Area Forecast Discussion
Issued by NWS Grand Junction, CO

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997
FXUS65 KGJT 100438
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
938 PM MST Fri Dec 9 2016

.UPDATE...
Issued at 933 PM MST Fri Dec 9 2016

Winter Storm Watches upgraded to warnings beginning at 10am MDT
in the Parks and Flat Tops, and beginning at noon in the Gore
range, Vail Pass, and the Central Colorado mountains. Warnings
last through 6pm on Sunday.

&&

.SHORT TERM...(Tonight through Saturday night)
Issued at 330 PM MST Fri Dec 9 2016

Afternoon instability is adding a last punch to the orographic
snowfall over the mountains near I-70 up past the Wyoming border. A
repositioning of the jet stream northward will the snow to end from
south to north tonight...but not totally relenting over portions of
the Park Range. Current warning running through sunset after which
the convective component should wane and snow rates lessen.
Generally 8 to 16 inches have fallen at many of the snotel sites
with the northern mountains the biggest winner so far. A short break
is anticipated late this evening through near the noon time hour
tomorrow when the next storm system begins to affect the region. No
big surprise that the northern and central mountains remain under
the gun in this pattern but the southern mountains will get some new
powder by sunrise on Sunday. Derived satellite imagery is showing a
strong Pacific moisture plume north of the Hawaiian Islands being
drawn to the PacNW on a strong upper jet. This jet will be
strengthening across the northern Great Basin tomorrow as cold
high latitude air descends into the northern Rockies and tightens
the thermal gradient aloft. The increasing westerly flow will
advect moisture substantial moisture to our doorstep by noon
tomorrow with PWAT pushing toward climatological max numbers for
early December. This forecast would be much easier if we were just
dealing with a cold air mass. However the strong influx of
moisture is coming with warmer air and mixing is very possible
into the valleys bring liquid precipitation into the mix.
Confidence extremely shaky in the 6000 to 8000 feet level where
accumulating snow forecast will highly depend on the snow levels
and possibly underlying cold air. This includes the upper Yampa
and Eagle Valleys...the Gunnison Basin and possibly the Uinta
Basin. Confidence much higher over the 9000 feet where the
moisture will be lifted by the hills and upper dynamics centered
over the northern and central mountains early in the storm with
instability coming into the mix over all of western Colorado early
Sunday morning. In the lower elevations of eastern Utah and far
western Colorado an inch or maybe two is possible including the
southern San Juan Valleys. This should be another good snow
producer for the northern and central mountains which is similar
to the seasonal outlook. There is some better news for the south
later in the forecast. Temperatures are another tricky part of the
equation. The Gunnison Basin seems to have some deeper air
entrenched and this is huge for the snow forecast. Guidance and
models want to mix it out into the upper 30s tomorrow...and not
buying attm and went below guidance. The same is true for the
upper Yampa and Eagle Valleys. Milder temperatures will be found
in the desert valleys tomorrow afternoon. Colder air arrives early
Sunday morning in the north and this will help boost snow ratios
and hence the better snow totals in the north.

.LONG TERM...(Sunday through Friday)
Issued at 330 PM MST Fri Dec 9 2016

So snow continues Sunday morning over much of the region being
enhanced by instability and ascent related to digging upper jet
aloft. As the day goes on this jet will be dropping into the
southern reaches of our area and flushing the moisture to the
south as well. Much of the precipitation that is left by the
afternoon with will be orographically forced snow showers in
northwest flow favoring the northern Colorado mountains. The jet
will actually begin to lift northward by Sunday evening and keep a
threat of high country snow going over the Park and Northern Gore
Ranges. In fact this threat of snow will likely continue well into
the mid week period for the northern mountains and extending into
the I-70 corridor at times as well. Unfortunately the models are
breaking down in consistency as early as Tuesday in the east
Pacific which will have a direct impact on our forecast. The GFS
wants to tear the energy out of the Pacific early and bring a
pickup in moisture and precipitation Tuesday night into Wednesday
while the EURO holds this back until late in the week. So this
trend will have to be monitored. Either way the pattern still
shows promise for continuing precipitation over the central
Rockies through the extended.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 447 PM MST Fri Dec 9 2016

Aviation challenges in the very short term appear minimal as
ceilings slowly rise overnight in the wake of the first in a
series of waves of low pressure. VFR conditions this evening with
light and variable winds overnight. Ceiling heights will lower
through Saturday morning, likely below ILS thresholds by noon at
ASE, EGE, and RIL. Precipitation moves in just after noon at those
aforementioned sites. Forecast challenges Saturday will include
precipitation type in valley locations as temperatures will be
marginal (at or slightly above freezing) when moisture arrives
after noon, and visibility when rain changes to snow during the
day.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Storm Warning from noon Saturday to 6 PM MST Sunday for
     COZ009-010-012.

     Winter Storm Warning from 10 AM Saturday to 6 PM MST Sunday for
     COZ004-013.

UT...None.
&&

$$

UPDATE...MAC
SHORT TERM...15
LONG TERM...15
AVIATION...MAC



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