Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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458
FXUS65 KBOU 120300
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
800 PM MST Thu Jan 11 2018

.UPDATE...
Issued at 800 PM MST Thu Jan 11 2018

As the high clouds from the approaching system have taken
slightly longer than anticipated, temperatures over the eastern
plains are cooling faster than forecast. Have adjusted the sky
forecast as well as overnight lows. Snow is pushing into western
Wyoming and should arrive by the advisory begin time of 11pm into
our mountains. No changes needed for the advisory and amounts.


&&

.SHORT TERM...(This evening through Friday)
Issued at 329 PM MST Thu Jan 11 2018

An upper level trough moving onto the Pacific Northwest coast
will race southeast tonight and Friday in the strong northwest
flow aloft. Snow will increase in the mountains tonight as
moisture increases ahead of the trough. Ample moisture, moist
adiabatic lapse rates, and ridge top flow northwest at 30-50 knots
will lead to very good orographic lift. Issued a Winter Weather
Advisory for this. Snowfall amounts of 4 to 8 inches are expected.
However since orographic lift will produce most of the snowfall,
amounts will greatly vary with a few locations seeing up to a
foot. Blowing and drifting snow may become a problem at higher
exposed areas and through the mountain passes because of winds
gusting to 50 mph. Conditions will slowly improve in the mountains
Friday afternoon and more so Friday evening.

For the foothills east to the eastern plains, gusty north to
northwest winds will spread from the foothills tonight to the
eastern plains Friday morning. Wind prone areas will likely see
gusts to around 50 mph. This will help keep overnight lows mild in
and near the foothills. A surface low over eastern New Mexico
with an inverted surface trough extending north along the I-25
corridor will bring varying weather. To the east of the inverted
trough, north to northeast flow will usher cooler conditions. The
lack of downslope and lift from the trough should produce a little
light snow over the northeast corner Friday morning. Drier air
moves in early in the afternoon and is expected to bring any snow
to an end. Snowfall will be light and is expected to be less than
an inch. To the west of the inverted trough, gusty downslope winds
will produce mild and dry conditions through the day Friday.

.LONG TERM...(Friday night through Thursday)
Issued at 329 PM MST Thu Jan 11 2018

The ridge over the west will shift eastward a bit going into the
weekend. Saturday will still be cool behind Friday`s shortwave, then
Sunday will be warmer. There will likely be another batch of high
clouds moving over Saturday, then less clouds on Sunday. Only
minimal mountaintop clouds/flurries in this period.

Another shortwave will drop south from central Canada into the
central and eastern U.S. on Monday. There`s pretty good agreement
on timing, but the important detail of how far west the cold air
digs still varies. Most models keep the deep cold east of
Colorado, though we would get a shallow front with some cooling.
There are some model solutions that show this a bit stronger and
bring a more solid shot of cold air and a little ribbon of snow
across eastern Colorado on Monday. A few runs have this further
east with little impact. Temperature guidance based on the EC
ensemble for Monday varies from the 20s to the 50s. GFS has
similar spreads. Blended solution actually looks like most of the
ensemble runs with 10-15 degrees of cooling and a slight chance of
snow across the northeast plains.

There is agreement in the ridge nudging eastward Tuesday with the
cold air retreating, though how much depends on how strong a push
we get in the first place. For Wednesday and Thursday the model
agreement falls apart as energy coming out of the Pacific jet
moves into the ridge. Many model runs show a weak to moderate
strength shortwave moving through the ridge which would likely
just be brief mountain snow without a lot of cooling. There are
several solutions with a much stronger system, closing off a low
over the southwest. This would threaten a wetter system, but would
also be slower, affecting us Thursday or Friday or possibly
dropping south of us altogether. And a few runs split this energy
enough to have little impact. We may not have a good handle on
this until we`re sampling the responsible jet streak a couple days
in advance. For now moderate temperatures with low PoPs in the
mountains is appropriate.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday afternoon)
Issued at 453 PM MST Thu Jan 11 2018

VFR conditions are expected through Friday at the Denver
airports. Gusty west winds at KBJC are decreasing, but should
reform 09-12z tonight and slowly turn northwesterly by 18z. The
west winds that reached KDEN should be decreasing as well over the
next hour or two. Winds eventually settle at a southerly
direction at KDEN and KAPA. Northwest winds with gusts to 30
knots are expected to develop at KDEN and KAPA around or just
after 15Z.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 11 PM this evening to 5 PM MST
Friday for COZ031-033-034.

&&

$$

UPDATE...Kriederman
SHORT TERM...Meier
LONG TERM...Gimmestad
AVIATION...Kriederman



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