Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 140510
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1010 PM MST Fri Jan 13 2017

.UPDATE...
Issued at 940 PM MST Fri Jan 13 2017

Temperature/Dew Point Spreads remain tight across much of the
forecast area with reports of fog, dense in spots already coming
in. KSBS and KVEL down to 1/4 mile and latest satellite imagery
showing plenty of low clouds across northeast Utah and northwest
Colorado, generally along and north of Interstate 70. Went ahead
an issued a dense fog advisory for a more widespread area tonight
as motorists will likely encounter visibilities under one mile
along stretches of I-70 and Highway 40. Winter highlights from the
central and southern mountains will remain intact for now as
moisture slowly creeps back north overnight with expectation that
snowfall will increase. The southern valleys will see rain develop
overnight, becoming mixed with and eventually flipping over the
snow by sunrise. Sloppy mess expected with ratios running 10:1 or
even 8:1, so more blue in the snow than white if we can drive the
temps down low enough. BUKIT soundings drive Durango to the
freezing mark between 2 and 3 am with Pagosa dropping sooner. As
has been the case all winter it will be a battle and snow
accumulation ranging from 1 to 3 inches with the higher amounts
above 7k. Current grids reflect this thinking and no plans on
expanding highlights attm.

&&

.SHORT TERM...(Tonight through Saturday night)
Issued at 315 PM MST Fri Jan 13 2017

A low pressure system currently situated off the southern
California coast continues to pump moisture across the region in a
diffluent southwest flow across our CWA. Some light snow is still
occurring but for the most part, we have seen a bit of a lull in
the action due to a dry slot working its way across the Four
Corners region into southwest Colorado. Models are in fairly good
agreement showing snowfall picking up in intensity this evening
through late Saturday afternoon across the southern and central
mountains, with favorable orographics and dynamics in play as the
low makes its track inland across northern Mexico. Isentropic
upglide is evident over the San Juans through this timeframe with
time heights over the San Juans, Grand Mesa, West Elks and La
Sals/Abajos showing strong lift and deeper moisture through the
dendritic layer this evening through Saturday afternoon. Expecting
the best snowfall to occur in the southwest San Juans with south
and southwest facing slopes being favored, as well as the La Sals
and Abajos getting in on the action. Given the above factors in
spite of the current lull, elected to keep the highlites for the
southern and central Colorado mountains going until the current
expiration time of 5 pm MST Saturday and also decided to extend
the Winter Storm Warning for the La Sals and Abajos through this
timeframe as well.

The flow begins to shift to the southeast by Saturday night into
Sunday morning with what appears to be a downturn in intensity,
even though some light snow may continue to fall across the
southern mountains. The NAM indicates some drier air and a loss of
strong lift, where the GFS keeps precip going through this
timeframe. Confidence is low at the moment with the discrepancy in
the details, so felt the times for the current highlites were
sufficient. Later shifts will need to have a look as we get closer
to expiration time whether these highlites need to be extended or
not. Valleys still appear fairly mild with rain in the lower
valleys, mixing in with snow at times during the nighttime hours,
but accumulations appear minimal.

.LONG TERM...(Sunday through Friday)
Issued at 315 PM MST Fri Jan 13 2017

An upper level shortwave combined with jet energy on the back side
of this upper level low pressure system will dive down the West
Coast and into northern Mexico on Sunday. This will force the low
pressure system over northern Mexico to quickly move eastward
Sunday afternoon and lift northeast through eastern New Mexico and
the Texas Panhandle by Monday morning. As it does, precipitation
will wraparound the low in an easterly flow favoring the eastern
slopes of the San Juan mountains during the day on Sunday. This
pattern does not tend to favor the western San Juans, however
enough dynamics may be in play to allow snow to pick up again
Sunday afternoon through Sunday night before tapering off to light
snow across the northwest San Juans as a drier northerly flow
pushes southward across the area. Additional highlites may be
needed depending on how much moisture spills over, so later shifts
will have to evaluate this scenario with later model runs.

The models are in good agreement that we will finally see a break
from this steady stream of moisture and active weather beginning
Monday afternoon and lasting through Wednesday night as high
pressure moves in over the Great Basin and lays over the region,
with a much drier airmass in place. As this occurs though,
another low pressure system will be moving into the Pacific
Northwest with moisture increasing across eastern Utah and western
Colorado by Thursday morning. So, after our brief dry spell Monday
through Wednesday, the weather will become more active again
Thursday through at least the weekend as Pacific storms barrel
across the region bringing more Pacific moisture which in turn
means more snow for the mountains, with a mix in the valleys.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 940 PM MST Fri Jan 13 2017

Unsettled southwesterly flow will continue to pump moisture into
the southern half of the forecast area through Saturday afternoon.
This will keep mountain snow and a rain/snow mix in the valleys
with areas of MVFR to VFR conditions around the forecast area.
Heavy snow will be a possibility for Durango and Telluride
Airports Saturday morning, however forecast confidence is low due
to warm southerly flow potentially lifting the snow levels.
Locally dense fog is also expected across the northern valleys
through 17Z/SAT.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Dense Fog Advisory until 10 AM MST Saturday for COZ001-002-
     005>007.

     Winter Storm Warning until 5 PM MST Saturday for COZ019.

     Winter Weather Advisory until 5 PM MST Saturday for COZ009-012-
     017-018.

UT...Dense Fog Advisory until 10 AM MST Saturday for UTZ024-027.

     Winter Storm Warning until 5 PM MST Saturday for UTZ028.

&&

$$

UPDATE...JDC
SHORT TERM...MDA
LONG TERM...MDA
AVIATION...JDC



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