Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 210335
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
835 PM MST Mon Nov 20 2017

.UPDATE...
Issued at 824 PM MST Mon Nov 20 2017

Only minor adjustments planned for the grids in this update.
Bumped the pops up to 100 percent, over the high exposed west
facing mountain slopes and added areas of blowing snow above
timberline. Overall timing and potential snow amounts look good
with the highest amount along the higher west facing aspects.


&&

.SHORT TERM...(This afternoon through Tuesday)
Issued at 235 AM MST Mon Nov 20 2017

Significant mountain wave event ongoing this afternoon across
northeast Larimer and extreme northwest Weld Counties. Frequent
gusts over 70 mph in that small corridor north of Wellington along
I-25 and US-85. Just north of the border the winds are gusting
over 80 mph. Expect these winds to continue into early evening and
have thus extended the RFW though 7 PM this evening. RH values
started low and will remain low area-wide with the strong
downsloping east of the Divide.  The jet streak is approaching
Colorado in the northwest flow and the nose of the streak should
enter our state around midnight tonight. GOES-16 showing the jet
nicely and the models appear to have a good handle on it`s
evolution. Looks to be 150-160 kts in strength and it will impact
our weather through Tuesday with mountain snow and winds. Details
immediately below.

First, precipitation chances.  Moisture continues to be slow to
arrive, current thinking is late this evening upslope flow will
initiate the snow showers across the Park Range and the northern
Front Range. Then the left exit region of the jet streak impacts
the northern Mountains between midnight and 8 AM, allowing for
several inches of accumulating snow. Despite subsidence from the
departing jet midday, upslope flow with some moisture sticks
around until Tuesday evening. Total snow amounts for the Park
Range are expected to be 3-6 inches, Front Range Mountains along
the Continental Divide 2-5, and the I-70 corridor across Summit
County 1-4 inches. There is a slight chance of rain showers across
the eastern plains associated with the mesoscale lift from the
left exit region of the jet. Best chance will be sometime between
5 and 10 AM Tuesday.

Second impact are the winds. Brute force across the high mountains
with a 150 kt should result in gusts 70 mph or greater overnight
tonight. There is also decent stable layer that develops late
this evening that should force winds to the surface across the
east slope foothills and surrounding plains from Jefferson to
Boulder to Larimer County. Today`s mountain wave near the WY
border is impressive to say the least. Best chance for strong
winds will be after 9 PM through around 3-5 AM Tuesday morning.
The winds most likely won`t extend too far east across the Plains,
typical for these wind events. Wind gusts 50-60 mph are possible
along the favored locations from west of Fort Collins to Boulder
along CO-93 to Golden. The strong winds could extend east as far
as I-25 tonight. It will also be windy across northern Park
County tonight with brute force northwest flow there. As the jet
departs Tuesday morning the mountain wave should break down and
winds relax across the area.

During the day Tuesday the weak shortwave trough and jet move
quickly east and in their wake a cool front will move across the
Plains from the north. Wind shift to the north expected across
the Plains late morning, keeping high temperatures near 50 but
upper 40s far eastern Plains. The mountains will also have a brief
cooldown as 700 mb temps cool 4 degC compared to Monday,
resulting in highs in the 30s to low 40s.

.LONG TERM...(Tuesday night through Sunday)
Issued at 235 AM MST Mon Nov 20 2017

Tuesday night through Friday...a large upper level ridge of high
pressure will be centered just off the Baja Mexican Coast with a
moderate northwesterly flow aloft over Colorado. This pattern should
result in unseasonably warm temperatures and dry weather across all
of the CWA. The only exception may be isolated to scattered light
snow showers mainly over the high mountain peaks...Tuesday night
into Wednesday...due to lingering moisture and orographic
northwesterly flow. The Front Range Mountains and Foothills should
also see periods of breezy conditions through the period
especially on Friday as a 100 KT jet sags south over northern
Colorado and a mountain wave sets up to the lee of the Rockies.
The dry airmass over the region combined with the warm downslope
flow should result in temperatures around 70 on the plains
Thursday and Friday. The MEXDEN shows a high of 69 at DIA on
Thanksgiving Day while the ECMDEN has a high of 72 degrees. The
record high temperature on Thanksgiving Day is 73 degrees which
was set way back in 1909. Due to the forecasted pattern...it is
not out of the question that Denver could equal or exceed this
record. The two factors that will determine if this happens are
the strength of the downslope flow and the development and extent
of wave clouds through the day.

Models are hinting at a weak cold front moving across the
northeastern plains late Friday bringing cooler temperatures east of
the mountains on Saturday. Temperatures warm back into the 60s on
the plains on Sunday as the upper level ridge of high pressure
shifts east over the Rocky Mountain Region.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 824 PM MST Mon Nov 20 2017

VFR ceilings and visibilities over the next 24 hours. Will
continue with generally southwest winds and KDEN through 11z,
then weak east/southeasterly ahead of the approaching cold front.
then gusty northeast winds. Timing wise, 15z seems reasonable. At
KAPA a light south/southeasterly overnight. Gusty westerly winds,
around 35 kts, will persist at KBJC until 10-11z then decrease
and become northwesterly ahead of the approaching front. Post
frontal ceilings around 060 kft agl at KBJC 15-18z Tuesday
morning.


&&

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

$$

UPDATE...Cooper
SHORT TERM...Schlatter
LONG TERM...Kalina
AVIATION...Cooper


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