Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 180345
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
845 PM MST Sun Sep 17 2017

.SYNOPSIS...Expect isolated afternoon and evening thunderstorms
mainly east to south of Tucson through Saturday then dry conditions
next Sunday. Near normal daytime temperatures will occur through
Wednesday followed by a cooling trend later this week. Gusty winds
will also occur at times, but should be most prevalent Thursday into
Saturday.

&&

.DISCUSSION...Aside from a few cirriform clouds east of Tucson,
clear skies prevail across southeast Arizona at this time. Dewpoints
at lower elevations valid 8 pm MST ranged from the lower 30s in
south central Pinal County to around 50 degrees near the
international border south-to-southeast of Tucson. 18/00Z KTWC
sounding total precip water value was only 0.67 inch, down 0.25 inch
since the 17/12Z KTWC sounding. The sounding depicted a very stable
environment with moderate-to-fast swly/wly flow throughout much of
the column.

Expect clear skies the rest of tonight, or perhaps just a few thin
cirriform clouds during the next hour or so for some locales.
Surface winds will be fairly light, and low temperatures around
daybreak Monday are forecast to be within a couple of degrees plus-
or-minus of seasonal normals. The official forecast was updated
early this evening to remove the mention/depiction of showers/tstms
near the White/Chiricahua Mountains.

Based on the 17/18Z GFS and a quick glance at the 18/00Z Univ of AZ
WRF-NAM, opted to increase PoPs slightly Monday afternoon and
evening southeast of Tucson, or across about the southern half of
Cochise County. The GFS was more robust with measurable precip
chances versus the NAM, but the WRF-NAM developed precip echoes
early Monday afternoon that subsequently move rapidly eastward.
Otherwise, precip-free conditions for much of the area Monday with
daytime temperatures quite similar to today.

Please refer to the additional sections for further detail.

&&

.AVIATION...Valid thru 19/06Z.
Isolated -TSRA/-SHRA south to southeast of KTUS Monday afternoon and
evening. Although the best chance of -TSRA/-SHRA for the three TAF
sites is at KDUG, forecast confidence is too low to include in the
KDUG TAF at this time. Otherwise, clear skies into early Monday
morning, then a few to occasionally broken clouds around 8k-12k ft
AGL late Monday morning into Monday evening. Surface wind Monday
afternoon swly to nwly 10-18 kts with gusts to 25 kts. Surface wind
less than 12 kts at other times. Aviation discussion not updated for
TAF amendments.

&&

.FIRE WEATHER...Expect isolated afternoon and evening thunderstorms
mainly east to south of Tucson into Saturday, then dry conditions
will occur next Sunday. 20-foot winds will generally be terrain
driven at less than 15 mph into Tuesday morning, then gusty winds
are forecast to occur especially during the afternoon hours Tuesday
into Saturday. The period when the strongest winds will likely occur
appears to be Thursday into Saturday, and elevated speeds should
continue overnight. Near normal temperatures through Wednesday
followed by a cooling trend later this week.

&&

.PREV DISCUSSION /255 PM MST/...A scattered flat cu field was
present on satellite imagery across areas mainly east of Tucson
early this afternoon. Nothing of consequence showing up on radar
yet, though the development of a few showers or thunderstorms may
still occur into this evening as a weak shortwave begins to push
through the state. Any showers or storms which do develop will
likely be over the terrain near the NM border and will be be fast-
moving and short-lived. The trough axis passes to our east overnight
into Monday as a more pronounced trough begins to affect the Pac NW
states. Isolated convection will be possible once again Monday
afternoon for areas east to south of Tucson as a scant bit of
moisture advects north into Cochise County.

Synoptically speaking, this Pac NW trough will influence our
sensible weather over the next few days as ridging over the eastern
CONUS blocks its progression. Most of the CWA will remain dry under
westerly flow through Wednesday, but a few storms south to east of
Tucson can`t be ruled out. We`ll see the backside of the
aforementioned trough carve sharply southward through CA Wednesday
through Friday, compressing the pressure gradient and increasing our
surface southwesterly winds. Additionally, a moisture increase ahead
of the trough will increase rain chances westward from the NM
border. PoPs were increased 5 to 15 pct across eastern zones
Thursday and Friday to better account for this potential. A few
isolated showers are possible next Saturday, but drier air moving
into the region will hinder most storm development.

Near normal temperatures will occur through Wednesday. Lower heights
will then result in a cooling trend Thursday through Saturday, when
daytime temperatures may be 4 to 8 degrees below normal.

&&

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

&&

$$

Francis/French

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