Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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221
FXUS65 KBOU 141751
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1051 AM MST Thu Dec 14 2017

.UPDATE...
Issued at 1010 AM MST Thu Dec 14 2017

There are still some jet related snow showers over the northeast
plains at this time. They are moving southward. Water vapor pictures
are showing subsidence behind the upper trough that is moving
south through Colorado. Will increase the pops and prolong them a
bit in appropriate areas. Will adjust winds as well based of
actual data.

&&

.SHORT TERM...(Today and tonight)
Issued at 340 AM MST Thu Dec 14 2017

A positively tilted upper trough stretched across northwestern
Colorado will continue its progression southeast through tonight.
Light snow continues across most of the forecast area early this
morning. Snow fall rates will begin to decrease as downward QG
motion on the backside of the trough pushes into the state.
Looking around area webcams, most of the urban corridor and
adjacent plains have some snow sticking to grassy surfaces but
mainly wet and/or slushy roadways due to road temperatures being
above freezing according to the RWIS sensors. Areas closer to the
foothills, mainly in Boulder and Jefferson Counties have slightly
more snow sticking to the pavement surfaces due to road
temperatures around 31 to 32 degrees F. Backside of the system is
visible on satellite and radar, just now moving into the northern
mountains. Timing this motion and looking at high res model
timing, all point to an end time of around 7 am for the Denver
Metro area, around an hour or two earlier to the north near the WY
border and the same amount later to the south near Monument Hill.
Expect another half inch to an inch from now until then over the
plains, and another 1 to 3 inches over the mountains. Morning rush
hour may be impacted due to wet slushy roads over the lower
elevations, some mix of snow and slush toward the foothills, and
snowpacked over the mountains. Care should be taken driving over
bridges and overpasses due to possible icy patches on colder
surfaces. Colder air will be spreading into the area today, with
highs in the upper 30s to lower 40s over the plains, and in the
20s in the mountains.

Another impact today will be the strong northerly winds that
develop on the backside of the trough, mainly over the eastern
plains. Winds will pick up speed this morning, maxing out early
this afternoon with gusts in the 35-50 mph range. This will make
travel difficult on east to west oriented roads, especially for
light and high profile vehicles. Last nights snow should decrease
the possibility of any blowing dust. A few isolated snow showers
may develop this afternoon across the plains, with little to no
accumulation.

Tonight, clear skies and decreasing winds will allow for cold
temperatures as an upper ridge builds in front the west. The
exception will be in and along the foothills where increasing
mountain top stability and winds turning more westerly will result
in some downsloping winds, which will help warm temperatures
locally.

.LONG TERM...(Friday through Wednesday)
Issued at 340 AM MST Thu Dec 14 2017

A flat upper level ridge will build over the area Friday into
Friday night. The westerly component aloft will increase during
this time, and there will be a few ingredients in place for
stronger west winds in/near the Front Range. While cross mountain
component is only near 30 knots, we will be close to having a mean
state critical layer in place Friday, and at a minimum a reverse
shear profile. Mountain top stability is moderate, and cross
mountain pressure gradient per the latest forecast Sangster data
is moderate as well. With these favorable mountain wave parameters
in place, we should see gusty winds spread down the Front Range
through the course of the day, possibly spilling out onto the
nearby plains and Wyoming border area. Have increased winds for
this period, with peak gusts expected to reach 45-60 mph in wind
prone areas of the foothills.

Temperatures will warm quickly with the westerly component and
warm advection under the ridge. Highs on the plains should be able
to reach the mid to upper 50s on Friday, barring too much
mountain wave cloudiness.

Saturday should start off mild with a downslope component still in
place, but then as the next trough approaches another cold front
will sweep southward across the plains. High temperatures
difficult to pinpoint as just a couple hours of difference in the
frontal timing could mean 10 degrees warmer or cooler than
present forecast.

By Saturday night, there should be at least shallow upslope to
help generate clouds and some light snow. Models are still
disagreeing with regard to how much the trough and energy split,
but there are splitting signs in all model diagnostics. ECMWF
remains one of the stronger, more compact systems, but overall
preference is still for more splitting in this pattern. Will
maintain a consistent forecast at this point with colder
temperatures and a chance of mainly light snow Saturday night into
Sunday.

For the first half of next week, there is reasonable agreement in
a flat upper level ridge building across the Central Rockies. We`ll
continue the dry and warmer forecast for this period with high
temperatures warming above normal by Monday and much above normal
by Wednesday. Then perhaps a more significant change and another
chance of snow toward next Thursday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1010 AM MST Thu Dec 14 2017

Winds at DIA will become northerly through 19Z as the upper
trough is moving across the area. There are some jet related
showers over the plains of Colorado and they may get close to DIA.
Will go with VCSH for a few hours. Ceilings should get above
BKN080 by 21Z this afternoon. Normal drainage winds are likely at
DIA by 04Z this evening.

&&

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

$$

UPDATE...RJK
SHORT TERM...Kriederman
LONG TERM...Barjenbruch
AVIATION...RJK



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