Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 211010

Area Forecast Discussion
National Weather Service Denver/Boulder CO
410 AM MDT Sat Oct 21 2017

.SHORT TERM...(Today and tonight)
Issued at 400 AM MDT Sat Oct 21 2017

An upper level trough moving across the eastern half of Colorado
is bringing active weather at this time. Snow moved across the
mountains overnight, most of it has ended already. Where it
hasn`t, light snow will remain possible through mid morning. Web
cameras show snow accumulating on the roads over the mountain
passes. With snow ending, conditions will rapidly improve early
this morning. Lift from the trough will bring a few showers to the
Front Range through 6 AM. The showers are expected to progress
eastward and should exit the state by 9 to 10 AM.

Behind the exiting trough northwest flow will bring subsident and
drier air. Other than some cloud cover over the mountains due to
orographic flow, mostly sunny skies will prevail by late morning.
Gusty northwest winds will develop late this morning and afternoon
across the area. Gusts to 45 mph will be possible over the
eastern plains and near the Wyoming border. Temperatures will be
much cooler today with highs in the 50s across northeast Colorado.

Overnight lows will be chilly over the eastern plains and along
parts of the Front Range with readings falling into the mid 20s to
lower 30s. Gusty downslope winds are expected to develop along
the east slopes and in/near the foothills. The normal windy
locations will see wind gusts to 60 mph tonight. The downslope
winds will keep temperatures warmer with lows in the 30s.

.LONG TERM...(Sunday through Friday)
Issued at 400 AM MDT Sat Oct 21 2017

Large scale
thermal and flow pattern over the western CONUS is progged to
undergo changes during the extended period, particularly beyond
Wednesday. To start out on Sunday, the fcst area will still be under
the influence of a cool, brisk northwest flow. There may be
sufficient mid-level moisture in the flow to fuel a few early
morning snow showers in the northern mtn ranges. Otherwise Sunday
looks dry with a 8-18kt west-southwest/downslope flow helping to
push afternoon temps up into the upper 60s/lower 70s on the plains.
Warmest readings will likely to observed in breezy areas along the
base of the foothills. The high country should also see an 8-12 deg
f jump in max temps with a 4.5-6 deg c 24-hr increase in the 700mb

On Monday, temperatures retreat a few degs with a weak mid-level
shortwave trough brushing by to our north. No precip associated with
this feature, but a weak cold front accompanying this wave is
expected to back into nern Colorado during the morning bringing with
it slightly cooler air on a nely breeze. For Tuesday, temperatures
should be a few degs cooler with the somewhat stronger north-
northwest flow overhead. Do not see any precip and probably not much
cloud cover.  Warmup will likely continue on Wednesday with a shift
to a warmer quasi-zonal flow aloft. Could see daytime temperatures
as much as 10-15 deg f warmer than those on Tuesday. Dry conditions
also remain in place.

Potentially significant changes in the large scale pattern begin
showing up on Thursday according to the medium range models with
a vigorous upper trough swinging down from the Pacific Northwest
and northern Rocky Mtns. The forward speed and strength of this
system differs from model-to-model. The GFS moves the trough
crossing nern Colorado Thursday afternoon and producing 6-8 hours
of light showery precip scattered around. The 00z run of the
European model now appears wetter with a deeper and slower moving
upper trough. The new EC doesn`t show the business end of this
storm system moving across the fcst area until Friday, and with
colder air and more moisture. The EC even generates a closed
upper low over the 4-corners by late on Friday, which if this
should happen would extend the period of cold and unsettled
weather well into the weekend. Meanwhile the Canadian model is
notably drier and not as cold during this period. For now will
trend cooler starting Thursday and introduce low PoPs starting
Thursday afternoon.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 400 AM MDT Sat Oct 21 2017

An upper level trough may bring a few rain showers and ceilings
down to 6000 feet through 12Z. Winds will be variable through 15Z
and could be gusty out of the northwest at times due to the
showers. Dry air and gusty northwest winds move in by mid morning
as the upper level system exits the area. Northwest winds gusting
to 30 knots will be possible from 15-00Z, then winds will settle
at a southerly direction by 05Z. Skies will become mostly clear by
16Z as drier air moves into the area.




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