Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 180938
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
238 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 -10 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 Friday 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 event.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 238 AM MST Mon Dec 18 2017

Expect VFR conditions and CIGS above ILS breakpoints through the
next 24 hours. Winds will be light and terrain driven.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...None.
UT...None.

&&

$$

SHORT TERM...NL
LONG TERM...MAC
AVIATION...NL



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