Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 212213
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
313 PM MST Tue Nov 21 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 313 PM MST Tue Nov 21 2017

Cloud layer is getting shallow over the mountains, with just some
very light showers remaining. Likewise the clouds east of the
mountains are eroding steadily. Northerly jet is passing over with
the accompanying subsidence inversion that will lower tonight.
This will create a favorable temperature and wind profile for a
mountain wave overnight. Only issue is that at mountaintop level
the flow is from a more northerly direction than is ideal for the
Front Range. This may limit the extent of any strong wave
amplification. The exception is areas prone to NNW mountain waves
in the higher elevations of Clear Creek county and especially
along US 285 in Park county. Winds there could approach high wind
criteria of 75 mph as the ideal conditions pass over during the
night. Across the rest of the higher Front Range elevations 40-60
mph winds should be more common. By Wednesday morning the warmer
air will have taken over with a steadier northwest wind. There
shouldn`t be much amplification, but 30-50 mph winds could become
more widespread with a more general downsloping. Some of this wind
should trickle into the lower foothills in the morning, and then
spread more generally onto the plains in the afternoon but with
lesser speeds than in the mountains. The mixing will offset the
likely mountain wave cloudiness that may be present much of the
time. I raised highs in the mountains under this pattern, and
lowered them just a little on the plains.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 313 PM MST Tue Nov 21 2017

Wednesday night dry Northwest flow continues with the center of
the mid and upper level ridge across Baja California. The models
don`t break down this ridge until late Monday. In generally the
weather will be dry and unseasonably warm area-wide until late
Monday. Details below.

Thanksgiving still looks like it could be a top 2 warmest
Thanksgiving Day, and close to the warmest November 23 in
Denver`s history. 700 mb flow is west across the Front Range on
Thursday, and temperatures warm 2 degC at 700 mb over Wednesday.
The thermal ridge is centered just lee of the Rockies with the
downslope flow, resulting in a local max in temperatures across
metro Denver. Warmest Thanksgiving Day is 73, while the warmest
November 23 is 74 degrees. It should warm to those levels across
metro Denver, with a chance of exceeding them. Elsewhere across
the Plains temps will be around 70 degrees as well, with mostly
50s in the mountains.

On Friday a clipper system moves across the northern Rockies and
cold advection aloft brushes the northern half of the state. There
will also be a surface cool front from the north moving across
the Plains around midday. Ahead of the clipper before the cool
front it looks really windy across the mountains and east slope
foothills pre-dawn Friday morning. Brute force with some possible
mountain wave amplification could bring the winds into the
adjacent Plains but right now the mountain top lapse rates look
marginal. Regardless, the favored foothill locations and higher
mountains will likely be blowing 50 mph with gusts to 70 mph
possible.  By afternoon mixing and the approaching surface high
pressure should result in gusty winds across the eastern Plains,
20-25 gusting as high as 35 mph. Doubt enough moisture will be
advected by the clipper so do not have any significant PoPs in the
mountains for this time. The cold advection and cool front only
marginally impacts highs across the Plains, ~5 degF cooler area-
wide than on Thanksgiving.

Saturday through Monday look warm and dry area-wide under the
influence of the stubborn ridge and dry northwest flow. By Sunday
the ridge axis is overhead, and on Monday a transition to
southwest flow as the ridge flattens and moves east ahead of a
strong long-wave trough moving across the Pacific Northwest U.S.
Temperatures will be cooler on Saturday but still far above
normal, because weak surface winds with a surface high just east
of CO mean an inversion will be hard to break with late November
sun angle and weak diurnal upslope flow...likely holding high
temps around 60 deg. Mountains remain in the upper 40s to mid 50s.
Sunday and Monday lee troughing and the return of some downslope
will warm temps near or slightly above 70 both days across the
Plains, with again upper 40s to mid 50s in the high country.

Models are advertising the start of a pattern change starting
late Monday with a trough digging southeast towards Colorado with
a decent southwest jet downstream of the trough. Mountain snow
chances increase late Monday into Tuesday, but none of the models
are bullish on available moisture ATTM.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 313 PM MST Tue Nov 21 2017

Remaining clouds near the mountains should dissipate by around
00z, with VFR conditions through tonight and Wednesday. A mountain
wave could create areas of LLWS over and near the foothills after
06z, with west winds spreading off the mountains to KBJC after 12z
and to KDEN/KAPA by afternoon. There is a chance of west winds
gusting to 25 knots at KDEN for a few hours during the middle of
the day, but the winds should become more northwesterly in the
afternoon.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Gimmestad
LONG TERM...Schlatter
AVIATION...Gimmestad



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