Area Forecast Discussion
Issued by NWS Grand Junction, CO

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917
FXUS65 KGJT 020504
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1004 PM MST Thu Dec 1 2016

Updated Aviation section

.SHORT TERM...(Tonight through Friday night)
Issued at 156 PM MST Thu Dec 1 2016

Visible satellite imagery this afternoon shows partly cloudy to
overcast skies from the I-70 corridor northward as a shortwave
trough approaches from the Great Basin. A shallow moist layer
within light westerly upslope flow has been just enough to result
in a few snow showers across the north. Surface temperatures in
valley locations are at or just slightly above freezing, with
little in the way of snowfall accumulation likely through the
evening. Higher elevations in the Uinta, Flat Top, and Park
ranges could see 1-3 inches of snow before precipitation tapers
off in the early morning hours on Friday.

The shortwave trough responsible for precipitation Thursday
afternoon and evening will dive southward and cut off well to the
south near the Arizona/Mexico border. As this happens, weak mid-
level divergence combined with light easterly flow over the
southern Rockies will open the door for a secondary maximum of
precipitation in central and southern Colorado. This maximum will
occur on Friday afternoon, however with only seasonably average
PWATs and a cold front quickly crossing the area by Friday
evening, snowfall totals will likely end up in the 2 to 4 inch
range - well below advisory criteria in the mountains.

.LONG TERM...(Saturday through Thursday)
Issued at 156 PM MST Thu Dec 1 2016

Dry weather continues to look like a safe bet on Saturday as weak
ridging aloft moves in from the west. Models have trended drier
through Sunday as well, with any mention of a snow shower Saturday
night limited only to the highest elevations of the Park range.
Afternoon high temperatures will run around 5 degrees below
average through the weekend.

Medium-range guidance took a rather dramatic turn overnight with
regard to the more potent trough progged to arrive on the west
coast early on Monday. The first piece of energy with this system
is expected to spread precipitation over the northern and central
mountains by daybreak on Monday. Both the 0z and 12z GFS now
depict a much flatter, progressive wave passing through the
central Rockies Tuesday and exiting the region by Wednesday
morning. The 12z ECMWF still depicts a deeper trough digging
across eastern Utah and Western Colorado Tuesday night and early
Wednesday morning, however 500mb heights no longer close off over
Utah as Wednesday`s guidance indicated. The end result for this
forecast package has been a slight lowering in PoP chances across
the entire region on Tuesday and Wednesday, and an increase
overall uncertainty in the chances for a significant winter
weather event during this time frame.

Global forecast ensembles still support a notably cold airmass
with this system with 700mb temperatures falling into the -15 to
-20 C range over the entire region from Tuesday morning through
Wednesday. It is worth noting that despite the consistent cold air
signal depicted by the GEFS, even ensemble means have relaxed
somewhat in the intensity of the trough moving into the west early
next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1004 PM MST Thu Dec 1 2016

Snow over north central Colorado is expected to end by sunrise
Friday morning. This will keep cigs and vis down across this
region, but should not impact any TAF sites except potentially ASE
overnight.

Additional snow is expected Friday afternoon as a trough slings
over the Eastern Great Basin during the day on Friday. Moisture
will filter in from the south, bringing showers over the San
Juans and south central Colorado mountains. KDRO and KTEX guidance
is indicating snow likely between 21 and 03z...and KASE may
possibly be impacted between 23 and 09z. Other TAF sites should
remain VFR over the next 24 hours.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.
&&

$$

SHORT TERM...MAC
LONG TERM...MAC
AVIATION...JAM



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