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Area forecast discussion 
National Weather Service Denver/Boulder Colorado
832 PM MST Thursday Dec 25 2014

issued at 832 PM MST Thursday Dec 25 2014

Weather scenario evolving much as expected as the center of the
upper low progresses toward The Four-Corners and eastern Colorado
lies under the diffluent region downstream from the upper trough.
At the surface...modest upslope flow continues as the main surface
low holds its position over the southeast corner of the state.
Snow accumulations of 2 to 5 inches have been reported up and down
the Front Range so far this evening. Farther to the
northeast...winds are a bit stronger...causing some blowing and
drifting snow in Logan...Sedgwick and Phillips counties.

Model soundings from this evenings NAM run show that a middle-level
inversion will be developing by 06z which should change the
characteristics of the snowfall for the latter half of tonight.
Quantitative precipitation forecast forecasts from the rap and hrrr models also show that
intensity will be falling off in the next few hours. The current
set of highlights and snowfall forecasts look like they will
handle the situation well no changes are forthcoming.

The evening run of the NAM now shows that the upper low is not
going to clear out of the state until Friday evening. Will
increase the probability of precipitation through Friday afternoon. Expected snow
accumulations through the daylight hours on Friday should be less
than an inch or two...given that model soundings are showing the
middle-level inversion with some drying aloft continuing through the


Short term...(this evening through friday)
issued at 314 PM MST Thursday Dec 25 2014

The main storm system continues to dig southeast into Utah this
afternoon. Northeasterly upslope flow has spread across the
plains behind this mornings cold front...with light snow beginning
to push south. The heavier snow occurring along the Wyoming
border today is expected to begin to push south as another surge
pushes into the state. The areas are mostly covered under
advisories and warnings...except Logan...Morgan...Phillips and
Sedgwick counties where 3-5 inches are expected to fall overall.
Current radar and hi-res model trends are hitting Weld County
more...may need to upgrade eastern Weld to a warning. However with
the more northerly push expected this evening...more than likely
the northern Front Range urban corridor and plains will have snow
greatly decrease as downsloping occurs off the Cheyenne Ridge.
Main push still expected to occur between 5pm and midnight with
heavier snowfall rates along and east of The Divide and Front
Range mountains. Qg vertical velocities are weak upward which is
not favored for high snow amounts...but good enough upslope flow
with the available moisture and cold advection should help it out.
West of The Divide...snow has taken a while to get going in the
unfavored south to southwesterly flow. This evening...orographics
are not the best but upward qg vertical velocities increase...will
allow the advisories to carry on.

The upper low will move over The Four Corners early tomorrow
morning then weaken and become an open trough Friday afternoon.
As this happens...qg vertical velocities will increase upward.
High surface pressure over the great northern plains will keep
north to northeasterly upslope flow over the Front Range
mountains...foothills and plains. Overall the speed of the low has
slowed down over past runs which will provide a more favorable
environment for snow to continue through tomorrow. Have increased
chances of precipitation and precipitation amounts as well. Snow amounts
over the mountains will be light. By afternoon...upslope
flow decreases which will start clearing in and near the
foothills then spread east.

Long term...(friday night through thursday)
issued at 314 PM MST Thursday Dec 25 2014

GFS and a majority of its ensemble members...NAM and European model (ecmwf) all show
the 500 mb low tracking northeastward over northeast Colorado by
00z/Sat. The Canadian Gem however still has the upper low over S-
central Colorado at that time. Overall models indicate gradual
weakening of the upper low and rising heights as the trough lifts
over western Nebraska Friday evening. In addition 10-20kt north-northeasterly 850-
700mb flow over northestern Colorado gradually becomes northwesterly overnight with
qg vertical velocity fields indicating increasing downward motion over the County Warning Area
after midnight. Moisture also thins out overnight according to model
cross sections with the shift to a flow off the Cheyenne Ridge.
However relative humidity fields and cross sections indicate slow clearing
consequently preventing temperatures from crashing with cold pool
aloft passing by. Any additional snow accumulation will be minimal
and largely confined to higher elevations of the Front Range and out
near the northestern corner of the state. Reduced probability of precipitation Friday evening look
good but not as low as earlier thought.

For Saturday...weak middle-level instability and moisture swing down
from western Wyoming Saturday morning which could generate a few light
snow showers over higher elevations. Otherwise drying will be
underway with daytime temperatures warming several degrees above
those on Friday. During the late afternoon and evening hours mountain top
stable layer intensifies with warming aloft. Models cross sections
show well develop mountain wave forming over and along the Front Range
with a 45-50kt cross barrier wind component after 06z/Sunday. Cross
sections show this momentum moving down the East Face of the Front
Range possibly resulting in 30-40kt wind gusts in the lower
foothills towards morning. This will be a warming wind so late night
and morning temperatures along the base of the foothills will be
quite mild for this time of year.

On Sunday...attention shifts to the extensive Arctic airmass pooling
to the west through north of Colorado. Also watching a progressive
upper trough on the nose of a powerful Pacific jet diving out of the
Pacific northwest. Models show this trough and this Arctic airmass
settling in over the central Rocky Mountain region late Sunday into
Monday setting the stage for what promises to be a very cold and
possibly snowy couple of days early next week. Temperatures by
Monday night and Tuesday could be as much as 35-40 degrees f below
average...esply east of the mountains snow changes also likely to go up
with a west-east elongated upper trough anchoring over the Great
Basin and central Rocky Mountain region. With the airmass as cold as it
may be...snowfall for the most part should be a light fluffy sort
of snow.

Drier and warmer conditions possibly return for midweek with the
upper trough nosing swwrd over Southern California.&&

Aviation...(for the tafs through 00z Friday late afternoon)
issued at 314 PM MST Thursday Dec 25 2014

Light snow over the area airports with MVFR conditions...expected
to get down into the IFR category soon. Best chance for heavier
snow will hold off until after 00z with a secondary cold push.
Then expect an increase in coverage and intensity through the early
evening hours. Snow intensity will decrease by midnight...but will
continue to see light snow...with periods of moderate...through
Friday evening. Ceilings/visible will likely stay IFR through then as
well. Right now will go with accumulations in the 4 to 6 inch
range for dia...6 to 9 inch range for kbjc and kapa through 00z
Friday evening.

Northerly winds expected to continue through the evening with
decreasing speeds after 06z. Northerly winds overnight will trend
northeast Friday afternoon.


Aviation...(for the tafs through 06z Friday evening)
issued at 832 PM MST Thursday Dec 25 2014

IFR conditions are expected through much of the next 24 hours as
the winter storm slowly crosses over the state. Most of the snow
accumulations should be in the next few hours...with up to 3 more
inches...then only an inch or two from midnight through Friday
afternoon. Winds will be northerly...but without any strong blowing snow is not anticipated.


Bou watches/warnings/advisories...
Winter Storm Warning until noon MST Friday for coz035-036-039-

Winter Weather Advisory until noon MST Friday for coz031-033-034-

Winter Storm Warning until 8 am MST Friday for coz038.

Winter Weather Advisory until 8 am MST Friday for coz042-043.



short term...kriederman
long term...Baker

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