Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 231708

Area Forecast Discussion
National Weather Service Tucson AZ
1008 AM MST Sat Sep 23 2017

.SYNOPSIS...Clearing from the west today with relatively cooler
temperatures and drier conditions by this afternoon. Breezy at
times. A few showers and thunderstorms may occur near the New
Mexico border Tuesday. A low pressure system will then bring
scattered showers and thunderstorms Wednesday into Thursday with
lingering showers continuing east of Tucson next Friday.


.DISCUSSION...As a trough digs into the area from the west, we`ve
seen a narrow band of light showers spread across the area from
the west this morning. Currently located in Cochise county and
extending northward through Graham into Apache county and even
western Colorado. This is associated with a 700hPa jet immediately
ahead of the trough and will continue to push eastward and out of
the area today. As usual with an early, weak transition season
system, the southern extent of the surface frontal boundary is
fragmenting and difficult to track in our terrain. It looks like
it`s draped through our area currently, with temperature and dew
point trends down compared to yesterday and a couple of locations
with northwesterly winds. The Phoenix dew point is definitely an
indicator, down 25 degrees compared to yesterday. Going strictly
from H7 trends it will clear our area overnight with a much drier
airmass in place Sunday.

Still on track for our coolest temps since May this weekend, and
our current forecast reflects this well. Please see the previous
discussion below for additional details.


.AVIATION...Valid thru 24/18Z.
Clearing from the west today with a few -SHRA/-TSRA east of KTUS
ahead of a weak frontal boundary. Remaining VFR with the -SHRA.
Surface wind this afternoon swly/wly 10-20 kts with gusts near 25
kts. Surface wind variable in direction less than 12 kts at other
times. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions will prevail through Monday night
except for scattered showers and thunderstorms across the White
Mountains today. A slight chance of showers and thunderstorms exists
near the New Mexico border Tuesday. Expect scattered showers and
thunderstorms Wednesday into Thursday, then lingering showers east
of Tucson next Friday. Some gusty southwest to west winds this
afternoon followed by terrain driven winds less than 15 mph Sunday
into Tuesday. Gusty east winds may occur Wednesday into Friday.


.PREV DISCUSSION...Any showers/tstms are expected to move east of
the area by this evening, then clear skies will likely occur at
least across much of the area tonight through at least Sunday
night and perhaps into Monday.

The synoptic scale upper pattern over the eastern Pacific and CONUS
by Monday will be characterized with a longwave trough continuing
over the western states, and strong high pressure over the eastern
Pacific and over the northeastern CONUS. Dry conditions will
continue Monday night and across much of the area into Tuesday as
the flow aloft becomes increasingly southwesterly in response to low
pressure developing over southern Nevada. Have maintained a slight
chance of showers/tstms mainly near the New Mexico border Tuesday
afternoon and Tuesday night.

Thereafter, the deterministic 23/00Z GFS/ECMWF/CMC and their
respective ensembles were in fairly decent agreement with the
development of low pressure aloft to occur generally over the lower
Colorado River Valley Wednesday. There were differences between the
GFS/ECMWF regarding how far south this upper low will dig Wednesday
night. The GFS was the furthest south, with the upper low progged to
centered near Yuma around daybreak Thursday. Meanwhile, the ECMWF
kept the upper low further north to near the Las Vegas Nevada area.
Given the further south upper low position, the GFS was definitely
more robust with PoPs and liquid rain amounts versus the ECMWF.

However, per coordination with neighboring WFO`s, opted to increase
PoPs substantially during the Wed-Thur period. Thus, chance-category
or scattered coverage of showers/tstms is now expected to occur
especially from Tucson eastward Wednesday afternoon and Wednesday
night. Dry conditions should still continue across western Pima
County. The upper low is progged to lift northward Thursday into
northwest Arizona (GFS) or southern Utah (ECMWF).

Drier southwesterly flow should move into at least western portions
of the area. PoPs were still raised into the chance-category east of
Tucson, and a slight chance of showers/tstms continues for Tucson
Thursday. Precip chances should then decrease from west-to-east
Thursday night into next Friday as the weakening upper low remains
over southern Utah. However, enough moisture should exist to support
a slight chance of showers and thunderstorms Friday, especially east
of Tucson. Dry conditions should prevail west of Tucson next Friday.

The longwave trough over the western CONUS will translate into
daytime temperatures that will generally range about 2-10 degrees or
so below seasonal normals during much of this forecast period.






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