Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 071741
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1041 AM MST Wed Dec 7 2016

.UPDATE...
Issued at 1041 AM MST Wed Dec 7 2016

The last moderate snow band has moved from north to south across
the I-25 corridor earlier this morning and had pushed into El
Paso and Elbert Counties at around 9:30 AM local time. Very light
snow resulting from lingering shallow upslope flow will likely
continue for a few more hours before ending completely by early-
afternoon. No additional accumulation is expected across the
Plains. In the mountains very light snow will also continue
through midday for primarily the Continental Divide across the
Medicine Bow Range and Rocky Mountain National Park. Very little
additional measurable snow is expected there. Skies will start to
clear by early evening across central and northern Colorado as the
trough axis move further east into the Central Plains and deep
subsidence spreads across the state.

Only minor changes to the forecast for today were needed for the
hourly temperature grids to reflect slightly falling temperatures
throughout the day across the Plains. The most recent surge of
arctic air dropped temperatures this morning into the single
digits most areas across the I-25 corridor, and 10-12 degrees
further east out to the Kansas and Nebraska borders. Temperatures
are not expected to warm at all today east of the foothills with
the low level cold advection and now the temperature grids
reflect this trend.

&&

.SHORT TERM...(Today and tonight)
Issued at 417 AM MST Wed Dec 7 2016

Weakening dynamics associated with the upper level trough over the
state is evident on satellite imagery. Drier air briefly shut off
snow across most of the area under the weakening dynamics as
well. A northerly surge of colder air has spread south along the
northern urban corridor and will increase snowfall through the
morning commute. Though with only lower level forcing, not
expecting much more snow accumulation across the area. A trace to
an inch over the plains, with 1 to 2 more inches across the
foothills. Colder temperatures are helping dendrites form which
may help accumulation. Downward QG forcing will be pushing in
later this morning to diminish snowfall. Cold temperatures and low
level upslope flow will likely allow for areas of light snow to
continue into the early afternoon however before diminishing. Snow
over the mountains is already diminishing but some impacts still
expected through the morning rush and around another inch in
accumulation. Will allow advisories to continue, though the
mountains will likely be able to be expired early.

Cold temperatures today, with readings in the low teens over the
plains and in the single digits over the mountains, will combine
with winds to keep windchill temperatures below zero all day.
Clearing skies tonight will allow temperatures to drop below zero
for lows. Besides some wind over the high mountain passes up to 30
mph, winds will be light across the area, but enough to bring
very cold windchill temperatures overnight. Readings will be 10 to
20 below zero over the plains and 20 to 35 below zero over the
mountains.

.LONG TERM...(Thursday through Tuesday)
Issued at 417 AM MST Wed Dec 7 2016

The 7 days ahead will feature strong west/northwest flow aloft
and periods of accumulating mountain snowfall.

For Thursday, it will be our one of our few/only totally dry days
in the next 7 as weak ridging and warm advection occurs aloft. It
will remain quite cold across the plains however with lack of lee
troughing and some light snow cover in most spots. Highs should
only be able to push to near 20F, with potential for a little
warmer than that right next to the Front Range/Palmer Divide.

Snow is expected to move into the mountains Thursday evening as
the next plume of Pacific moisture streams into the state. Warm
advection usually results in models overdoing moisture, but
instability is abnormally high in this particular warm advection
pattern so will have some high PoPs and light to moderate
accumulations in the mountains. Zone 31 appears best favored for
highlights (an advisory) for Thursday night through Friday given
the more westerly flow and better orographic component there.

By Friday, lee trough strengthens which should allow warmer
westerly flow to develop across the plains. Have increased temps a
few degrees for this but potential for even warmer readings
through Saturday depending on how fast low level cold air gets
scoured out. Winds are also expected to increase as the cold air
scours to the east. Despite the strength of winds with 40-50 knot
cross mountain flow, mountain wave amplification appears to be
limited by lack of a mountain top stable layer (and orographic
snowfall), so threat of high winds or warning criteria during
this period seems low in all but the highest mountain tops.

Saturday will likely see strong winds and heavier snowfall in the
mountains. This appears to be a higher impact event as an embedded
short wave, deep and rich Pacific moisture plume, instability,
and cold advection all join forces. Gusty downslope winds will
likely spill onto the plains but could see anticyclonic shift with
another brush of arctic air/surge from the northeast late in the
day or evening so have included some PoPs on the plains as well.

For Sunday through Tuesday, while the overall synoptic pattern is
well agreed upon, there are a lot of variations amongst the
models in the low level surges of arctic air across the plains.
This could lead to large temperature variations over short time
frames, so for now had to play middle of the road scenarios as
timing of these fronts is very difficult for now. I do think we`ll
have a pretty good chance of an arctic air resurgence toward
Monday/Tuesday so have nudged down those temps a few degrees from
model averages. Mountains should remain unsettled with the
West/northwest flow aloft resulting in more orographic snow to
reduce our early season snowpack deficit.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1041 AM MST Wed Dec 7 2016

Very light snow ongoing at DEN, BJC, and APA will continue for a
a few more hours with no additional accumulating snow expected.
Visibility and Ceilings will continue to improve between now and
19Z. Winds will remain north-northeast for the remainder of the
afternoon but speeds will be on a decreasing trend. By late
afternoon speeds should be less than 10 kt all terminals, and then
between after 02Z they should be around 5 kts (or less) and turn
generally out of the south. On Thursday skies will be mostly sunny
with VFR conditions and weak winds at the surface much of the
day, less than 10 kt all terminals as the ridge of high pressure
builds across the Plains of Colorado. Temperatures will remain
very cold at the terminals, struggling to reach 20 degrees.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MST today for COZ031-033>036-
038>041.

&&

$$

UPDATE...Schlatter
SHORT TERM...Kriederman
LONG TERM...Barjenbruch
AVIATION...Schlatter


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