Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 241713

Area Forecast Discussion
National Weather Service Tucson AZ
1013 AM MST Sun Sep 24 2017

.SYNOPSIS...Dry conditions will prevail across much of the
area through Tuesday with a few record low temperatures for the
coldest valley locations southeast of Tucson. Expect scattered
showers and thunderstorms Wednesday, then isolated to scattered
showers and thunderstorms mainly east of Tucson Thursday into


.DISCUSSION...Low temperatures were in the 40s and 50s across
southeast Arizona this morning, several degrees below average for
late September. A drier air mass is now in place with some modest
cool air advection continuing under the trough. 80s again for
most valley locations this afternoon, then a few degrees cooler
tonight. Near record lows in a couple of colder eastern valley
spots such as Tombstone and Douglas, but we aren`t expecting
freezing temperatures in any valley locations. Precipitable water
around .4 inches and surface dew points in the 20s and 30s, so
mostly clear with no precip. Our current forecast looks great, so
no updates this morning. Please see the previous discussion below
for additional details.


.AVIATION...Valid thru 25/18Z.
A few to broken clouds at 5k-8kt ft AGL southeast of KTUS mainly
this morning, otherwise clear skies. Surface wind wly/nwly 10-15
kts with gusts to 20 kts this afternoon. 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 across much of the area
through Tuesday. Scattered showers and thunderstorms will occur
Wednesday, then isolated to scattered showers and thunderstorms
especially east of Tucson Thursday into next Saturday. 20-foot winds
will mainly be terrain driven less than 15 mph through Wednesday,
then gusty east winds at times Thursday into Friday.


.CLIMATE...The current low temperature forecast at Tucson
International Airport for Monday morning is 54 degrees. That would
be the coolest low temperature in September since 2004 when 54
degrees occurred on the 22nd. The last time Tucson recorded a low of
53 degrees (which may occur Monday morning) in September was in 1988
on the 23rd.


.PREV DISCUSSION...The 24/00Z GFS/ECMWF/CMC were fairly
consistent versus their respective previous solutions regarding
the development of a closed upper low within the base of the
longwave trough Tuesday night into Wednesday. These solutions
progged the upper low to become positioned over west central
Arizona (ECMWF) or southwest Arizona (GFS) by midday Wednesday.
These positions were slightly further east of the progged upper
low positions via the 23/00Z GFS/ECMWF/CMC solutions. The model
PoPs and measurable liquid rain amounts were subsequently somewhat
further east as well versus model solutions of 24 hours ago.

Thus, forecast confidence at this time is fairly high that scattered
coverage of showers/tstms will occur east of Tucson Wednesday
afternoon and evening. Forecast confidence is considerably lower for
measurable rainfall of significance to occur within much of the
Tucson metro area, and especially for locales west of Tucson.

At any rate, this issuance of the official forecast depicts
scattered showers/tstms east of Tucson Wednesday afternoon/evening,
and isolated showers/tstms from Tucson westward into central Pima
County and south central Pinal County. Dry conditions should occur
Wednesday across western Pima County. The upper low is progged to
lift northeastward into northeast Arizona/southeast Utah by midday
Thursday. Isolated to scattered coverage of showers/tstms should
occur east of Tucson Thursday into Friday night.

Deeper moisture may be shunted somewhat further westward Saturday as
a shortwave trough moves across the area. The 24/00Z GFS/ECMWF were
supportive of this notion, but a quick glance at the 24/006Z GFS
kept deeper moisture further east due to a less amplified shortwave
trough. At any rate, have maintained a slight chance of
showers/tstms from Tucson eastward Saturday with dry conditions
across western locales.

Expect near record low temperatures tonight and Monday night across
portions of Cochise County, including Tombstone and Douglas.
Otherwise, daytime temperatures that will be below seasonal normals
through Monday, then high temperatures will trend closer to seasonal
normals Tuesday into next Saturday.






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