Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 181724
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1024 AM MST Thu Jan 18 2018

.SHORT TERM...(Today through Friday)
Issued at 246 AM MST Thu Jan 18 2018

The ridge over the Great Basin will slide east and over Colorado
today as a deep trough off the west coast begins to move inland.
the 500 mb ridgeline will be east of the area by sunset today and
that will open the Great Basin and Central Rockies to increasing
southwest winds aloft and at the surface after the morning
surface-based inversions mix out Friday.

Friday should see the 500 mb trough over the west coast at noon and
moving inland. Ahead of that trough will be a 120 kt jet stretching
from southern CA through western Montana. Strengthening southwest
surface winds will spread across Nevada and Utah and nudge upward
over western CO as well. High clouds will also stretch over the
forecast area with precipitation reaching the eastern Uinta
mountains in northeast UT by sunset Friday as the surface cold
front approaches.

.LONG TERM...(Friday night through Wednesday)
Issued at 246 AM MST Thu Jan 18 2018

The 120 knot-or-so upper level jet will be on our doorstep Friday
night and begin to usher some isolated to scattered precipitation
into northeastern Utah and northwestern Colorado. The latest Euro
run looks to be about 6 hours behind the GFS and NAM but,
regardless, the coverage of precipitation looks to increase
significantly in the mid morning/early afternoon hours on
Saturday as the trough and associated cold front swings into Utah.
Numerous showers are expected to continue through Saturday night
as the cold front pushes through before showers begin to taper off
from west to east on Sunday. Both specific humidity and
precipitable water will remain well above normal throughout the
storm with PWATs climbing to almost half an inch on Saturday
afternoon, 200% above the climatological normal of 0.25 inches.
Available moisture has been quite promising and relatively
consistent between model runs with this storm, but what about snow
levels?

As the previous forecaster mentioned, strong and mild
southwest flow ahead of the arrival of the trough will keep snow
levels high at the onset of the event, especially in the San Juan
mountains, where snow levels will rise to approximately 9500 feet
Friday afternoon. 700mb temperatures are still on track to
plummet to -10 to -14 degrees C with the passage of the cold front
which will help drop those snow levels down to valley floors. As
far as snowfall totals go, the potential is there for advisory-
level criteria in most mountain areas with some light to maybe
even moderate accumulations in many valley areas, especially
further north.

On Sunday the upper level trough will lift northeast into the
Plains, leaving significantly cooler northerly flow in its wake.
Most snow showers should be confined to the along the Divide by
mid afternoon Sunday where orographic showers will linger into the
new work week. Temperatures will begin to moderate slightly on
Monday under weak transitory ridging. Extended models then bring a
progressive shortwave into the forecast area on Tuesday which may
bring some isolated mountain snow showers in addition to increased
cloud cover. At the midweek point, a strong and broad upper level
Pacific trough will elongate off the west coast so the cool and
progressive weather pattern looks to continue!

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1004 AM MST Thu Jan 18 2018

VFR at all terminals through the forecast period.

&&

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

$$

SHORT TERM...CC
LONG TERM...MMS
AVIATION...MAC



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