Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 191022

Area Forecast Discussion
National Weather Service Tucson AZ
322 AM MST Fri Jan 19 2018

.SYNOPSIS...Dry rather warm again today. A Pacific storm system
will then bring a good chance of valley rain and mountain snow
showers, slowly spreading in from the west Saturday before exiting
the area Sunday morning. Much cooler temperatures this weekend,
with a widespread freeze including Tucson likely Saturday night
and/or Sunday night. Dry conditions with a warming trend next


.DISCUSSION...The next couple of days will be dominated by a
strong and broad trough of low pressure currently off the coast
of British Columbia digging southward off the California coast.
Ahead of this a short-wave ridge is phasing quickly east and south
of our area this morning. The northerly flow we saw on the 00Z
KTWC sounding will be weaker and start to become westerly with the
12Z observation. Even as heights and thicknesses recede today
we`ll have room for another degree or two of warming in valleys.
Precipitable water still running around 1/4 of an inch and dew
points in the teens to lower 20s. Just patches of cirrus level
moisture at times.

Good model agreement with handling the southern branch of the
trough as it comes onshore tonight and pushes through the central
and southern Great Basin into the central and southern Rockies
Saturday. An equal blend of GFS/ECMWF is fine, along with regional
and national blends. The timing of the associated cold front has
slowed about 3 or 4 hours (and may be a trend to watch with room
to slow down another 3 hours before it gets here). That pushes it
through our area a little later in the day on Saturday, with time
for temperatures to get a little warmer in the warm sector ahead
of it. A tough call around Tucson, where we pushed daytime highs
up a few degrees Saturday. Best moisture and dynamics couplet for
our area will be just ahead of and with the frontal boundary, with
prime time between 18Z Saturday and 03Z Sunday. It still won`t
have a lot of time to tap into a southerly fetch of moisture even
with a slightly more favorable trajectory, so still looking at
storm total precip values around .02 to .2 inches in valleys, and
.2 to .4 inches in mountains. Snow levels will drop as low as 5000
feet with the best precip period Saturday afternoon. Snow amounts
of around 1 inch at 5000 to 6000 ft, with 2 to 4 inches above
6k/7k ft. Snow levels will drop below 4k feet Saturday night
behind the front, but most of the moisture will be east of the
area. A few flurries with little or no accumulation in some
valley locations of Cochise and Graham counties Saturday night.

Freezing conditions possible across much of the area including the
greater Tucson area Saturday night into Sunday morning, but that
will be somewhat dependent on clearing from the west overnight.
Our coldest valley temperatures will probably be Sunday night
into Monday morning with the cold air mass in place, clear skies
and light winds. Teens to lower 20s in colder valley locations
south and east of Tucson, with upper 20s to lower 30s from Tucson
westward. It has been about a month since our last freeze around
Tucson, with most of that period spent well above average; a
freeze warning may be warranted.

Shortwave ridge to zonal flow the first half of the new week for a
steady warming trend and dry conditions. Morning lows will remain
on the cool side for a few days.


.AVIATION...Valid thru 20/12Z.
FEW-SCT clouds above 20k ft AGL through this evening, becoming BKN
late in the period. SFC wind generally ely/sely at less than 10 kts
this morning. Expect sly/swly winds at 8-12 kts this afternoon into
Saturday morning. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions through tonight. Valley rain and
mountain snow chances will increase from west to east on Saturday.
The system will clear out of the region quickly, with precipitation
chances decreasing Saturday night and Sunday morning. Dry conditions
will return Sunday afternoon and continue into next week. Gusty west
to southwest winds will occur Saturday afternoon and evening.
Otherwise, 20-ft winds will be terrain driven at mainly less than 15






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