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Area forecast discussion 
National Weather Service Grand Junction Colorado
915 am MST Thursday Dec 18 2014

issued at 910 am MST Thursday Dec 18 2014

Went ahead and dropped the advisories this morning with clearing
moving into the southern mountains. Across The Mesa into the
central Colorado high mountains light snow showers and fog likely
to persist through the day though the nature of the precipitation
will become more showery. Upstream energy...phased from the more
defined system in Arizona to the northern rockies...will move
through the County Warning Area. Instability and residual moisture should be
enough to keep isolated to scattered showers in the picture
through sunset. Expect some of the showers could dump a quick half
inch but with coverage limited do not expect significant impacts
at this time. Fog may be more of a problem through the late morning hours
then again overnight so will be monitoring this as well.


Short term...(today through friday)
issued at 445 am MST Thursday Dec 18 2014

Though returns on radar are weak and satellite imagery indicates
low cloud tops...the snow is still flying across parts of west
central Colorado. At midnight we received reports of an ongoing
gorge event with 7 inches of snow reported in The Gorge on Highway
550 just south of Ouray but only 3 inches just south of The Gorge
area at Red Mountain Pass. Expecting this event will wrap up around 6
or 7am as negative Omega support slides to the north.

Deep atmospheric moisture continues today which will keep the
forecast area in an unsettled pattern. The main bulk of precipitation
is expected to shift north and east today as the wind shifts to a
more west- northwesterly direction...however the San Juans will
still remain showery throughout the day. Upper level divergence
increases this evening which should keep showers going into the
overnight hours at higher locations. Fog will be the biggest
challenge of the day and tonight as relative humidity remains high and surface
moisture has a tough time drying out with continued cloud cover.

On Friday...upper level ridging builds into the area for a brief
lull in active weather. Temperatures will remain near normal the
next couple of days. The ground should get a chance to dry out on
Friday during the lull of activity so fog will be less widespread
Friday evening.

Long term...(friday night through wednesday)
issued at 445 am MST Thursday Dec 18 2014

..significant snowfall possible for the northern and central
mountains Sunday afternoon through Tuesday night...

Pacific energy kicking out of the Pacific northwest Friday night
will drag across the northern zones and bring a quick shot of snow
through Saturday morning. System is unorganized and will weaken as
it encounters weak ridging over the region...limiting potential
impact on our forecast area. This wave exits Saturday afternoon
with a brief break ahead of what is looking like a long
duration...persistently moist northwesterly flow event that
starts Sunday afternoon and continues through Tuesday night.

Medium range models in good agreement this morning with sending a
series of fast moving waves through the Pacific northwest with
abundant Pacific moisture and slamming this into our northern and
central mountains. Specific humidity exceed 3 g/kg for a large
duration with strong jet support as a 120+ jet drives the moisture
into our area. Based on current set-up in the models...a prolonged
period of moist Pacific flow and very favorable orographics to
bring substantial snowfall over a 2+ day period beginning Sunday
afternoon. The heaviest snow is expected to fall across the
northern and central mountains in western Colorado...with focus
from Vail Pass northward to Steamboat Springs. Still time to work
out the details...but appears we will be looking at several winter
highlights by late this weekend into early next week.

However...and there is always a however...warm air advection could and most
likely will create some problems with the model generated quantitative precipitation forecast
values...thus snowfall accumulations may be overblown. Will get a
better handle on this as the event draws closer. Even with that
concern...will still be issuing an Special Weather Statement later this morning as this
will be a heavy travel week and potential impacts could be large
as snowfall may be measured in feet across some of our
mountains...with the GFS generating over 3 feet in some areas by
midweek. Even half of that amount will cause impacts on Holiday
travel plans. Other areas will get into the action as well...but
the emphasis looks to be across the northern zones at this time.
Temperatures moderate a bit...especially as the colder valleys mix
Monday and Tuesday.

And then there is the potential Christmas day storm...although
models diverge a bit on this storm with the European model (ecmwf) closing a low
over the Great Basin whereas the GFS remains more open. Either
solution brings more snow to the region...with the ec more
favorable for snow into the valleys. Will just ramp probability of precipitation up for
now with a cool down in temperatures expected.


Aviation...(for the 12z tafs through 12z Friday morning)
issued at 445 am MST Thursday Dec 18 2014

Widespread IFR ceilings and visibilities will persist across much of western
Colorado through 16z this morning...before conditions slowly
improve from west to east. Expect areas of rain...snow...low
clouds and fog with mountains remaining obscured. All terminal
sites will be impacted for several hours this morning...before
conditions improved this afternoon. The storm system responsible
for the inclement weather lifts out of the area with moist
northwest flow keeping abundant cloud cover in place through
tonight...although ceilings are expected to lift above Cat thresholds
at all sites.


Gjt watches/warnings/advisories...


short term...jam
long term...jdc

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