Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 212132

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

.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 reamplifying 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 18Z TAFS through 18Z Wednesday afternoon)
Issued at 232 PM MST Tue Feb 21 2017

VFR over the next 24 hrs with no restrictions. Gusty westerly
winds will continue but the stronger gusts should lessen by 23z.
The wind will decrease this evening and go to drainage at DEN and
APA but may pick up after 06z. Another mountain wave tonight will
likely allow for gusty wly winds of 30-35kts develop at KBJC
after 08z, with some gusts to 20kts at KDEN and KAPA. Southwest to
west winds Wednesday morning, will transition to north/northeast
by late aftn as a cold front approaches fm the north.


Issued at 232 PM MST Tue Feb 21 2017

Stronger winds in the foothills have become more localized but
will continue with the Red Flag Warning through 6 pm,due to the
dry conditions and low relative humidities. Some concern regarding
stronger winds overnight as another mountain wave develops. Could
see some gusts to 60 mph develop between 0300-0900 am wednesday
morning with not much of an rh recovery overnight. The rest of the
Red Flag Warning will continue as well with the strongest gusts to
around 40 mph north of Highway 34 so far this aftn.

Temperatures will remain warm on Wednesday but not as much as
today. Although the fire danger will remain elevated across much
of the region tomorrow, the main area of concern will be south and
southeast of Denver with the best combination of warm
temperatures, gusty winds and low rh will exist. As a result, will
issue a Fire Weather Watch for this area from late Wednesday
morning through early Wednesday evening, prior to the arrival of
the front.


Red Flag Warning until 6 PM MST this evening for COZ215-216-

Fire Weather Watch from Wednesday morning through Wednesday
afternoon for COZ241-246-247.

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



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