Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 190229
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
729 PM MST Sat Nov 18 2017

.UPDATE...
Issued at 705 PM MST Sat Nov 18 2017

No significant changes planned this evening. Only adjustment
involved increasing the wind speeds in and near the Front Range
Foothills Sunday night, where the NAM12 reflects a fairly decent
mountain wave ahead of an approaching jet on Monday. As a result,
I increased the winds a bit more in the mountains and higher
foothills at that time.


&&

.SHORT TERM...(This evening through Sunday)
Issued at 136 PM MST Sat Nov 18 2017

Benign weather and seasonable temperatures forecast tonight and
Sunday. 500mb ridge axis centered over the Pacific coast states at
00z tonight will shift east and align over the Rockies by this
time Sunday, although upper flow flattens out when a weak vort max
moves across Idaho/Montana on Sunday. Model soundings showing an
increase in moisture above 500mb on northwest flow after mid day
Sunday, and will result in a scattered to broken layer of cirrus.
At the surface high pressure centered over the intermountain
west, while a lee trough will set up on the Colorado plains.
Generally west wind in the mountains, west to southwest in the
foothills to I25, and southerly surface wind on the plains.
Temperatures driven by radiational cooling tonight and solar
warming on Sunday, and will be very close to seasonal normals.
Lows in the teens in the mountains and low to mid 20s on the
plains are right on the normal mark. Sunday max temps will be near
to just a few degrees above normal, with highs in the 40s in the
mountains and mid to upper 50s on the plains.

Northeast Colorado seeing wind 20-25 mph gusting over 30 mph in area
northeast of Fort Morgan, and higher mountain elevations in similar
wind speeds. Wind to subside this evening as daytime mixing
decreases and high pressure builds in. Previous forecast had patchy
blowing snow in windier areas of high terrain, and continued that
the rest of the afternoon until wind subsides.

.LONG TERM...(Sunday night through Friday night)
Issued at 136 PM MST Sat Nov 18 2017

Northwest flow continues Sunday night into Monday, and increases
throughout the day as the nose of another strong Pacific jet
comes onshore across Oregon and then moves into Colorado late
Monday. Ahead of the jet a weak short wave trough in the
northwest flow looks to impact our area during the day, resulting
in increased winds area-wide. Strong mid-level subsidence on the
backside of the trough should aid in transporting strong winds
(45-50 kts at 700 mb) to the surface most areas Monday afternoon
and evening. Neutral temperature advection at all levels mean
temps will be similar to Sunday, close to 60 Plains and 40s in the
mountains.

The 120-136 kt jet stream out of the northwest translates across
the state late Monday and throughout much of Tuesday, carving out
a long wave trough across the Mississippi Valley. Synoptic scale
lift is negligible, remaining far east of our area. The available
sources of lift are from the left exit region of the jet and
Northwest upslope flow. Best chance of precipitation will be
confined to the west slope mountains (though still pretty low),
and should begin late Monday evening into Tuesday as the core of
the jet approaches northern Colorado, which is coincident with the
arrival of some moisture in the northwest flow. Models differ on
amount of moisture available but they all agree the QPF will be
light. Most of the snow will be confined to the northern
mountains, along and north of I-70. Despite the strong northwest
flow, neutral temperature advection occurs throughout Tuesday
with 700 mb temps remaining between 0 and -4 degC during the best
chances of precipitation, though still supporting snow everywhere
in our area west of the Continental Divide. The jet departs late
on Tuesday, and then just light upslope snow showers could
continue into early Wednesday. The GFS advects more moisture into
the northern mountains Wednesday morning than all the other
models. Given a lack of synoptic lift and the departure of the
jet, feel precipitation should shut off everywhere but maybe the
Park Range by late Tuesday. Light snow may continue across the
Park Range through midday Wednesday.

With strong northwest downslope flow Tuesday through Wednesday,
everywhere below ~9000 feet east of the Continental Divide should
continue to be dry. Temperatures briefly drop into the mid to
upper 50s across the Plains on Tuesday with a weak cold front and
out of the northwest and more cloud cover, but rebound into the
mid 60s on Wednesday with the departure of the jet and a
strengthening ridge aloft. Like Monday, Tuesday`s winds will be
strong (but not as strong as Monday) coming off the Cheyenne Ridge
and along and west of I-25 into the Foothills. Winds will
decrease area-wide Tuesday afternoon with the departure of the
jet. Wednesday will again have winds similar to Tuesday`s winds
with a weak short wave trough passing by the northeast Plains,
transporting 20-30 kt NW winds to the surface.

Thanksgiving Day and Friday will be dry area-wide and well above
normal temperature-wise with a ridge aloft and downsloping east
of the Continental Divide. Denver`s normal high for November 23
is 49, expect highs in the upper 60s Plains and upper 40s to mid
50s in the mountains both days. Models then differ on what happens
to the ridge after Friday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 705 PM MST Sat Nov 18 2017

VFR over the next 24 hrs with no significant impacts. Generally
light south/southwest winds at or below 10 kts at KDEN and KAPA,
but occasional westerly winds to 15 kts possible at BJC tonight.

&&

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

$$

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



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