Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 220342
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
842 PM MST Tue Feb 21 2017

.UPDATE...
Issued at 831 PM MST Tue Feb 21 2017

Current satellite imagery shows push of moisture moving SE from
Western Wyoming. Models show snow making it into NW portions of
the CWA between 05 and 06z with timing tool backing off until
between 08 and 09z. Current grids show main push later in the
period so have maintained current forecast. Strong SW push prior
to onset with increasing component along winds over the mountains.
Increased winds gusts into the 50 to 60 mph range overnight to
account for this. Model cross sections show mountain wave
development around midnight with an increasing mountain top
stable layer and mountain top winds peaking around 70 mph by the
morning hours. Expect wind gusts up to 45 mph possible closer to
the foothills with less further east into the early afternoon
hours on Wednesday.

Fire danger continues to be a concern with NAM...RAP and GFS
showing RH values over the southerN counties of the CWA dipping
into the low teens by the afternoon. Further information in the
fire weather discussion.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 232 PM MST Tue Feb 21 2017

For tonight...a strong west/southwesterly flow aloft will remain
over the region tonight. The models show moisture increasing over
northwest CO overnight, but have slowed the timing down a bit.
Will keep pops primarily in zones 31 this evening, and hold off
introducing pcpn to the znes 33 and 34 until late tngt with a
gradually increase fm northwest to southeast across the mountain
zones Wednesday aftn. Overall just some light snow amounts
expected primarily in 31. Strong winds have become more localized
in the foothills this aftn but NAM12 spatial cross-sections show
a mountain wave re-amplifying after 06z. Mdls also have a 120Kt
upper jet stretching fm central CA into southern WY by 12z
Wednesday morning so not a typical mountain wave set up. Could
see gusts in the 50-60 mph range develop in the 09-15z window. No
highlights regarding the wind but continued concern surrounding
the elevated fire danger. Overnight lows in and near the foothills
will be on the mild side once again. Across the northeast plains,
generally dry but enhanced southwesterly winds over the Palmer
Divide. On Wednesday, not as warm but still well above normal.
Main chc for pcpn will remain in the mountains, possibly
developing along the northern border south of Cheyenne toward 00z
Thursday. Dry and stable across the rest of the cwa. Fire weather
concerns will be south and southeast of Denver for Wednesday aftn,
see the fire wx discussion below.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 232 PM MST Tue Feb 21 2017

Long wave trough will be over the western part of North America
Wednesday night through Friday. An intense short wave trough over
the Great Basin Wednesday night will lift northeast into Wyoming
and strengthen into a close low Thursday. Southwest flow aloft
ahead of the trough will turn westerly late Wednesday night.
Expect this to produce orographic snow over the mountains. Lapse
rates will be good at 7-9 C/km. Expect mountain snow to increase
Thursday due to deeper moisture and cold air advection keeping
steep lapse rates in place. Best snowfall is expected over the
northern mountains, closer to upper level low. Will issue a Winter
Storm Watch for zones 31 and 33 where 8 to 14 inches of snow will
be possible.

For the Front Range and eastern plains, the GFS and ECMWF are very
similar regarding the upper level details. However, the 12z GFS
has the surface low farther north, over northwest Kansas, where
the 12z ECMWF indicates the surface low will be over southeast
Colorado at 18z Thursday. Will go with the ECMWF solution which
pushes a cold front through eastern Colorado Thursday morning.
Upslope flow behind it is then expected to produce snow, mainly
north of I-70. The GFS keeps the cold front over the northeast
corner of Colorado with dry downslope winds for the Front Range
and eastern plains. Will go with mainly 1 to 3 inches of snow for
the Front Range. The highest amounts will over the north. If the
GFS pans out, snowfall amounts will be much less.

Northwest flow aloft will prevail behind the low on Friday with
northerly surface winds. Temperatures will be cooler with highs
expected to stay below freezing. There may be enough moisture for
snow showers over the plains. Orographic lift combined with the
moisture should produce scattered snow showers over the mountains.

For weekend, westerly flow aloft is forecasted to prevail. The
models are still trying to work out the details. Expect temperatures
to be below normal Saturday and perhaps warm to normal Sunday as
weak ridging in the westerly flow aloft drifts across Colorado. Will
have isolated to scattered pops over the mountains. Not expecting
significant snow at this time.

Models are advertising a trough will move onto the California coast
Monday. It then tracks across the Four Corners region and Central
Rockies Tuesday. Timing still uncertain, which is not usual this
far in advance. Will go with normal temperatures and a chance for
mountain snow.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 831 PM MST Tue Feb 21 2017

VFR conditions expected over the next 24 hours with gusty WSW
winds being the main forecast challenge. For the evening hours
winds will be from the SSW with increased speeds ahead of next
weather disturbance. This will help to usher in another round of
gusty SW winds over the field between 06 and 12z. Winds speeds
could reach upwards of 35 kts at BJC with 25 possible at DIA and
APA. At this time do not anticipate the winds to change to a
westerly direction but confidence is only medium that they will
stay to the SW. By 17z expect that wind switch but wind speeds
will decrease through the afternoon and evening with an increasing
mid-level cloud deck. Expect a wind shift to accompany this
increasing moisture from an approaching cold front from the north
by late Wednesday into early Thursday.

&&

.FIRE WEATHER...
Issued at 831 PM MST Tue Feb 21 2017

Decided to upgrade the fire weather watch to a red flag warning
due to model consistency on placement of dry conditions over
portions of Elbert and Lincoln counties for tomorrow afternoon. RH
values will dip into the lower teens with winds possibly gusting
from 25 to 35 mph. The NAM does place a boundary over the area
that could help to keep winds under RFW criteria but decided to
upgrade given the dry history for the plains and continued low RH
values. Expect that if a fire were to start gusty winds could
carry it out of control quickly. Burning in these areas is not
suggested.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 11 AM to 6 PM MST Wednesday for COZ241-246-
247.

Winter Storm Watch from Wednesday evening through Friday morning
for COZ031-033.

&&

$$

UPDATE...Bowen
SHORT TERM...Cooper
LONG TERM...Meier
AVIATION...Bowen
FIRE WEATHER...Bowen



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