Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 081837
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1137 AM MST Thu Dec 8 2016

.UPDATE...
Issued at 1122 AM MST Thu Dec 8 2016

Satellite pictures are showing snow on the ground over nearly all
of the CWA. The Arctic airmass is fairly shallow but it will not
mix out today. Highs should not make 20 degrees anywhere in the
CWA this afternoon. Upper level cloudiness is moving in from the
northwest at this time. There is no snow falling in our mountains
yet, but that should change as the afternoon wears on.

&&

.SHORT TERM...(Today and tonight)
Issued at 454 AM MST Thu Dec 8 2016

Arctic cold is fully entrenched across the forecast area this
morning with almost all locations below zero. There was just a
narrow band of low clouds still keeping temps up above zero
in the Boulder county area and southern Lincoln county. Meanwhile
a couple spots near Fraser and Tabernash plunged to -30F or
colder!

The cold will remain in place across the plains through tonight
with arctic high pressure only slowly retreating to the east.
Mountain areas will be cold with increasing winds later today.
Then warm advection will likely bring warming temps overnight to
the high country but also a return of snow. A few showers may
begin to develop as early as late afternoon, with more widespread
snow expected to reach the mountains this evening as pacific
moisture plume moves in. Despite warm advection, lapse rates
gradually improve to greater than 5 C/km with further improvement
into Friday morning. Winds also turn more westerly so even the
I-70 corridor of the mountains should be impacted by accumulating
snow. Amounts might be marginal for advisories there, but
considering strength of winds with gusts 60 mph or more over the
higher passes feel an advisory is warranted for all of the
mountains. Highest totals in this pattern certainly shaping up for
the mountains north of Rabbit Ears Pass.

High temperatures on the plains today will struggle to push
through the teens with only a couple spots from downtown Denver
south possibly reaching 20F. High mountain valleys are tough with
an extremely cold start and strong inversions, but should be some
moderation late as clouds move in. The clouds will also make
overnight lows difficult to forecast tonight as some areas across
the higher elevations will begin warm air in earnest, while plains
locations remain stuck in the arctic air but increasing clouds
should result in some moderation.

.LONG TERM...(Friday through Wednesday)
Issued at 454 AM MST Thu Dec 8 2016

The active period of weather will continue into the medium range
periods of the forecast. Moist and fast westerly flow will
continue through Friday and into Saturday, generating more
orographic snow in the mountains. Another 4 to 8 inches of snow
will be possible by Friday evening. Blowing snow will also be
possible over the higher peaks. Will continue the Winter Weather
Advisory into early Friday morning, but it may need to be extended
if the airmass remains moist enough.  The core of the upper level
jet will be centered over Wyoming, placing Colorado under the
right exit region which is normally subsident. However, the moist
nature of the flow pattern will interfere with the development of
an amplified mountain wave. Therefore, gusty winds should remain
over the ridges and high foothills. Cold air lingering at the
surface will also keep gusty winds from moving further down the
foothills. The westerly flow aloft will also result in dry
conditions over the northeast Colorado plains. And that is just
Friday and Friday night.

For Saturday, snow will continue in the mountains as the moist
westerly flow will continue. Cross sections still show a moist
airmass up through 500 mb with drying downstream of the mountains.
The continuing westerly flow will bring gradual warming along the
i-25 corridor and eventually out onto the plains. Warming should
then continue through Sunday and into Monday.

For Tuesday, there are differences between the GFS and ECMWF
models related to how strong the next surge of Arctic air is going
to be. The GFS run is much stronger, bringing very cold 850
temperatures all across Colorado. The ECMWF is more restrained and
keeps the Arctic air over the eastern half of the state. All of
the low level cold advection is going to take place under
continuing westerly flow aloft. The MOS guidance from the GFS
shows high temperatures Tuesday some 25 degrees colder than
previous MOS guidance values. Until this scenario is confirmed by
later runs, will stick closer to the ECMWF solutions. The chances
for snow could continue in the mountains through next Wednesday,
and the latter half of next week looks like yet another trough
embedded in the westerly flow aloft moving over the state. It
looks like the mountain snowpack will be in good shape for the
near future. The plains are going to need to wait for the flow
pattern to become a little more meridional.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1122 AM MST Thu Dec 8 2016

With a snow field in place and a weak low level pressure
gradient too, winds will stay light and mostly easterly this
afternoon. Tonight`s drainage winds may be a little east of due
south with the snow field in place. There should be no ceiling
issues.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 7 PM this evening to 5 PM MST
Friday for COZ031-033-034.

&&

$$

UPDATE...RJK
SHORT TERM...Barjenbruch
LONG TERM...Dankers
AVIATION...RJK



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