Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 132159

Area Forecast Discussion
National Weather Service Tucson AZ
259 PM MST Fri Jan 13 2017

.SYNOPSIS...A few sprinkles or light showers possible tonight,
then a low pressure system will bring a good chance of valley rain
and mountain snow over the weekend into early Monday along with
cooler temperatures. A few showers near the mountains early next
week, then another storm system will impact the area later next


.DISCUSSION...We continue to moisten from mid levels downward with
moisture increasing over the next 36 hours.

The storm system generating this increasing fetch from the south
and southwest is currently spinning near the southern California
and northern Baja coast. Still on track to kick eastward through
Sonora and southern Arizona Saturday afternoon through Sunday
morning. We`ll have some weak upslope activity before this, but
our best dynamics and moisture couplet will be associated with the
northeast and northern portions of the low, so our Winter Weather
Advisory time mirrors this (18z Sat to 18z Sun above 6500 feet).

A second, weaker piece of energy will carve into the back side of
the trough position as the main low exits into the southern and
central plains Sunday night and Monday, keeping things showery
and cool around Southeast Arizona.

Storm totals through the entire weekend into Monday include valley
rain amounts in the .3 to .8 inches range (best chance for most of
that Saturday afternoon through Sunday morning). As for the
mountains, snow levels ahead of the system will start around 8000
to 8500 feet tonight and Saturday morning, fall to about 7000 to
7500 feet early Saturday evening, then bottom out around 5500 to
6500 feet Sunday morning. Snowfall amounts in the advisory period
around 6 to 12 inches above 7000 feet with with 1 to 5 inches
between 6000 and 7000 feet. Some of the initial precip will fall
as rain, even at higher elevations.

A dirty shortwave ridge will push temperatures back up to average
levels the middle of next week.

That system we`ve been talking about for the second half of next
week could be a stronger and colder storm for our area. Once
again, medium range operational models as well as ensemble mean
and spread trends continue to show increasing confidence in a
more significant and deeper split the second half of next week.
This would open our area up to a couple of strong storm systems
from the north Pacific in the Thursday-Saturday time frame next
week. We don`t see very many systems that manage to sustain and
even reinforce their cold air on a long wet trajectory like this,
maybe once or twice per winter. We continue to respond
accordingly with increasing precip chances next Thursday night
into Saturday, and snow levels near some valley floors as early as
late next Friday or Friday night.


.AVIATION...Valid through 15/00Z.
VFR with SCT-BKN clouds at 8-13k ft AGL and BKN-OVC clouds above
15k ft AGL. A few light showers possible overnight, then clouds
and showers increase from the west after 14/15z. Surface wind
from KTUS westward generally less than 12 kts. Surface wind east
to south of KTUS (including the KOLS/KDUG terminals) sly at 12-18
kts with gusts to 25 kts. Aviation discussion not updated for TAF


.FIRE WEATHER...Relatively mild temperatures with increasing
cloudiness through late tonight. A low pressure system will then
move into the region this weekend, resulting in a good chance of
valley rain and mountain snow by Saturday afternoon and Sunday.
Some showers lingering into Monday. Dry conditions are expected
to return areawide Tuesday and Wednesday before another storm
system brings more precipitation the second half of next week.
Outside of some elevated southerly 20-foot winds at 10-20 mph
today and Saturday, 20-foot winds are expected to be terrain
driven and generally less than 15 mph.


Winter Weather Advisory from 11 AM Saturday to 11 AM MST Sunday
for AZZ510>514.




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