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Area forecast discussion 
National Weather Service Denver/Boulder Colorado
940 PM MST Friday Dec 19 2014

issued at 940 PM MST Friday Dec 19 2014

Have updated short term forecasts with recent observation. Sterling is
reporting some fog currently...expecting this to remain patchy mainly
along the South Platte valley. Will have a mention of patchy fog
only for the northeastern corner of the state at this time.


Short term...(this evening through saturday)
issued at 109 PM MST Friday Dec 19 2014

A short wave ridge of high pressure will shift over northern
Colorado tonight with the ridge axis shifting to the eastern
border by late Saturday. Some upper level moisture will increase
over northwestern Colo late tonight...then shifts east across the
northeast plains on Saturday. In the mountains...some middle level
moisture develops during the day will allow for the chance of snow
to redevelop...especially in the mountains north of Interstate 70
by late afternoon. The flow aloft will remain fairly weak so wind
should not be an issue in the high country. Across the northeast
plains...the pressure gradient remains fairly relaxed which will
translate to continued light winds. There will be some
modification of temperatures with highs generally in the middle to
upper 40s...a little cooler where snowfield still exists north and
east of Akron.

Long term...(saturday night through friday)
issued at 109 PM MST Friday Dec 19 2014

..moist and cold for the week ahead will lead to multiple
opportunities for snow...

The next major snow producer will move into the mountains on Sunday.
Strong northwest flow dominates in the upper levels with increased
moisture from the approaching trough impacting the park and Gore
ranges. Another important ingredient for a good snow producer is
the upper level jet. The latest GFS has the jet dipping into the
western portions of the state leaving our western borders exposed
to the coldest and most unstable portion of the currently 160 knots
jet streak. This will aid with getting snow in the best layer in
the atmosphere. Qg is not impressive with this system but with the jet as
well as middle level lapse rates from 7-8 c/km the instability will
be sufficient. The only possible hitch to the system are the
temperatures. Current cross sections show temperatures near
mountain top in the upper teens to low 20s which will create a
more slushy and heavy snow mix to start. Once the cooler air moves
in with the front early Monday the snow will become more dry and
accumulations will increase. Current models are indicating
possible amounts of 12 to 20 inches for higher elevations in the
northern mountains and passes and possible 4 to 8 inches in the
valleys. In addition to the snow cross section component along
flow is indicating high winds for the mountains as well thanks to
the approaching jet. Current models are indicating 60-70 kts at
mountain top early Monday. This will create areas of blowing snow
further complicating mountain travel along and north of I-70 from
Sunday afternoon through Monday. A watch will accompany this
forecast package for the mountains and be valid from Sunday
afternoon to Monday evening.

For the plains downsloping down the foothills will keep snow at Bay
for Sunday into early Monday before a cold front pushes through
and instability and moisture increase. Cloudy conditions will
prevail on Sunday with temperatures in the middle 50s for NE Colorado.
Models are indicating measurable snow for the plains most likely
Monday afternoon into the evening but exact amounts are difficult
due to possible banding. An advisory might be needed as snow
amounts become clearer...especially around dia. Temperatures will
cool behind the cold front bringing overnight temperatures into the lower

On Tuesday flow aloft will become more northerly as the low pressure
center moves off to the east. Moisture will decrease through the
day but will maintain probability of precipitation in the mountains due to lingering
moisture and decent low level lapse rates. Temperatures will be
cooler with highs only able to get into the lower 40s for the
plains and single digits for the higher mountains.

On Wednesday into Thursday morning an approaching ridge moves into
the region bringing winds back to a more northwesterly direction.
Moisture is weak so clouds will scatter out and temperatures will
bounce back into the lower 50s for the plains. Kept some low probability of precipitation
in the mountains during this time mainly due to orographics but
there is little moisture to produce much accumulation.

The next major system is currently forecasted as a weak upper
level trough that will be over Utah by 18z Thursday. Both the ec
and the GFS currently have a an open wave that is less favorable
for snow. I have kept chance probability of precipitation for the mountains increasing
slightly overnight Thursday due to strong SW flow aloft combined
with decent moisture ahead of the trough Thursday morning and
continued qg ascent of -35 to -40 from the ec. For the plains on
Thursday clouds will move in with increasing moisture Thursday
morning and afternoon with temperatures in the upper 40s for
Christmas day cooling overnight with the next trough. Models
diverge on the possibility of snow over the plains with this
system as the main energy according to the GFS sits to our
north...greatly reducing snow chances without upper level
instability and jet support. Will continue to watch models and
fine tune forecast over the next few days.


Aviation...(for the tafs through 06z Saturday evening)
issued at 940 PM MST Friday Dec 19 2014

VFR conditions over the next 24 hours. Expect south-southwesterly drainage winds
through the night and early morning before switching slightly to
the southeasterly. Airmass will remain hazy with limited mixing but do not
anticipate fog late tonight into Sat morning. The latest hires models paints
some fog right along the Platte River northeast of Denver...from
Fort Morgan and northeast...but that is it. As a result...will keep
fog out of dia at this time.


Bou watches/warnings/advisories...
Winter Storm Watch from Sunday afternoon through Monday evening
for coz030>034.



short term...Cooper
long term...Bowen

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