Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 231020

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

.SYNOPSIS...Scattered showers and thunderstorms across the White
Mountains today followed by dry conditions tonight through Monday
night. 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...IR satellite imagery and surface observations depicted
clear skies across western/central Pima County west of Tucson early
this morning , and clear skies were also across much of Cochise
County. Meanwhile, lower and mid-level clouds were noted generally
in an arc from the White Mountains southwestward into southeast
Pinal County, and these clouds continued southward in a narrow band
across the Tucson metro area and to near the international border
west of Nogales. These clouds were ahead of a mid/upper level trough
axis that extended from low pressure centered over Nevada southward
into far southwestern Arizona.

Several HRRR solutions as well as the 23/00Z NAM12 and the SREF
suggest that enough moisture will exist today to support scattered
showers and thunderstorms across the White Mountains with precip-
free conditions elsewhere. The 23/00Z Univ of AZ WRF-NAM and WRF-GFS
were markedly less robust with measurable precip potential across
the White Mountains. At any rate, the HRRR has been consistent with
depicting about a 6 hour or so period for measurable precip to
occur, essentially from late morning through about mid-afternoon.
Any showers/tstms are expected to move east of this forecast 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

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.


.AVIATION...Valid thru 24/12Z.
Scattered -TSRA/-SHRA mainly across the White Mountains northeast of
KSAD this afternoon. Otherwise, mainly clear skies west of KTUS into
Sunday morning. KTUS vicinity eastward scattered to broken clouds at
7k-12k ft AGL then clear skies tonight into Sunday morning. 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 will potentially occur Wednesday into






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