Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 181741

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1041 AM MST Mon Dec 18 2017

.SHORT TERM...(Today through Tuesday)
Issued at 238 AM MST Mon Dec 18 2017

Eastern Utah and western Colorado will be between systems in the
near term. Mid and high level moisture in northwest flow will
move over the area, especially north, generating clouds but no
precipitation. Moisture/clouds decrease tonight and Tuesday.
Expect temperatures to remain near persistence/above normal early
this week.

.LONG TERM...(Tuesday night through Sunday)
Issued at 238 AM MST Mon Dec 18 2017

Zonal flow will slowly turn southwesterly as we approach the
midweek timeframe as our well-advertised winter weather system
approaches from the west. Forecast guidance remains in excellent
agreement through the 0z model cycle and confidence is high that
most locations in eastern Utah and western Colorado will see at
least some snowfall in the Wednesday night-Thursday timeframe.
Light orographic precipitation will begin in the northern
mountains by sunset on Wednesday, with the bulk of the
precipitation falling along and just after the passage of a cold
front on Thursday morning.

The 700mb low pressure center looks to pass from Vernal through
Grand Junction and southward to the San Juans by Thursday
evening. The track of the 700mb low is typically quite influential
to snow prospects in the lower valleys of western Colorado,
including the Grand Junction-Montrose corridor. As winds in the
lower third of the column decrease, the impact of downslope flow
is minimized and snowfall accumulation chances generally increase
in many valleys. Additionally, frontogenetic forcing will be
maximized just ahead of the center of the 700mb low. It is for
these reasons that some value can be added to deterministic model
forecasts, which only indicate a few hundredths of an inch of QPF
in the lower valleys of the western slope. Some potential does
exist for a bit more than that, perhaps a few tenths of an inch,
which would equate to Advisory-level snowfalls in the 2-4 inch
range. While confidence is high in the overall storm, exact storm
totals are medium- confidence for now. All mountain zones should
do well with this event, especially considering the potential for
excellent snow ratios as 700mb temperatures crash into the -10C
to -20 range. High confidence exists for at least 6" of snow in
all ranges of western Colorado and eastern Utah, with higher
totals possible in the central and northern mountains.

Precipitation should come to an end around midnight Thursday
night/Friday morning in the central and southern mountains as
this system departs to the east. After a dry day on Friday, the
next trough will dive quickly southward and spread orographic
snows into the eastern Uintas northern Colorado ranges by Friday
evening. Strong northwesterly flow will result in another shot of
accumulating snow in the mountains, however confidence is low in
exact totals due to wide variations in run-to-run QPF totals. With
another strong frontal passage, there will be another chance for
some snow in the valleys as well, however guidance has not been
quite as robust on low- level moisture compared to the Thursday


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1041 AM MST Mon Dec 18 2017

High clouds will increase across the region, more prevalent north
of KTEX, KMTJ, and KGUC with light, terrain-driven winds over the
next 24 hours. VFR conditions will prevail with no precipitation




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