BKN clouds KTUS vicinity sewd
thru this afternoon, then cloud decks tonight into Monday mainly
6k-10k ft AGL. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...A dry regime today will be followed by increasing
moisture and rain chances by this evening and continuing through
much of the upcoming week. There is a slight chance of showers and
thunderstorms this afternoon and evening south-to-southeast of
Tucson, then scattered showers with isolated thunderstorms late
tonight into Monday. Stronger thunderstorms may occur Tuesday,
especially from Tucson westward into western Pima County. A chance
of showers and thunderstorms exists Wednesday into Thursday mainly
east of Tucson, then showers and thunderstorms Friday and Saturday
should be limited to near the New Mexico border.

The strengthening low pressure system south of the area will also
bring strong and gusty east winds tonight into Monday night. 20-foot
winds will likely be mainly in the range of 15-25 mph with higher
gusts. Winds should begin to decrease Tuesday followed by elevated
southwest winds during the latter part of the week.

&&

.PREV DISCUSSION /305 AM MST/...Water vapor imagery this morning
showed a developing upper level low just east of Douglas. This
feature will be the main driver of our weather for the next four to
five days. As this upper low continues to sag southwest across
Sonora Mexico, the flow aloft will become easterly. Back door cold
front will push low-mid level moisture back across the area today.
Best chances for showers and a few t-storms will be along the
international border as far west as San Miguel. Mid-level gradient
peaks later tonight into Monday morning with strong gusty east winds
in favored areas from Tucson south and east. MOS guidance would
suggest that some areas might meet wind advisory criteria. Something
for later shifts to ponder. Chance of light showers/slight chance of
thunder will push west northwest tonight into Monday morning.

The threat of showers and t-storms will continue Monday night into
Wednesday as the above mentioned upper low, after bottoming out near
the Baja Spur, lifts north into western Arizona. The 25/00z runs of
the UofA WRF NAM and GFS suggest that afternoon thunderstorms on
Tuesday may get rather frisky west of Tucson.

The later part of this week will see some drier air push in with
chance of showers mostly east of Tucson on Thursday and Friday.

Below normal high temperatures will be around through Wednesday.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

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