Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 151109

Area Forecast Discussion
National Weather Service Tucson AZ
405 AM MST Fri Dec 15 2017

.SYNOPSIS...Sunny and dry conditions will prevail across southeast
Arizona through Saturday. A quick moving weather system will bring
colder temperatures, valley rain and mountain snow showers Saturday
night and Sunday. Slightly warmer temperatures and dry weather will
return early next week.


.DISCUSSION...IR satellite imagery revealed clear skies across
southeast Arizona this morning. This will generally be the rule
today, along with some periodically elevated easterly winds.

Winds will weaken overnight into Saturday in conjunction with
increasing cloud cover from west to east ahead of our next weather
maker. A trough of low pressure will strengthen as it sinks south
along the CA coastline. Contrary to model solutions 24 hours ago,
this trough is progged to elongate from Montana to southern CA,
with the base reaching into northern Sonora, Mexico by Sunday
morning. Neither the 15/00Z GFS or ECMWF produce a distinct cutoff
low anymore, but they have come to a much better consensus over the
last 2 model runs on the evolution of this trough. This development
combined with increased MOS guidance numbers lead the previous shift
to increase PoPs, QPFs and snowfall amounts. For this package,
slight upward adjustments in the 5-15% range were made to the
inherited PoP grids, along with with slight increases in QPFs and
snowfall amounts. Currently, the best chance for precip looks to be
from 17/12z-18/00z Sunday. Snow levels will fall to around 5k ft agl
by Sunday afternoon near Tucson and the nearby Catalina and Rincon
Mountain ranges. Didn`t want to issue any headlines just yet, but if
confidence continues to increase, wouldn`t be surprised to see a
watch or advisory issued by subsequent shifts for at least the White
Mountains of Graham and Greenlee Counties.

The aforementioned trough will spin slowly to the east, with
lingering rain and snow showers continuing into Monday morning. The
system will bid the state farewell by Monday evening, giving way to
dry conditions under northwesterly flow for Tuesday. We remain rain-
free into Wednesday, though more zonal flow will allow for a few
degrees of warming. We`ll then turn our attention to another
potential trough to dig out of the Pac NW by the end of next week.

Temperatures generally 1-3 degs above average today and Saturday,
with below average temperatures occurring on Sunday and possibly
Monday. Increasing heights by Tuesday and Wednesday of next week
will put highs above normal once again.


.AVIATION...Valid thru 16/12Z.
Clear with increasing clouds from west to east late in the period.
Sfc wind becoming ely at 8-15 kts with gusts to 20 kts at times,
through this afternoon. Wind speeds diminish overnight into Saturday
morning. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Easterly 20 foot winds will develop overnight and
persist into Friday with speeds of 10 to 15 mph and gusts to 25 mph
at times. Otherwise, mild and dry weather will prevail Friday and
Saturday. A stronger weather system will bring cooler temperatures,
valley rain and mountain snow to the area Sunday. Drier air and
warmer temperatures will then return next week.





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