Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 070841
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
141 AM MST Wed Dec 7 2016

.UPDATE...
Issued at 141 AM MST Wed Dec 7 2016

Some drying is evident on satellite, radar and observations moving
into the area. Most of the heavier snowfall is over the Palmer
Divide and east. The surge of colder northerly flow is up near the
northern state border and is already enhancing radar echoes up
there. Expect this surge to push south across the area over the
next few hours to increase snowfall amounts again slightly, though
the forcing will be shallow as upper forcing seems to be weakening
now. Expecting to see the radar echoes increase from the north
over the next few hours, though visibility may not be as low as we
saw earlier in the evening.

UPDATE Issued at 811 PM MST Tue Dec 6 2016

Snow has spread onto the Front Range, mainly west of Interstate
25. A couple of snow bands are stretching across the area with
heavier snowfall. Will make a couple of adjustments to the
forecast. First will be to trim snowfall amounts by an inch over
the eastern plains. The latest GFS, ECMWF and HRRR all show
precipitation amounts less than 0.15 of an inch. This would result
in less than 3 inches. An exception will be under snow bands
where an extra inch or two is possible. Also, increased snow
accumulation by about an inch over Boulder county where heavier
snow has started. Not a fan of the 00z NAM. It is not far enough
east with the current snow and doesn`t show any of the snow bands.
Also made minor changes to overnight lows to line up with current
trends.

UPDATE Issued at 605 PM MST Tue Dec 6 2016

Atmosphere along the Front Range continues to moisten from the
20 to 30 deg dewpoint spreads observed earlier this afternoon.
In addition KDEN and KCYS radars have detected an expanding area
of light precip...i.e. snowfall over and along the Front Range
especially within the past hour. The moistening is originating
from aloft with mid and high level moisture spreading in from Utah
in advance of the 700-500 mb trough axis. Recently updated the
weather and pop grids to delay measurable snowfall within the i-25
corridor until after 01z which appears to be working out. Also
delayed snowfall farther out on the plains by a couple of hours
even though radar is now indicating some banning out there. In
addition...the gusty southeasterly winds acrs eastern metro area
and points east should ease as they turn easterly with arrival of
the cold front presently slipping south through central
Weld...northern Morgan and Logan counties. At its present
speed...should see it in the metro area in the next hour or so.
That`s when snowfall should enhance along the I-25 corridor range
including the Denver metro area. Although the heavier snowfall not
expected until later tonight when the upstream jet enhanced
forcing arrives.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 154 PM MST Tue Dec 6 2016

Light radar echoes have blossomed over the mountains and northern
Front Range...foothills...and northern front range.  Not much has
been reported at the ground...but the process has begun.  I had
considered delaying the start of the mountain advisory...but snow
will begin impacting roads during the afternoon. For the lower
elevations...the Ruc13 shows snow starting in the metro area
around 3z...the HRRR around 07z...the Nam after 03z...the GFS
after 03z. Will mention mid evening as a starting time...this is
good news because the evening rush should not be severely
impacted. The morning rush is another story, as snow is expected
to fall through mid morning Wednesday and roads will likely become
snowpacked.

snow accumulations are another difficult forecast today...latest
SREF plume forecasts give a mean of 2-3 inches over much of
northeast Colorado...with barely an inch at Limon. In the Urban
Corridor the numbers were a bit higher. I was considering an
extension of the advisory to cover parts of northeast Colorado...but
these snow amounts in eastern Colorado were below criteria. snow
will begin to decrease across the area during the afternoon on
Wednesday.

Temperatures will be frigid tonight and tomorrow. With clouds the
mountain valleys will be warmer...but eastern Colorado will have
lows in the single digits tonight...and mid teens on Wednesday.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 154 PM MST Tue Dec 6 2016

By Wednesday night subsidence will be moving in over the region
bringing clearing skies and snow to an end by late Wednesday.
Winds will be light and combined with clear skies bring overnight
temperatures below zero over the forecast area. Mountains could
see temperatures fall overnight to -20...especially in the
valleys.

On Thursday increasing NW flow aloft will bring a continual supply
of moisture to the mountains with some polar jet influence keeping
orographic snow possible. Temperatures will moderate on the plains
but with snow on the surface and some lingering cloud cover
expect temps to be slow to increase so kept highs lower then
guidance. by late Thursday the jet stream will drop south
bringing instability to the Northern Mountains and increase
snowfall intensity slightly. The strong westerly flow will create
downsloping off the foothills and keep the plains dry.
Temperatures on Thursday will continue to be below normal with
highs in the teens increasing back into the 40s by Friday.

Conveyer of moisture will continue to move into Colorado from the
NW as the jet moves south over the state by the weekend. This
will create increasing snow for the mountains with a slight chance
on the plains by Saturday evening with the passage of a cold
front late Saturday. The front will turn surface winds more
northerly...however, model guidance has just limited moisture for
the plains so do not expect much in the way of accumulation.
Temperatures for the weekend will drop once again but only into
upper 30s and lower 40s. winds will increase in the mountains with
the help of the upper jet with gusts up to 50 mph possible at
higher elevations. This will cause areas of blowing snow and
possible hazardous driving conditions for weekend travelers.

The weather pattern will remain active into the start of next week
with winds at lower elevations as the jet remains over the state.
Temperatures will average closer to normal by monday into Tuesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 141 AM MST Wed Dec 7 2016

Snow, visibility and ceilings are improving as brief drying has
moved over the area. Ceilings are up to 5000 feet over several
airports. The expected surge of colder temperatures and northerly
winds is up near the northern state border and should push south
over the next few hours. Ceilings are expected to lower then and
snowfall rates increase slightly again after 11-12z. Low clouds
with ceilings less than 2000 feet should hang on through about 18z
Wednesday with reduced visibility. Conditions will slowly improve
through the day, though ceilings are expected to stay below 6000
feet before skies will likely clear rapidly after 00z.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MST today for COZ031-033>036-
038>041.

&&

$$

UPDATE...Kriederman
SHORT TERM...RTG
LONG TERM...Bowen
AVIATION...Meier


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