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fxus65 kbou 130330 

Area forecast discussion
National Weather Service Denver/Boulder Colorado
830 PM MST Thu Dec 12 2019

issued at 709 PM MST Thu Dec 12 2019

Radar, satellite pictures and web cameras show decent snow
falling in the mountains at this time. Will update some sky cover
grids. I upped the pops to over 85-100%s in the high mountains
with real data and the warning in place in mind. Some of the latest
model data indicates that a 170-180 knot northwesterly jet will
be over the western County Warning Area Friday morning at 12z. This evening's 00z
soundings, notably Medford, Oregon had some measured 160 knot
winds at 300 mb and just above. Its a strong jet heading this
way. Will leave the highlights as is for now.


Short term...(this evening through friday)
issued at 218 PM MST Thu Dec 12 2019

We have a weak shortwave ridge the next few hours for a relative
lull in the mountain snow. Another shortwave in the Salt Lake City
area at this hour will move over the mountains by mid to late
evening. The qg lift is weak, but there's great instability and
the moisture depth and wind both increase. We expect several hours
of heavy showers followed by a steady moderate snow in areas that
are favored in westerly flow. This is mainly the mountains north
of Berthoud Pass and to a lesser extent Vail Pass. Other areas
will still get significant snow though as a steady light snow
should be widespread. For the valleys it will be more about the
showers during the night with pretty light snow after that.
Ridgetop winds should increase to 40-60 mph creating near blizzard
conditions in exposed areas. On Friday the winds will continue
with some shallowing of the moisture and stabilization. Steady light
snow will continue Friday across the mountains, but favored areas
could still have a steady moderate to heavy orographic snow. There
should be some increase again late in the day, especially around
Rabbit Ears Pass.

For the plains, westerly winds will increase above the inversion
affecting the ridges and areas near the foothills. The inversion
over the cold pool northeast of Denver will be weakening, and some
areas may have lows in the middle of the night, then warming.
Clouds should increase again which will also help keep
temperatures up a bit. There may be a few light rain or snow
showers that make it to the base of the foothills, but any amount
should be minimal. On Friday, west to northwest winds should
surface although areas from about Denver southward may be
sheltered. Cold advection behind tonight's trough (a developing
cold front) should offset any gain from the mixing and highs will
wind up being a little colder, except for the Greeley area which
will be the same as everywhere else for the first time in a couple
of weeks.

Long term...(friday night through thursday)
issued at 218 PM MST Thu Dec 12 2019

Impacts will continue over the higher terrain through the weekend
with snow and winds creating hazardous travel conditions.

A shortwave trough will be pushing through central Colorado Friday
evening bringing continued increased levels of moisture streaming
into the region. With the expected 115 to 130 kt upper level jet
snow intensity should pick Friday evening and continue through the
overnight hours with models showing an increase in jet speed up to
150 kts over the central and southern mountains. The overall pattern
of the trough and shortwave downstream with predominately west-northwest upper
flow will help with snowfall amounts staying in the 1 to 2 inch per
hour range across the mountains through Saturday. Models are hinting
at some snow across the northern Colorado border of Wyoming as winds
wrapping around a surface low butted up against the southern
foothills helps to create some upslope conditions along the Cheyenne
Ridge Saturday afternoon. By Saturday evening the upper jet pushes
southern more along the central and southern mountains. This will
help to decrease snow intensity mainly over the northern mountains
by the overnight hours. The surface low that was against the
foothills will transition southward pulling in colder northern
winds onto the eastern plains.

Overnight into Sunday a trough will start to push onshore in
California bringing zonal flow across the region. The jet will
stretch from Utah into Kansas and then switch to a more southerly
Route as the trough continues to move eastward on Sunday. This will
bring the higher intensity snow across the southern and central
mountains and portions of southern Colorado. Models continue to show
some differences in the exact placement of the upper jet which could
change where the higher intensity snow falls. This will also
determine where the bigger impacts will ultimately be. However,
there is consensus of the trough deepening over The Four Corners
region by mid-day Sunday pushing upper level flow more southwesterly
which does favor the southern mountains. As the trough continues to
move ENE, it will produce additional snowfall mainly for the central
and southern mountains and especially west-facing slopes. By the
time the snow comes to an end early Monday there will be significant
snowfall amounts across the higher elevations with west-facing
slopes of the central mountains seeing possibly up to 2 feet by
Monday and 1 to 2 feet over the higher elevations of the northern
and central mountains. With regards to highlights, warning will stay
in place through 5 PM Friday with the watch picking up after the
warning expires and continuing into Sunday morning. Will hold off on
any upgrades for now to monitor a few more models runs to allow for
better consensus on the placement of the upper jet for the weekend.

At lower elevations, the west to northwest downslope flow will allow
for the plains to stay dry through Saturday morning. There is a
slight chance of snow across the eastern plains Saturday depending
on what model you look at. Will keep a slight chance in the forecast
for now just with the uncertainty. Either way, amounts should be on
the lighter side for the system on Saturday. The next system
however, that is impacting the state Sunday will have much better qg
across the entire state with slightly better upslope over the plains
that could result in better snowfall amounts for Denver southward.
Confidence continues to be low with the overall track and intensity
of this system so will continue to monitor over the next few
forecast periods. Temperatures will reflect the pattern with 30s
possible Saturday and below freezing on Sunday and Monday.

A pattern shift for the start of the week will allow for drying and
warming temperatures through Wednesday.


Aviation...(for the 00z tafs through 00z Friday evening)
issued at 709 PM MST Thu Dec 12 2019

Some of the models have the decent west-northwesterlies winds going
at dia until about 09z, other show more normal drainage patterns
by this time now. Will lean towards the west-northwest winds until after
06z in the taf. There will be no ceiling issues at dia.


Bou watches/warnings/advisories...
Winter Storm Warning until 5 PM MST Friday for coz031-033-034.

Winter Storm Watch from Friday evening through late Saturday
night for coz031-033-034.



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