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Area forecast discussion 
National Weather Service Denver/Boulder Colorado
856 PM MST Sat Dec 20 2014

issued at 856 PM MST Sat Dec 20 2014

Light snow had developed over the mountains in the last few hours
per latest radar returns and webcams. Most light though and this
wave already seen passing by on satellite. Cant rule out an
isolated brief sprinkle or flurry on the plains.

The main weather story will certainly be the significant mountain
winter storm and rather high impact event given its arrival time
Sunday afternoon. Should see snow first start to increase in the
Rabbit Ears Pass area Sunday morning and then a quick increase
across the Front Range mountains and I-70 corridor through Summit
County by noon...perhaps a little faster than previous forecast
given strength of flow aloft. Conditions are then expected to
deteriorate rapidly with heavy snow and strong winds developing in
the afternoon and continuing through evening. This will coincide
with additional ski traffic for most likely very difficult travel
conditions. Given the expected intensity of the snow/blowing snow
and near blizzard conditions in open areas...Road closures would
certainly be possible.

Adjusted wind forecast in the foothills and adjacent plains higher
based on impressive Cross Mountain pressure gradient developing
on Sunday. Not too impressed with any mountain wave enhancement
especially with increasing mountain snow by noon so gusts should
stay below any warning criteria.


Short term...(this evening through sunday)
issued at 320 PM MST Sat Dec 20 2014

Expect snow to increase late this afternoon and early evening over
the mountains as the next Pacific wave moves across the state. Main
forcing will be orographic...though there will be help from a jet
streak. Expect up to 3 inches of snow by Sunday morning. Snow
will likely decrease late tonight before it ramps up on Sunday.
Snow will become widespread and increase in intensity Sunday
afternoon as lift from the jet and orographic lift increase.
Expect widespread moderate to heavy snow around sunset 4-5pm
Sunday afternoon. Snowfall rates may reach 1-2 inches per hour.
The heavy snow combined with gusty winds will cause Road
conditions to rapidly deteriorate. Many roads may become
impassable Sunday evening.

For the Front Range...a mild airmass and cloud cover tonight will
result in a mild night. A Lee side trough will deepen Sunday. By
afternoon...expect breezy conditions across the Front Range and
eastern plains. Near the Wyoming border...gusts to 50 to 60 miles per hour
will be possible. The downslope winds will also result in mild
temperatures with highs in the 50s along the Front Range. It will
be cooler over the northeast corner where snow remains on the

Long term...(sunday night through saturday)
issued at 320 PM MST Sat Dec 20 2014

The northern Colorado high country is in for a blast of wintry
weather Sunday night through Monday. A deep plume of moist
Pacific air extending well off the Oregon coast will be driven
sewrd into northern Colorado by a powerful jet stream during this
period. Mountain areas above timberline and west-northwest facing
mountain slopes could see 2-3 inch per hour snowfall rates Sunday
night accompanied by 30-40kt sustained winds and peak gusts
to as high as 70kts. Heavy snowfall and fierce winds are likely
to produce treacherous whiteout conditions potentially leading to
Mountain Road closers. Mountain valleys not likely to see as much snowfall
or winds...although the upper Colorado River Basin in Grand
County could possibly pick up as much as 8 inches of snow by
Monday afternoon. Meanwhile North Park will probably see lighter
snow amounts due to less favorable orographics...and South Park
even less snow for the same reason. Although North Park could see
several hours of moderate snowfall on Monday when winds aloft turn
more northerly which is a more favorable direction for snow
production in zone 30.

As for the plains...a strong westerly flow moving down off the Front
Range will likely suppress snow production along the foothills
Sunday night. This same area could see west-northwesterly winds gusting in
the 25-35kt range late Sunday night with strong middle-level cold
air advection kicking in. Farther out on the plains jet banded
snowfall could lay down a few ribbons of 1/2-1 inch snow
accumulation across the northeast corner of the state late Sunday

On Monday...models show a cold front sweeping down from Wyoming
early causing a shift to north-northeasterly surface winds. Post-frontal boundary
layer air does not look all that moist. Still areas close to the
foothills could see some banded precipitation in the form of rain/snow
showers during the day Monday. Any snow accumulation should
generally be under an inch. In the high country...snowfall and
wind speeds will gradually decrease from north to south during
the day as the jet stream sags south of the forecast area by
evening. Could see another 3-7 inches of snow in the high country
by Monday evening...with storm total snowfall amts of 12-24
inches by late Monday evening.

Tuesday through early Wednesday look dry and slightly warmer overall
with shortwave ridging. By Christmas day...models show yet another
potent upper air weather system dropping down from the Pacific
northwest. Unfortunately there/S quite a bit of uncertainty in
the models regarding the timing...trajectory and magnitude of
this system. For now...will hang onto a slight chance of snow at
lower elevations and a chance of snow in the high country
beginning late on Thursday and Friday.


Aviation...(for the tafs through 06z Sunday evening)
issued at 856 PM MST Sat Dec 20 2014

VFR conditions will persist through Sunday evening. Lower cloud
deck around 6000-9000 feet early this evening will quickly break
up by 06z. Then clouds may lower again to 6000-9000 feet Sunday
afternoon and evening. Fairly normal south/southwest winds
expected tonight...but should strengthen on Sunday afternoon as
mixing improves. That would allow a stronger west winds of 12-20
knots with a few gusts to 25-30 knots at kden and kapa. At
kbjc...peak gusts could reach 35 knots if sufficient mixing


Bou watches/warnings/advisories...
Winter Storm Warning from noon Sunday to midnight MST Monday
night for coz031>034.

Winter Weather Advisory from noon Sunday to midnight MST Monday
night for coz030.



short term...meier
long term...Baker

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