Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 212046

Area Forecast Discussion
National Weather Service Denver/Boulder CO
146 PM MST Sat Jan 21 2017

.SHORT TERM...(This evening through Sunday)
Issued at 137 PM MST Sat Jan 21 2017

Main upper level trough will move fm the srn rockies into the srn
plains however thjere is a weakter secondary feature that will move
across nrn co this evening.  This feature will bring snow to the
mtns this evening which will decrease late tonight.  Winds will
increase late tonight in the mtns so there will be area of blowing
snow at higher elevations into sun morning.

At lower elevations as the wk upper level trough moves across there
could be a slight chc of -shsn over nern CO due to favorable lapse
rates this evening.  Late tonight into Sun morning winds will
increase in the foothills and across the plains nr the WY border.
Component along winds are in the 35-40 kt range so expect some gusts
fm 50 to 60 mph in the foothills late tonight.

On Sun a flat upper level ridge will build over the area.  There
will still be some lingering moisture in the mtns in the morning so
keep in a chc of -shsn. By aftn drier air in wly flow aloft will be
across the area.  Thus expect dry conditions except in zn 31 where a
few -shsn may occur.  As for highs 850-700 mb temps change very
little so will keeps highs in the 40s across nern CO.

.LONG TERM...(Sunday night through Saturday)
Issued at 137 PM MST Sat Jan 21 2017

Upper ridging moves across Colorado Sunday night. There is a
130-160 knot west-southwesterly jet maximum over the CWA early
Monday morning into Monday night. On Monday night, models have a
mean upper trough over much of the western United States. This
gets into Colorado by early Tuesday and upper troughiness remains
over us well into the extended period. The models all show a
closed circulation with the upper trough early Tuesday morning
into late afternoon, but it is located all over the place
depending on the model. It is not in a favorable position on any
of the models for a decent snowfall on the plains. The flow aloft
weakens considerably by mid day Tuesday and stays weak into the
extended period. There is overall upward QG vertical velocity for
the forecast area Sunday night through Tuesday night, but it is
somewhat convoluted depending on the model. The boundary layer
winds are dominated by normal drainage patterns Sunday night, then
southwesterly downsloping flow is progged on Monday. Models have
a cold front move into the CWA Monday night, with some of them
showing upslope on Tuesday and north-northwesterlies Tuesday
night. For moisture, the models are not in good agreement Sunday
night into Monday afternoon. By Monday afternoon, the NAM and GFS
do have fairly deep moisture in our mountains, but the ECMWF does
not. Monday night`s moisture solutions are not very agreeable
either. On Tuesday, the NAM has the best and deepest with
moisture, especially over the plains where the GFS and ECMWF have
very little moisture in the lower levels. The QPF fields show
nothing for the CWA much of Sunday evening...then a tad by 12Z
Monday morning. On Monday, there is some measurable alpine snow,
with the NAM having more than the rest. There is a tad on some of
the models for some of the northern plains Monday afternoon. There
is a tad again Monday night in the mountains and some on the
plains. On Tuesday, the NAM has a decent amount of measurable snow
for most of the CWA with the other models showing very little to
none. All over the place. Concerning pops in the mountains...will
go with "likely"s from late Sunday night into Tuesday evening. The
orographic enhancement is not great, but it is not bad Sunday
night at least concerning wind speeds. Lapse rates are not that
good Sunday night. Moisture is sketchy between the models. When
moisture is more consistent on Monday, the orographic enhancement
decreases but the lapse rates are okay. There is also some warming
on Monday. By late Monday overnight and Tuesday, the NAM has
easterly mountain top flow, the GFS is northwesterly. The great
lack of agreement for most parameters means no highlights in the
mountains at this time. For plain`s pops, the best model, by far,
is the NAM and the day is Tuesday. The NAM and the Canadian have
upslope on Tuesday, the others do not. So will go with 20-40%s
mainly on Tuesday for the plains. For temperatures, Monday`s highs
are 3-5 C warmer than Sunday`s. Tuesday`s highs are 8-12 C colder
than Monday`s. For the later days, Wednesday through Saturday,
models have the upper trough axis finally pushing through the CWA
early Friday. Then weak northerly flow aloft is progged well past
Saturday. The CWA stays in the cold air. Moisture is sparse,
maybe some in the mountains, especially the northwestern corner of
the CWA.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 137 PM MST Sat Jan 21 2017

Winds were light NNE this aftn but should become more WNW by 02Z.
There still could be a slight chc of snow showers fm 02z-07z but no
accumulations are expected.  Ceilings may drop to around 8000 ft as
well during the evening hours.

After 07Z winds will become more swly with ceilings aoa 12000 ft. By
12z on Sun winds will be SSW and then become more ese by early
aftn with VFR conditions thru the day.




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