Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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726
FXUS65 KBOU 130341
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
841 PM MST Tue Dec 12 2017

.UPDATE...
Issued at 833 PM MST Tue Dec 12 2017

No significant changes planned in the grids for tonight`s
forecast. One short wave embedded in the flow aloft will pass to
the north and east of the state on Wednesday morning. It`s
associated cold front is still expected to move into the Denver
area around sunrise, with increasing clouds over the northwest in
the afternoon ahead of a stronger system in the northern Rockies
dropping into the Front Range Wednesday night.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 310 PM MST Tue Dec 12 2017

Upper ridge upstream of Colorado will lead to a pleasant evening
and into the overnight hours. A few high clouds and light winds
tonight will allow overnight lows to cool off into the 20s across
the Plains, near 30 across the I-25 urban corridor, and teens in
the mountains.

A weak short wave trough will move across the northern Plains
early on Wednesday and force a cold front south across the Plains
of Colorado. Expect high temperatures to occur late morning to
near noon just before frontal passage as the front enters Colorado
around sunrise, reaches metro Denver mid-morning. Steady temps
around 50 should occur Wednesday afternoon. The mountains will
see similar temperatures compared to today, with 700 mb temps
cooling 1-2 degC, so expect highs in the upper 30s to low 40s
there.

Chances of precipitation increase significantly Wednesday
evening in the mountains (and eventually Plains) as a short wave
trough moves quickly south-southeast towards central Colorado. The
trough will dig south given a 100-120 kt jet on the backside of
the trough. These features will impact Colorado weather and the
potential for snow overnight into Thursday, discussed below.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 310 PM MST Tue Dec 12 2017

The upper trough passing over the state Wednesday night and
Thursday morning should produce a period of snowfall for the
mountains, with snow accumulations being limited by the short time
frame that the snow is expected to stick around. The upper trough
will be moving out of the state by Thursday afternoon, with strong
subsidence expected to be over the state by Thursday evening.
Surface high pressure will be building in from the north behind
the trough and this will tighten up the pressure gradients across
the plains. Gusty north-northwest winds are expected across the
plains, which might be approaching high wind criteria for a few
hours. The northerly winds will also be bringing in chillier
temperatures through the day.

On Friday, pressure gradients will relax and warmer temperatures
will move back in. Another stronger trough is forecast to drop
over the state Saturday night. The ECMWF and GFS are in good
agreement with the storm as it moves across the state in a track
similar to Thursday`s trough. This trough will also be driven by a
110+ knot upper level jet. Plains areas may also pick up some
precipitation from this trough. After Sunday, the flow aloft will
become more westerly as it looks like the blocking ridge pattern
than has been over the western U.S. will begin a retrogression out
over the eastern Pacific. If the ridge does retrograde, then the
storm track over the Rocky Mountain Region may become a little
more active than we have had the past several weeks. In the
meantime, Monday and Tuesday still look dry.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 833 PM MST Tue Dec 12 2017

VFR through Wednesday evening. Diurnal winds tonight into early
Wednesday. Expect a weak cold front to impact the metro area
terminals with a wind shift to the north and northeast 15-16z,
and increase in speed to 10-15 kts through through 00Z.
Precipitation chances go up late Wednesday night along
with likely MVFR ceilings after 00z with a chance of rain/snow
turning to snow in the evening.

&&

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

$$

UPDATE...Cooper
SHORT TERM...Schlatter
LONG TERM...Dankers
AVIATION...Cooper



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