Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 080326
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
826 PM MST Wed Dec 7 2016

.UPDATE...
Issued at 820 PM MST Wed Dec 7 2016

Arctic high pressure moving over the area will bring dry
conditions and light winds tonight. A weak east/northeast flow
around the high has kept low clouds along the foothills and Palmer
Divide. Most of these will dissipate by midnight. Only made minor
changes to line up with current trends.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 222 PM MST Wed Dec 7 2016

The upper level trough that produced snow across our area is
migrating east this afternoon and winds aloft will transition to a
dry west northwest flow. At the surface a strong ridge of high
pressure will gradually build across Colorado. There is still just
enough upslope component and instability within the base of the
trough to generate some lingering light snow showers along the
north side of the Palmer divide and along the east slopes of the
Front Range. Any snow showers should be brief and should diminish
this evening as the trough axis moves out of the state and the
upslope flow weakens with the building of the surface ridge. Do
not expect additional snowfall accumulation this
afternoon/evening.

Tonight a strong surface ridge of high pressure will build into
far NE Colorado with maximum sea level pressure getting to about
1043mb. This will act to weaken the winds across the Plains
overnight. Weak winds, clearing skies, and a fresh snow cover will
cause already cold temperatures to plummet shortly after sunset
area-wide. Expecting minimum temperatures to drop below zero
across the Denver metro area for the first time since February
2015. All model blends and most of the deterministic models do not
have a good handle on the low temperatures. The bright spots in
terms of guidance are the high res models: RAP and HRRR, as well
as the ESRL HRRR, all seem to zero in on the really cold min
temperatures. For low temperatures tonight have followed the HRRR
and RAP closely, which are at least 10 degrees colder than any
other blend or deterministic guidance out there. Low temperatures
should bottom out in the -5 to -10 range in the Denver metro area,
close to -15 in the Platte River Valley, and in the -10 to -20
range in the mountains. Temperatures alone are near wind chill
advisory levels across the BOU CWA. However, the only areas
expected to have winds above 5-10 mph and exceed advisory criteria
should be the far eastern counties. Have hoisted a wind chill
advisory for Logan, Washington, Sedgwick, and Phillips counties
from 11 PM to 8 AM local time, in line with surrounding offices.
Despite the weak winds in the metro area, it will still be
exceptionally cold so it will be a good idea to take cold weather
precautions for sensitive/vulnerable people and animals.

On Thursday temperatures will struggle to hit 20 degrees across
the Plains of northeast Colorado given the cold start, snow cover,
and presence of the surface high pressure preventing any downslope
component to the low level winds. Should remain dry across the
Plains and mountains with light winds.

.LONG TERM...(Thursday night through Wednesday)
Issued at 222 PM MST Wed Dec 7 2016

Continued moisture will flow into the mountains from the WNW through
the extended period bringing accumulating snowfall.

Thursday night into Friday models show increasing moisture over the
mountains with warm air advection moving in with WNW flow aloft. QG
increases through the day Friday which will help with snow making
properties. Will keep high pops for this period with confidence in
some accumulation on Friday...especially for west-facing slopes of
the Northern and Central mountains. Will double-down on the previous
mention of the need for a possible advisory for zone 31 given
current QPF model accumulation amounts.  On the plains a lee side
low will form at the base of the Foothills keeping the area dry.
Downslope winds will heat the plains and help to increase snow melt.
Cold air near the surface will take some time to scour out as
melting and evaporation will lower temps before they increase so
will maintain temps in the 40s with even warmer temperatures
possible by Saturday. Friday the jet will start to drop south over
the state increasing mountain top winds. Model cross sections show
component along values upwards of 50 knots at mountain top level but
do not see a distinct stable level with adequate shear that would
allow for those winds to make it down into the foothills and plains.

For the weekend the upper jet will continue to drop south over the
Central mountains increasing winds over the higher terrain. Moisture
continues to flow into the mountains increasing snowfall production
through the weekend. Elements such as QG instability,shortwave, high
levels of moisture and upper jet dynamics could result in a winter
storm event in the mountains with high levels of accumulation. The
front will move onto the plains by late Saturday bringing some
increased moisture and cooler NW flow. This is a quick shot with
little accumulation shown at this time...however if surface winds
switch just a little more NNE the foothills and western plains could
see some light snow so will keep pops in place.

For next week the upper level NW flow will continue with multiple
embedded shortwaves keeping the orographic snow machine churning.
The main difference in the models is the amount of cooler air that
makes it onto the plains. With continued NW flow in the synoptic
picture I do think that colder arctic air could make it down so
keeping temps an average between the two models with some nudging to
the cooler side.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 820 PM MST Wed Dec 7 2016

VFR conditions will prevail through Thursday evening with light
winds.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory until 8 AM MST Thursday for COZ048>051.

&&

$$

UPDATE...Meier
SHORT TERM...Schlatter
LONG TERM...Bowen
AVIATION...Meier



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