Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 062352

Area Forecast Discussion
National Weather Service Grand Junction CO
452 PM MST Fri Dec 6 2019

.SHORT TERM...(Tonight through Saturday night)
Issued at 223 PM MST Fri Dec 6 2019

Quiet and dry weather will continue this afternoon, as the ridge of
high pressure moves overhead. A deck of high clouds will continue to
stream across the region this evening, with lower clouds lingering
across the north. Some clearing is likely tonight, while winds aloft
transition to west-southwest flow.

Low level clouds and fog will likely develop again across
northeastern Utah and western Colorado Saturday morning. Light winds
at the surface combined with lingering low level moisture will
support fog forming across the Gunnison and Yampa Valleys, portions
of the Grand Valley, and near the Uinta Basin. Patchy areas of fog
will quickly clear after sunrise; however, other regions with a more
consistent stratus deck aloft (such as Gunnison and Vernal) may
experience poor visibilities and low cloud ceilings persisting into
afternoon hours.

Expect winds to shift towards the west Saturday afternoon, as upper
level flow turns zonal ahead of the next system. Zonal flow
accompanied with increased winds aloft will briefly improve mixing
at low levels for most of western Colorado and eastern Utah. Areas
with decent mixing will result in slightly warmer temperatures and
could see stronger winds reaching the surface. Mixing should also
help conditions improve across areas with lingering low level clouds
and morning fog. Otherwise, weather conditions will remain dry
across the region into Saturday evening, however cloud clover will
begin to spread across the region ahead of the next system. Chances
for precipitation also increase late Saturday night, mainly over
higher terrain, while the rest of the region will experience mild
overnight lows due to the insulation from a blanket of cloud

.LONG TERM...(Sunday through Friday)
Issued at 223 PM MST Fri Dec 6 2019

Precipitation will break out over much of the higher terrain of
eastern Utah and western Colorado early on Sunday morning. The
trough responsible for the unsettled weather is not particularly
strong, but it will make up for this with a broad shield of
moisture and favorable westerly upslope flow through the day on
Sunday. A diffuse frontal boundary does pass east through the
western slope on Sunday afternoon as well, with 700mb temperatures
dropping 2-4 degrees across the board by the end of the main
event on Sunday evening. This will help snow ratios, especially
in the central and northern mountains. Latest QPF guidance in the
12z forecast cycle today showed a slight uptick in precipitation
totals, with snow totals possibly exceeding 6 inches in the Park,
Flat Top, Gore and Sawatch ranges. Farther south in the Elks and
San Juans, close to 1" of liquid is forecast by some
models...which could result in nearly a foot of snow in favored
westerly and northwesterly slopes. Lower elevations are a bit more
tricky, as snow levels and warm air in the valleys will play a
role in Precipitation Type early in the event. If enough cold air
filters in on Sunday afternoon, an inch or so will be possible in
places like Craig, Rifle and Montrose, with 1-3 inches in
Steamboat (town), Vail, and Gunnison. The US160 corridor is a
particularly challenging forecast, as the San Juan Mountains may
block the progress of much of the colder air from the north.
Durango could easily see 0" in this case, however a slushy inch or
so in the grass is also possible if colder air makes it in before
precipitation ends Sunday evening.

The trough axis with this system passes east of the Continental
Divide by early Monday morning with flow turning northwesterly
over eastern Utah and western Colorado. A shallow stream of
moisture will remain in place over the northern Colorado mountains
in this northwest flow pattern from Monday through Tuesday
morning. Favorable upslope flow will result in additional snow
accumulations in the northern Parks and Elkhead mountains, however
model consistency has not been high in this timeframe. Things
finally dry and clear out on Tuesday evening up north as a ridge
builds in over the Four Corners for Wednesday. Both the ECMWF and
GFS indicate a weak, dry trough moving through on Thursday
resulting in a mainly cloudy day. The next opportunity for
precipitation will arrive by Friday morning as a fast Pacific jet
moves inland and into the Great Basin. Both the ECMWF and GFS
support the idea of unsettled weather from Friday into Saturday
however specific details with this system will have to wait for
better model consistency on both temperatures and the position of
the best moisture stream.

Overall, forecast confidence is moderate throughout the long term.
Winter weather headlines will likely be needed in the mountains for
the storm arriving on Sunday morning, and will be addressed in
forthcoming forecast packages.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 445 PM MST Fri Dec 6 2019

Mid level ceilings around 4K to 5K feet persist over the northern
valleys at this hour and cross sections suggesting this will
continue through the overnight. HRRR data thinks otherwise though
as RH values drop off significantly. Satellite data showing some
movement to the low clouds so gave the nod to the HRRR with some
clearing over the next few hours. KVEL will likely come back down
though as dewpoint depressions are very similar to yesterday when
fog dropped the airport to 1 1/2 miles and ceilings near 200 feet.
Outside of those areas, expect some high clouds especially down




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