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Area forecast discussion 
National Weather Service Denver/Boulder Colorado
848 PM MST sun Dec 21 2014

issued at 847 PM MST sun Dec 21 2014

Showers have also managed to spill across the Front Range as
instability has increased this evening. There may be some
potential instability with drying noted aloft...and cells have
become quite convective with isolated lightning indicated in the
Colorado lightning mapping array. Meanwhile...front is moving
across the Colorado/Wyoming border as of 830 PM with another area of
precipitation...mostly snow...behind it. Have increased probability of precipitation for
most of the plains to account for both of these features. Short
range rap and hrrr are not handling this precipitation well at
all. A few spots could see an inch or two of snow on the plains
if showers become more persistent as intensity/convective
parameter is there.

Update issued at 652 PM MST sun Dec 21 2014

Band of showers is organizing on the eastern plains early this
evening...from near Cheyenne to Fort Morgan and east of Limon.
This feature seems rather slow moving. Most of this will be rain
before cold front arrives later in the evening. The arrival of
this feature and a more northerly low level component could shift
the band back toward the I-25 corridor overnight...but still
favoring locations east of I-25. Mountain heavy snow event is
underway with mostly 3-6 inch amounts but a few spots already
approaching a foot toward the Columbine and tower snotels...and
also near Copper Mountain. The heaviest snow is still on track to
occur this evening but there was some drying noted on the Riverton
Wyoming sounding between 700 mb and 500 mb...which should result
in lower snow intensity after midnight. Until half to
one inch per hour snow amounts will continue over most of the high
country. Hazardous and slow driving conditions will remain in
place through Monday with continued Winter Storm Warning.


Short term...(this evening through monday)
issued at 326 PM MST sun Dec 21 2014

Snow will continue in the mountains overnight tapering off
starting Monday evening. Upstream observation show visibilities reducing
down to half a mile with the heavier snow. Current warning
products still on track but overnight shifts may need to re-assess
end times due to recent model runs moving the jet out sooner. The
lingering moisture and orographic flow will still produce some
snow showers over the mountains tomorrow but the intensity will
decrease. Continuing to urge travelers to use caution as winds
over the higher mountain passes will gust up to 60 miles per hour this
evening. Considerable blowing and drifting snow along with
visibilities below a quarter of a mile will occur over areas such
as Loveland Pass...Berthoud Pass and Rabbit Ears Pass. Other well
traveled areas along I-70 such as the Tunnel will also see greatly
reduced visibilities with icy and snowy roadways. Travelers are
urged to use caution and delay plans to travel over the mountains
this evening if at all possible.

For the plains downsloping winds will keep areas along the
foothills 4-5 degrees warmer and dry with winds gusting up to 60
miles per hour over the wind prone areas. Winds will move to a more northerly
direction early tomorrow morning bringing cooler conditions along
with a chance of snow east of I-25. In addition to the influx of
moisture over the plains there is some instability so some areas
could have some light accumulation...with up to 2 inches possible
under the heavier bands. Temperatures for tomorrow will be cooler
with highs only getting into the lower 40s for the plains and 20s
for the mountains. Both the NAM and GFS are showing increased
winds over the NE plains starting tomorrow afternoon. GFS
currently has 65kts at 700 mb that could translate to warning
criteria. Will consult with the next shift and future model runs
on possible issuance.

Long term...(monday night through sunday)
issued at 326 PM MST sun Dec 21 2014

Northeast Colorado remains under the influence of a broad upper
level trough centered over Nebraska and a strong and cold northwesterly
flow Monday night. Bands of instability and moisture moving down
the west side of this trough should keep skies mostly
cloudy/overcast and snow chances in the forecast for the High
Plains overnight. However areas along the base of the Front Range
may escape from any snowfall due to the drying effects of
downslope flow off the foothills and Cheyenne Ridge. In the high
country...snowfall will likely continue overnight...with moderate
snowfall at times on the higher ridges and northwest facing slopes
along the Continental Divide. Gusty northwest winds may also
continue to produce some blowing snow in open terrain. With
snowfall letting up after midnight...the Winter Storm Warning for
mountain zones 31...33 and 34 will be allowed to expire at midnight
MST. Meanwhile strong northwesterly flow aloft gradually mixes downward
overnight as cold advection increases. By early Tuesday morning
models indicate north-northwesterly surface winds gusting to around 40 miles per hour across
the northeast corner of the state. With several inches of snow
covering the ground out that way...good bet we/ll see some blowing

During the day Tuesday...models show the upper trough tracking
slowly newrd away from Colorado with northerly flow over the plains
becoming even stronger during the day. Model wind speeds across
the northestern corner in the 35-50kt range for much of the day.
Blowing snow will be a good bet especially in open areas of
Logan...Washington...Sedgwick and Phillips counties where several
inches of snow on the ground. No highlights at this time...however
a High Wind Warning may be necessary should later model runs
indicate even stronger wind speeds. As for snowfall...should see
an end to it by morning at lower elevation and by late afternoon
in the high country with the start of warming and drying aloft.

On Wednesday..large upper ridge over the Great Basin nudges eastward
over Colorado clearing skies and warming temperatures. Could see an
8-10deg f warmup in most areas on Wednesday. Wednesday night upper
ridge shifts east as the next in a series of long wave troughs
sweeps over the Pacific northwest. Latest indications...and they are
in better agreement this trough tracking nearly due
east across the northern rockies. Models show the south side of
this trough brushing northern Colorado Christmas day. As it does
mountains and high valleys should see a shot of snow and gusty winds
while the plains remain mild and dry with gusty downslope flow
off the Front Range. As the trough passes late on Thursday...models
show a band of light precipitation associated with cold front slipping
south across northeast Colorado during the evening hours. Some
areas may see snowfall...but any amounts should be light. By late
that night...drying sets in on cold and gusty northwesterly winds.

Friday and Saturday look dry but with below average temperatures.


Aviation...(for the tafs through 06z Monday evening)
issued at 847 PM MST sun Dec 21 2014

Cold front is expected to reach the Front Range airports toward
05z with winds turning more northerly and gusting to around 25
knots. Should see a few rain showers with ils Landing conditions
until around 06z...then snow showers with IFR/MVFR ceilings and
visibilities with snow showers. Potential for lower visibilities
of one mile or less if showers maintain there present
intensity...with local snow accumulation up to an inch or so
possible mainly on grassy surfaces. Those would likely be ending
toward 12z as drier air moves in from the northwest.


Bou watches/warnings/advisories...
Winter Storm Warning until midnight MST Monday night for

Winter Weather Advisory until midnight MST Monday night for



short term...Bowen
long term...Baker

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