Area forecast discussion 
National Weather Service Denver/Boulder Colorado 
925 am MST sun Dec 21 2014 

issued at 925 am MST sun Dec 21 2014 

Satellite imagery showing snow making its way into western 
Colorado as the pool of moisture and jet moves southeast. Will 
not make any updates to the on-going warning products as timing 
and snow fall amounts are still on track...however have decided to 
hold off on any possible blizzard warnings as the valleys will 
most likely not be hit with the highest wind gusts. Model cross 
sections are still indicating most of the higher winds will be 
regulated to the higher mountain passes and above timberline with 
gusts to 60 to 70 miles per hour between 4 and 8 PM this evening. This will 
create extremely hazardous driving conditions for travelers on 
I-70 this evening with greatly reduceded visibilities. Pushing 
the effects of the heavy snow at this time in addition to the 
increased winds over well traveled passes through conference calls 
and on social media. Will re-assess the Blizzard Warning this 
afternoon based on upstream observation over northwest Colorado. 


Short term...(today through tonight ) 
issued at 523 am MST sun Dec 21 2014 

Moisture plume moving into Colorado this morning on the nose of a 
very strong jet. For this morning there is weak warm advection and 
increasing winds...with deepening moisture but with some middle level 
stability. This should generate steady snow over and west of the 
Front Range...but not heavy yet. This afternoon the lift will 
increase a bit and cold advection will set in making for a much 
more productive environment this evening. It looks like the best 
lift...strongest winds...and best instability will be over the 
Interstate 70 corridor in the late afternoon and evening. Bad 
timing for the impact could be substantial. We will 
be hitting this message hard today. Winds go more northerly later 
tonight with the best lift sinking south...but still reasonably 
good conditions for moderate to heavy snow the rest of the night. 
Current highlights look alright...tweaked the timing of the snow a 
bit to hit this evening the hardest. This may be a candidate for a 
Blizzard Warning later today if it pans out. 

For areas east of The Divide...downslope winds but plenty of snow 
spilling over into the foothills. In addition some forcing with 
the jet so we will need some accumulations in the higher foothills 
and South Park at the height of the event. Some threat of high 
winds...though not enough for a warning. High winds appear likely 
for the higher mountain areas that are in the Winter Storm 
Warning...such as Berthoud Pass...Niwot Ridge and other above 
timberline areas sticking up into the jet. For the more populated 
foothill areas gusts in the 50-70 miles per hour range look likely tonight. 
Jet related banding may be strong enough to overcome the downslope 
at some point...especially later tonight as the deep wind 
component becomes more northerly and there could be some low level 
convergence somewhere on the east side. Probably a better chance 
further away from the mountains...but some threat in Denver as 
well. This stuff will probably be light but with the strength of 
the jet and some instability there could be some places that get a 
couple of inches as a shower band comes over. 

Boosted high temperatures a couple of degrees today...warm airmass 
and windy. 

Long term...(monday through saturday) 
issued at 523 am MST sun Dec 21 2014 

There is northwesterly flow aloft for the forecast area on Monday. 
The flow aloft becomes almost due northerly by 12z Tuesday morning 
with jet level speeds on the increase. The flow aloft is northerly 
around 100 knots Tuesday and Tuesday night. Downward synoptic 
scale energy is prognosticated over the County Warning Area all four periods...Monday 
through Tuesday night. The boundary layer flow is all northwesterly 
for the County Warning Area Monday through Tuesday. Concerning stays 
fairly deep over the mountains Monday and Monday night. By Tuesday 
..moisture is decreasing in the high country. But not totally 
gone until Tuesday evening. For the is pretty dry 
Monday morning...then moisture increases in the middle and upper 
levels...lowering...especially over the eastern half by Monday 
evening continuing into Tuesday. All areas are pretty dry Tuesday 
night. The quantitative precipitation forecast fields have pretty decent amounts prognosticated in the 
mountains Monday...decreasing Monday night. There is little to no 
measurable precipitation prognosticated for the plains Monday...but a tad 
for Monday night into Tuesday morning. So for probability of precipitation...the 
highlights are out and we are expecting a healthy snowfall for 
the mountains. Widespread snow and some blowing snow too are 
expected in the mountains Monday into Monday evening before 
decreasing into Tuesday. Plains probability of precipitation will be 0-30%s Monday into 
midday Tuesday. Temperatures on Monday are prognosticated to be 6-10 c 
colder than today's highs. Tuesday is another 1-4 c cooler than 
Monday for highs. For the later days...Wednesday through Saturday 
..models have strong northerly flow aloft on Wednesday with upper 
ridging to move over the County Warning Area through Wednesday night. Then a 
broad upper trough moves in Thursday into Friday night. The flow 
aloft is west-northwesterly and moderate in speed. On Saturday 
strong north-northwesterly flow aloft is prognosticated. The European model (ecmwf) has 
better parameters overall for snow Thursday into Friday... 
especially in the mountains. 


Aviation...(for the tafs through 18z Monday morning) 
issued at 925 am MST sun Dec 21 2014 

VFR conditions will prevail for the morning and afternoon time 
periods today. Winds will increase this afternoon with possible 
gusts up to 25 miles per hour increasing around 03z Monday to 30 miles per hour. After 
03z ceilings may drop below 6000 feet with light snow possible. Snow 
is not expected to be heavy enough to reduce visibilities below a 
mile but some reduction in visible will be possibe around 09z when 
models indicate the heaviest snow could be. 


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

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



short term...gimmestad 
long term...rjk 

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