Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 100440
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
940 PM MST Fri Dec 9 2016

.UPDATE...
Issued at 940 PM MST Fri Dec 9 2016

A weak cyclonic undulation in the northwest flow aloft is
responsible for the mid-level cloudiness over northeast colorado
this evening. more moisture immediately upstream of the
Continental Divide will probably delay clearing east of the mtns
until after midnight. Meanwhile mountain areas along and west of
the Divide may also see a modest uptick in snowfall over the next
few hours. New snow accumulations should be on the low
side...generally 1-3 inch range mainly on upper northwest slopes.
Also counting on an increase in winds over higher elevations
during the next several hours with the nose of a 120kt jet diving
down across Wyoming. Have updated the overnight sky grids to
account for greater cloud cover as referred to above. Also made
minor adjustments to winds mainly at higher elevations and to
nudge up overnight low temps a deg or two along the Front Range.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 335 PM MST Fri Dec 9 2016

Downsloping over much of the metro area has allowed temperatures to
get into the lower 50s this afternoon.  Dia`s high temperature so far
today has come up 51 degrees F from the morning low of 3 F. Radar
pictures continue to show snow along the divide at this time. Models
keep 60-90 knot northwesterly flow aloft over the CWA tonight and
Saturday. The QG Omega fields have benign synoptic scale energy
tonight, with upward motion progged Saturday. The low level winds
should adhere to pretty normal drainage patterns overnight, with a
downslope component to the surface winds on Saturday. For
moisture, all the models have it decreasing fairly good overnight
into Saturday morning, and the satellite pictures agree with that.
Moisture increases again on Saturday afternoon. The QPF fields
have a tad of measurable snow over the northwestern CWA this
evening, then nothing overnight and Saturday morning. QPF returns
to the mountains after 18Z Saturday afternoon. For pops, will
diminish them significantly this evening in the mountains and keep
0-20%s going along the divide overnight. By Saturday, will
increasing them again by late morning, with near 100%s in the high
mountains after 21Z Saturday afternoon. By later Saturday
afternoon, the moisture is deep, there is synoptic scale energy
and decent orographic enhancement in the high mountain zones. A
watch will be needed. For temperatures, Saturday highs are 1-4 C
warmer than this afternoon`s.

.LONG TERM...(Saturday night through Friday)
Issued at 335 PM MST Fri Dec 9 2016

A strong and moist zonal flow aloft over Colorado will continue
through much of the period with the potential for heavy snow in
the mountains. The plains should see colder temperatures and some
snow by the middle of next week.

Saturday night into Sunday  morning...strong 120KT+ jet sags
south into northern Colorado. A deep and moist westerly
flow...combined with 40 to 50 kt winds at mountain top level and
QG ascent from the jet should produce quite a bit of snow and wind
in the mountains. The orographic snow model shows 8 to 14 inches
of snow across the high country with up to 2 feet in zone 31.
Therefore...have issued a Winter Storm Watch from late Saturday
afternoon through noon Sunday. The combination of wind and
snow...should result in quite a bit of blowing and drifting snow
as well with low visibilities at times...especially above
timberline. The foothills could see some snow and wind with up to
7 inches possible across the higher foothills. Isolated to
scattered light snow showers will also be possible across the
adjacent plains and Palmer Divide associated with a weak cold
front and weak upslope flow.

Snow intensity and wind should decrease  Sunday afternoon as the
moisture becomes more shallow and some subsidence moves into the
area. A strong west to northwesterly flow aloft will continue
through Friday...with a good chance of snow in the mountains each
day. Models diverge somewhat by Wednesday...with the position of
the upper jet. The GFS has a 170 kt jet over central Colorado...
while the ECMWF has the strong upper jet over northern Wyoming. If
the GFS verifies...the mountains could see another major storm
Tuesday afternoon into Wednesday...with snow and colder
temperatures on the plains. However...the ECMWF run would result
in lighter snowfall in the mountains and dry conditions on the
plains.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 940 PM MST Fri Dec 9 2016

The cloud deck up around 9000-12000 ft agl may hang over the
greater Denver metro area a while longer with mid-level moisture
continuing to spill over the Front Range. Do not see any
precipitation from these clouds. This mid-level cloudiness is
expected to thin out towards morning with the formation of
leeslope mtn wave. Therefore anticipate IFR ceilings and
visibilities next 24 hours. Meanwhile...the light and variable
winds in the Denver metro area at the present time are still
projected to gradually become south-southwesterly (the prevailing
nocturnal drainage flow around here) within the next hour or so.
Already see signs of this happening of the south side of the metro
area. Overall speeds for the remainder of the night should remain
at or below 12 kts.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday afternoon through Sunday
morning for COZ031-033-034.

&&

$$

UPDATE...Baker
SHORT TERM...RJK
LONG TERM...Kalina
AVIATION...Baker



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