Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 191651

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

.SYNOPSIS...Dry conditions with unseasonably warm temperatures
today. A Pacific storm system will bring scattered valley rain and
mountain snow showers especially east of Tucson, and mainly
afternoon and evening. Showers will end east of Tucson Sunday
morning. Much cooler temperatures this weekend, with a widespread
freeze including Tucson likely Saturday night and Sunday night.
Expect dry conditions with a warming trend next week.


.DISCUSSION...IR/visible satellite imagery and surface observations
depict clear skies west of Tucson at this time. Meanwhile, a few to
scattered cirriform clouds were noted especially east to south of
the Tucson metro area. Surface temps at lower elevations valid 9 AM
MST ranged from the mid-upper 30s across eastern sections, and to
the mid 40s-lower 50s elsewhere. A very dry surface-500 mb regime
continues across southeast Arizona this morning as per the 19/12Z
KTWC sounding, and the total precip water value was just 0.26 inch,
or nearly identical to the previous two soundings.

Expect clouds to gradually move eastward during the next several
hours allowing for sunny skies to mostly sunny skies areawide this
afternoon. High temps this afternoon from Tucson westward will be
just a few degrees warmer versus Thursday, but afternoon temps
across eastern sections will be about 5-10 degrees warmer versus
Thursday. These temps will generally be nearly 6-10 degrees warmer
than normal for January 19.

Have noted that the 19/12Z NAM12 has depicted a fairly pronounced
decrease in precip chances associated with the low pressure system
and frontal boundary to move eastward across the area Saturday. In
fact, the METTUS PoP is only 6 for Saturday afternoon, and this is
contrasted to a 34 PoP for KTUS for Saturday afternoon via the
18/12Z METTUS guidance. A quick glance at the 19/12Z GFS and
associated MAVTUS shows a similar downward PoP trend versus 24 hours

Based on these solutions, may opt for some downward precip chance
adjustments with the afternoon forecast package. Will await receipt
of the ECMWF and associated ensembles prior to making any PoP
adjustments. Cooler temps and gusty winds still on tap Saturday with
any precip ending across eastern sections Sunday morning. Freezing
temps for the Tucson metro area also appear to have a fairly high
likelihood Saturday night and again Sunday night.

Please refer to the additional sections for further detail.


.AVIATION...Valid thru 20/18Z.
FEW-SCT clouds above 20k ft AGL into tonight, then SCT-BKN clouds
around 6k-10k ft AGL Saturday morning. Isolated to scattered
-SHRA developing after 12Z Saturday. MVFR conditions especially
higher terrain and increasing coverage of -SHRA particularly KTUS
vicinity eastward after valid period, or Saturday afternoon and
evening. Surface wind sely to swly 5-15 kts thru valid period.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions through tonight. Expect scattered
valley rain and mountain snow showers especially east of Tucson, and
mainly Saturday afternoon and evening. Precipitation will end east
of Tucson by midday Sunday, then dry conditions will prevail Sunday
afternoon into next Thursday. Gusty southwest to northwest winds
will occur particularly Saturday afternoon and evening. Gusty
northwest winds will favor the Upper Gila River Valley including
Safford on Sunday. Otherwise, 20-foot winds will be terrain driven
mainly less than 15 mph at other times.


.PREV DISCUSSION /322 AM MST/...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.






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