Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 231558

Area Forecast Discussion
National Weather Service Tucson AZ
858 AM 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 and 23/12Z upper-air plots this
morning show an upper low spinning over southeast Idaho into
northeast Utah, with a trough axis extending south into Arizona. IR
and visible satellite imagery along with surface observations and
radar imagery indicate that a cold front currently lies across
Arizona and generally extends from east of Casa Grande to east of
Ajo. Dewpoint temperatures to the east of this line generally range
from the 50s to lower 60s, with readings in the 20s and 30s west of
this line. Radars are detecting isolated showers over eastern
portions of Pinal and Pima counties and making their way into
western Graham county. The inherited POP forecast shows isolated to
low end scattered type POPs for this morning then generally isolated
POPs for the mountains this afternoon as the front marches east
during the day as significantly drier air moves into the region.
This mornings 23/12Z KTWC sounding revealed a PW of 0.92 inches.
However, the 15Z CIRA LPW Total imagery shows values now ranging
from 0.5-0.75 inches across much of southern Arizona and southern

As of 15Z (8 AM MST), temperatures across the region ranged from the
mid 60s to the mid 70s and these readings seem reasonable with
respect to the expected/inherited afternoon high temps but will
continue to monitor and will make changes if needed.

See previous discussion below for details beyond today.


.AVIATION...Valid through 24/18Z.
Isolated -SHRA/TS with cloud decks mostly ranging from 4-7k ft AGL.
Most activity will be done after 24/03z. Sfc wind become breezy at
times this afternoon with strongest gusts occuring at KDUG.


.FIRE WEATHER...Isolated showers and storms are possible through
this evening as an upper level trof moves across the state. Gusty
southwest winds of 10 to 20 mph will be around this afternoon before
subsiding this evening. Dry most of the weekend with mainly light
winds. Moisture returns to the area from the east as an upper level
low, which drops south along the Arizona/New Mexico border moves
west across Sonora Mexico. Thus early next week will see a chance of
showers and thunderstorms across the area.


.PREV DISCUSSION...Over the weekend a piece of energy will break off
the upper low and move south along the AZ/NM border. Over the past
several days, the models have been flip-flopping on this idea.
Now the NAM/GFS/ECMWF/Canadian all are on the same page. However
what happens when this upper low finally digs south into Sonora
Mexico and starts to retrograde to the west early next week is very
much uncertain. How much moisture does this upper low pick up from
Texas and New Mexico and does it potentially tap into an eastern
Pacific tropical system. Have made some changes to the inherited
forecast by introducing slight chance PoPs as early as Sunday across
the eastern areas and slowly push them west early next week.





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