Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 052243
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
343 PM MST Mon Dec 5 2016

.SHORT TERM...(This evening through Tuesday)
Issued at 305 PM MST Mon Dec 5 2016

Strong downslope flow east of the Continental Divide will continue
this evening and overnight. Changes to our weather are already
underway as the initial cold front has moved south into Colorado
from Wyoming with very strong northerly winds impacting areas
east of I-25 across Weld, Logan, Washington, and Morgan Counties.
Winds are briefly high enough to warrant a high wind warning this
afternoon into the evening for a few hours. These winds should
weaken significantly after 8 PM. Behind the cold front very cold
air, the coldest of the season so far, will build in. Expect low
temperatures tonight in the 10-15 degree range east of the
foothills and below zero on the West Slope. Light snow showers
currently over the Continental Divide will diminish overnight as
slightly drier air impacts the northern half of the state.

On Tuesday the strong mid and upper level trough will impact
Colorado. This storm is cold enough that it will be all snow area-wide
from the onset of precipitation. Snow showers will redevelop
Tuesday midday across the higher mountains and western slopes
because of moderate synoptic- scale ascent coupled with westerly
upslope flow. Snow should hold off until Tuesday evening across
the I-25 urban corridor likely impacting the evening commute.
Temperatures will only reach the upper 20s across the Plains and
teen in the mountains. Winds will remain fairly light all areas
other than above 9000 feet on Tuesday given the approach of the
trough axis in the mid levels and a surface low pressure center
developing across far southern Colorado. Pressure falls with the
low are modest so 10-20 mph winds across the eastern Plains look
reasonable.

.LONG TERM...(Tuesday night through Monday)
Issued at 305 PM MST Mon Dec 5 2016

Cold air and snow will define the start of the long term.
For Tuesday night into Wednesday the upper level flow will
be zonal with a 95 kt jet over the state. A surface low will
deepen over the southern mountains helping to increase ESE
flow into the foothills. This flow combined with moderate
moisture and upward QG of -30 mb/hr will cause Tuesday night
into Wednesday to have the heaviest snowfall rates of the storm.
Model cross sections show increasing colder easterly winds into
the foothills that could set up a cold air damming scenario.
This would create lower pressure butted up at the base of the
foothills providing for winds to change to a more northerly
direction. The main impact of this would be ENE winds over
the Cheyenne ridge that could hinder some accumulations in
that area and bring localized higher amounts along the lower
foothills and Palmer Divide. The biggest player will be the
sfc to 700 mb level as at this point favorable easterly winds
do not deepen to 700 mb so it stays rather shallow. Have
2-5 inches for the Urban corridor with less eastward.
A winter weather advisory has been issued for the foothills
and plains for impacts related to snowfall starting around
rush hour on Tuesday with the heaviest arriving by Tuesday
evening. In the Mountains a winter storm watch will start
Tuesday morning and go through Wednesday morning for amounts
of 12 to 15 inches in an 18 hour period. Moderate to heavy snow
will hinder travel conditions in the mountains on Tuesday
evening through Wednesday morning.

The upper trough will push east on Wednesday with sfc flow turning
more northerly on the plains by Wednesday afternoon. Snow will
diminish by Wednesday late afternoon and evening. Highs on Wednesday
will stay in the single digits to teens across the region. Clearing
out Wednesday night under subsidence will allow for overnight
temperatures on Wednesday to drop below zero over the Denver area with
temperatures reaching to -20 in the mountains. Upper ridging will move
in Thursday but areas will take time to warm even under increasing high
pressure.

Flow aloft will become more zonal by the end of the week with a series
of shortwaves and continual push of moisture keeping some snow falling
in the mountains. Temperatures will warm back to seasonal normals by
the end of the week. The next major weather disturbance will arrive
between Friday and Saturday bringing a good shot of snow for the mountains
and possibly some for the plains. Stay tuned.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 305 PM MST Mon Dec 5 2016

No Visibility or Ceiling issues through 00Z Tuesday evening. Main
issue will be the winds. Strong northerly winds arrived as 2200Z
associated with the initial surge of cold air at KDEN. The
northerly winds are stronger than many of the high res models
estimated and are blowing to 50 kts across Weld County. Expect
very strong northerly winds . Winds will remain northerly at KDEN
overnight and weaken in speed by 06Z into the 10-12 kt range.
Winds will pick up out of the north at KAPA after 23Z but not be
as strong as at KDEN. Both KBJC and KAPA will have winds weaken
overnight.

Light snow should begin at the terminals around 00Z
Tuesday evening and increase in intensity into the late evening
and overnight hours with 3-6 inches of snow for KBJC and KAPA, 2-4
inches expected for KDEN through Wednesday midday. VIS and
ceilings along with temperatures in the low teens are in the
forecast after 00Z for the terminals.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 PM Tuesday to noon MST Wednesday
for COZ038>041.

Winter Weather Advisory from 11 AM Tuesday to 6 AM MST Wednesday
for COZ035-036.

Winter Storm Watch from Tuesday morning through late Tuesday
night for COZ031-033-034.

&&

$$

SHORT TERM...Schlatter
LONG TERM...Bowen
AVIATION...Schlatter


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