Weather Service Report


260 
FXUS65 KGJT 110550
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1050 PM MST Sat Dec 10 2016

.SHORT TERM...(Tonight through Sunday night)
Issued at 315 PM MST Sat Dec 10 2016

The strong Pacific jet guiding our storm track has been slightly
slower to shift southward than guidance indicated on Friday. The
anomalously high precipitable water plume stretching into the
central Rockies from the Pacific coast has also remained a bit
farther north that previously forecast. As a result, precipitation
has been slower to develop over the forecast region by a few
hours. Observational and forecast soundings reflect these subtle
changes, with the morning GJT sounding indicating a substantial
dry layer below 600mb and forecast soundings along the I-70
corridor a bit drier through the column early this Saturday
evening. The result of these changes has been a slight reduction
of snow totals area wide on the front-end of the current storm
system. Current forecast totals are still well within warning and
advisory criteria, so no changes are expected to current
headlines through the weekend event.

Once the jet shifts south this evening, all systems still appear
go for significant snowfall in the northern and central mountains.
As noted in previous discussions, snow levels remain a bit of a
concern through Saturday night and Sunday morning until colder air
filters in from the north at the end of this event. 700mb
temperatures will average around -3 to -5 through the heart of the
precipitation from 06z through 12z Sunday. Temperatures in this
range would indicate snow levels from 6500 to 7000 feet, dropping
to around 6000 feet by late morning Sunday.

A caveat to the forecast is cold air trapped in valley locations.
Strong inversions have been slow to break in some locations (such
as Gunnison). If these inversions remain in place and and the warm
nose aloft breaks down quicker than expected overnight into Sunday
morning, snowfall may make it further down into the valleys than
expected. High temperatures on Sunday have been adjusted downward
by a few degrees in valley locations to reflect this idea.

.LONG TERM...(Monday through Saturday)
Issued at 315 PM MST Sat Dec 10 2016

Precipitation will come to an end in all but the highest
elevations of the northern mountains on Sunday evening. While the
persistent Pacific Jet will shift northward, our region will be
close enough to the storm track to keep clouds, and a few snow
showers, in the forecast through Monday as low-level moisture
lingers north of the US40 corridor.

Our next significant weather-maker for eastern Utah and Western
Colorado comes later on Tuesday. As of the noon hour on Saturday,
medium-range guidance is in good agreement on a closed low at
500mb diving south off the coast of British Columbia on Tuesday
afternoon. As it does so, renewed Pacific moisture will surge east
along the California coast and into the central Rockies. The
central and northern mountains should see another round of
snowfall from early Wednesday morning through Thursday morning.
Given the Pacific airmass in place and west to southwesterly flow
ahead of this system, the Wednesday event will share some
similarities to the current weekend storm with regard to concerns
over snow level and precipitation type in valley locations.

Beyond the Wednesday system, substantial differences in medium-
range guidance evolve. The ECMWF merges the closed-low at 500mb
into the mean western longwave trough, resulting in a quick-moving
shortwave traversing the Four-Corners region on Friday. The GFS
keeps the 500mb low cut-off and digs a much deeper trough into the
California coast. As a result a much slower solution is depicted
with warm southwest flow overspreading the region Friday morning,
and significant precipitation lingering through the first half of
next weekend. With such dramatic differences in the evolution of
the western trough, confidence remains quite low in the forecast
after midweek.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1050 PM MST Sat Dec 10 2016

Current radar and satellite imagery showing snowfall occurring
over the northern and central Colorado mountains with rain
changing over to snow in the higher northern valleys around KCAG,
KSBS and KEGE as lift enhances snowfall rates and top down
saturation occurs. ILS breakpoints likely to be reached at KASE
and KEGE. Mountains will remain obscured with snowfall continuing
through 18z Sunday before lessening in intensity and coverage.
Expect some light showers over the lower valleys with some lower
ceilings possible by daybreak Sunday.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Storm Warning until 6 PM MST Sunday for COZ004-009-010-
     012-013.

     Winter Weather Advisory until noon MST Sunday for COZ002-003-005-
     008.

UT...None.
&&

$$

SHORT TERM...MAC
LONG TERM...MAC
AVIATION...MDA

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Western Regional Climate Center, wrcc@dri.edu