Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 131740

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1040 AM MST Tue Feb 13 2018

Issued at 1034 AM MST Tue Feb 13 2018

Snow over the Park range and Rabbit Ears pass has diminished
through the morning. Expect the possibility of light flurries over
SW facing slopes through the afternoon. On the plains, conditions
have cleared out with warmer WSW flow helping to bring
temperatures back into the low 50s. The upper jet will slowly move
North increasing speeds over the mountains tonight. Made some
adjustments to sky, wind and temperature grids. Otherwise
forecast is on track.


.SHORT TERM...(Today and tonight)
Issued at 230 AM MST Tue Feb 13 2018

The closed upper low over central CA will track southwestward
off the southern CA coast by 00Z. As it does, increasing
subsidence will occur as an upper level ridge develops over
northwest CO. Snowfall will decrease over the northern mountains
as mid level warm air advection occurs. Snowfall later today will
be light and confined generally to the higher west facing slopes.
Will either cancel or allow the Winter Weather Advisory for zone
31 by 6 am. Across the I25 Corridor and Northeast Plains, latest
GOES satellite imagery this morning shows the edges of the
stratus deck gradually eroding away. Low clouds and patchy fog may
linger a bit, but around 12z window, skies should clear, then
mostly sunny and warmer for the rest of the day. For tonight,
NAM12 spatial cross-sections show a good mountain wave developing
after 06z. Sangster models output for from the NAM/GFS runs
suggests watch category regarding the wind gusts. No highlights at
this time but will make sure gusts at or above 60 mph will be
included in the grids.

.LONG TERM...(Wednesday through Monday)
Issued at 230 AM MST Tue Feb 13 2018

Active weather pattern will prevail Wednesday through the early
part of next week with a system affecting the area roughly every
other day. On Wednesday, a west-southwest flow aloft will prevail
over Colorado ahead of a pair of systems along the Pacific coast.
Main thing to watch will be windy conditions Wednesday. During the
morning hours, a mountain wave may be present and help accelerate
winds. Models showing warm air advection at ridge top level which
will help produce a stable airmass. The west-southwest flow aloft
isn`t the best direction for strong winds, but appears gusts to
50 to 70 mph will be possible in the foothills and mountains. It
will be warm Wednesday and with good mixing gusty winds will
spread onto the near by plains during the day.

Moisture will be shallow over the mountains Wednesday. A few
light snow showers can`t be ruled out, but no accumulation is
expected. Moisture will increase Wednesday night and Thursday.
Cold air advection will help to steepen lapse rates, leading to
better orographic lift. In addition to this, lift from the jet is
expected to enhance lift. Should see a good period of snow for the
mountains Wednesday night and Thursday. Light rain/snow will be
possible over the plains Thursday/Thursday night due to the jet
and upslope lift behind a cold front. Any precipitation across the
lower elevations is expected to be light.

Friday and Saturday look to be quiet weather days as the state
will be between systems. Saturday should be a warm and windy day
ahead of an upper level trough diving south across the Pacific

From Sunday on, forecast is highly uncertain. Models show a cold
front dropping south across the area Sunday ahead a Pacific
system. If the front pushes through early highs will only be in
the 30s, if not 50s will be possible prior to the frontal passage.
Models are even more divergent Monday. The ECMWF digs the Pacific
system south over Grand Canyon area, allowing arctic air to spill
into the area. Meanwhile the GFS keeps the trough north of the
state and shows a warm downslope flow. If the ECMWF is correct,
temperatures will struggle to get out of the teens, but if the GFS
is right highs in the 50s would prevail. Will split the
difference and go with highs in the 30s for Monday with a chance
for snow.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1034 AM MST Tue Feb 13 2018

VFR conditions are expected through the period. Flow from the
from the SSW will continue through 22z before moving to a more NE
direction. Speeds will stay from 8 to 11 kts through the period.
By 02z flow will veer to the SE before moving to drainage by 04z.
Winds will pick up at BJC into the morning hours with the increase
of the upper jet and speeds over the foothills. Expect gusting to
reach into the 40-45 kt range between 11 and 15z but not to APA or
DEN. There doesn`t seem to be enough amplification to bring the
winds further east on the plains.




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