Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 120319
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
819 PM MST Tue Dec 11 2018

.UPDATE...
Issued at 759 PM MST Tue Dec 11 2018

Main concern over next 24-36 hours revolve around potential for
high winds and snow in the mountains. Initially for tonight,
Colorado remains under a very dry and moderate northwest flow
aloft. The next incoming trof just coming onshore along the
Pacific Northwest and will approach Colorado later tonight and
Wednesday. Ahead of the trof, the mountain top flow will increase
and shift more westerly and set up some mountain wave activity
later tonight and Wednesday morning. Cross barrier flow increases
to around 50-60kt by 12z with some mountain top stability
developing. However, there is neutral to weak upward ascent noted
on QG fields so may hard to transfer the strongest winds down the
foothills. Strongest winds likely to remain over mountain passes
and exposed east slopes. Current forecasts showing only 40kt so
over winds in the mountains look underdone and will increase in
forecast through Wednesday morning. Expect snow showers to
increase in the mountains during the afternoon with increasing
orographic flow, QG ascent and lapse rates. Expect a quick burst
of a few hours of moderate/to heavy snow which could cause some
travel issues in the higher mountains.

Stronger winds will also develop over the plains with cold frontal
passage in the late afternoon and evening but moisture appears to
be limited with the strong downsloping flow behind the front.
Again, main changes will be to beef up the winds a bit.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 336 PM MST Tue Dec 11 2018

Today`s gusty winds have been brought to the surface by subsidence
associated with a trough axis. This evening, winds will decrease
across the plains, but gusts of 50 to 60 mph will continue over
the peaks and high passes through Wednesday afternoon. Winds will
increase over the plains again late Wednesday afternoon as a cold
front pushes across the northeast plains.

The next disturbance will progress from the Pacific Northwest and over
the Columbia River Basin, reaching Colorado by late afternoon
Wednesday. Snow will move into the mountains Wednesday afternoon.
Small accumulations are expected with up to 4 inches. This snow
will continue into the evening. Plains will remain dry through
the afternoon due to strong downsloping.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 336 PM MST Tue Dec 11 2018

Models are now in better agreement with the upper air features and
surface frontal system propagating across Colorado Wednesday night.
The surface portion of this system in the potential for
high winds and snow in the mountains. Initially form of a well defined
cold front blows southeast through northeast Colorado during the
late afternoon/early evening hours. The strong CAA, stiff northwest
flow and a narrow band of moderate QG ascent with passage of the 700-
500 mb trough axis are expected to produce moderate snowfall across
the high country during the evening hours. Travel could become a bit
nasty on the higher mtn passes. Right now, believe we could see
marginal winter advisory criteria snow and blowing snow for a few
hours on the higher passes, however for now will hold off on
hoisting one. Spatial cross-sections show the moisture waning and
strength of the synoptic scale forcing decreasing over the mountains
after midnight. Expect total snow amounts of 2-6 inches with the
higher amounts on orographically favored west-northwest facing
slopes.

Across the northeast plains, cold air advection kicks in with
frontal passage, and strong momentum aloft transports downward on
the back side of the passing trough resulting in gusty north-
northwest winds overnight. Strongest winds are likely to remain east
of the I-25 corridor where gusts of 25-35 mph may persist for much
of the night. However, the strongest winds will concentrate along
the Palmer Divide in Elbert and Lincoln Counties where gusts to
around 45 mph will be possible. The stiff northerly winds in
combination with spotty light to moderate snow showers during the
overnight hours could generate some light snow accumulation and
blowing snow, but not enough to warrant any winter weather or high
wind high wind highlights. In and near the Front Range Foothills,
will stick with 10-20 percent PoPs Wednesday evening.

The trough races east of the state Thursday morning as a strong
upper-level high pressure builds over Colorado for Friday.
Thursday temperatures will be cooler, but still around average
levels for the date. Temperatures rebound several degrees on
Friday with ample sunshine and subsidence aloft. Friday night into
Saturday morning the ridge exits the area allowing a weak mid-
level shortwave trough to race across the state possibly
generating a few snow showers in the high country and gusty winds
up along the Continental Divide and Front range foothills. System
should have little impact on the plains, if only to warm
temperatures a degree or two with a warming and drying downslope
flow.

For Sunday into early next week, another upper ridge builds over the
region keeping things dry and temperatures several degrees above
average. Finally, models throw a monkey wrench in the fray with some
showing a moisture laden Pacific storm system spinning up over the
Desert Southwest and spreading moisture and precip up over Colorado
on Tuesday. While, other models hang onto potential for high
winds and snow in the mountains. With low confidence in the
models out that far, will stay the course with dry and mild
conditions through Tuesday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 759 PM MST Tue Dec 11 2018

Surface winds shifting to drainage tonight with mainly clear
skies. Some increase in high level cloudiness later tonight with
development of some mountain wave cloudiness. Increasing northwest
winds later Wednesday afternoon with frontal passage and upper
trof.

&&

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

$$

UPDATE...Entrekin
SHORT TERM...Sullivan
LONG TERM...Baker
AVIATION...Entrekin


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