Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 232156

Area Forecast Discussion
National Weather Service Tucson AZ
256 PM MST FRI SEP 23 2016

.SYNOPSIS...A strong low pressure system moving north of the area
will bring a slight chance of showers and/or thunderstorms, cooler
temperatures and gusty winds at times today. Mostly dry conditions
will then prevail this weekend. Moisture and a slight chance of
showers returns to the area early next week as an upper level low
moves west across Sonora Mexico.


.DISCUSSION...Water vapor imagery continues to show the upper low
spinning over the conjunction of Idaho, Wyoming and Utah, with the
trough axis extending across eastern Utah into northeast Arizona. IR
and visible satellite imagery show a swath of cloudiness that extend
from southwest Colorado into northwest New Mexico and east central
Arizona and then into portions of southeast Arizona.

The cold front is generally aligned with the back side of this swath
of clouds with dewpoint temperatures still in the upper 50s to lower
60s ahead of the front and generally in the teens to 20s behind the
front. Radar mosaic is showing isolated showers continuing to move
east northeast across eastern portions of Pinal county and extreme
northeast Pima county and headed into western portions of Graham and
Cochise county.

As the front continues to make eastward progress during the
remainder of this afternoon and into the evening, skies will be
clearing from the west and northwest as the very dry air behind the
front moves into southeast Arizona. The GPS Precipitable water
values from the Univ of AZ shows 0.75 inches at Organ Pipe and 0.35
at Yuma as of 1845Z, but still around 1.1 inches for Tucson, but
this will change throughout the evening.

Models indicate the upper low will continue to move east into the
weekend, reaching the front range of the northern Rockies and
northern Plains by Saturday with the associated trough axis trailing
the low into eastern Arizona and western New Mexico.

By late in the day Saturday, a vort max will dive down the back side
of this trough over eastern Arizona and eventually form a cutoff low
over northern Sonora/Chihuahua Mexico by late in the day Sunday.
This will result in a northeast to easterly flow across my forecast
area, which will pull moisture from our east into Arizona. This low
will then move slowly west and eventually northwest toward the
Colorado river valley by late Tuesday into early Wednesday. As this
occurs we will see a moisture increase from the east with an
increase in rain chances beginning late in the day Sunday, with a
continued small threat through late Wednesday. There are significant
differences among the models with regard to rain chances, with the
GFS just showing isolated activity for Tucson throughout all of next
week, although the best chance will be Monday through Wednesday. The
ECMWF shows scattered activity Monday and Tuesday, with isolated
activity Wednesday through Friday. For now, my POP forecast is lower
than GFS MOS numbers (just slight chances through the middle of the
week and single digit POPs late next week), so if it continues to
look like this low will materialize as the models advertise, then we
have plenty of room to nudge POP numbers higher.


.AVIATION...Valid through 25/00Z.
Isol`d -SHRA/-TSRA mainly N to NE of KTUS through this evening.
Expect a band of SCT-BKN clouds at 3-7 ft AGL to gradually make its
way across the region from W to E as well. Gusty SWLY sfc winds at
10-15 kts with gusts up to 25 kts will subside this evening and
become terrain driven less than 10 kts overnight. FEW-SCT clouds
Saturday around 10-15k ft AGL mainly SE of KTUS with sfc wind
WLY/NWLY around 10 kts. Aviation discussion not updated for TAF


.FIRE WEATHER...Isolated showers and a stray thunderstorm are
possible through this evening, especially from Tucson north and
northeastward as a cold front moves across the area. Southwest winds
of 15-20 mph and occasional 25 mph gusts will subside this evening.
Otherwise, mainly light winds will occur Saturday, with elevated
easterly breezes Sunday through Wednesday of next week. After some
drying this weekend, moisture will return to the area starting
Sunday afternoon as an upper level low begins to affect Arizona. A
chance of showers and thunderstorms will then persist across the
area into the middle of next week.






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