Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 241653
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
953 AM MST Sat Feb 24 2018

.UPDATE...
Issued at 953 AM MST Sat Feb 24 2018

Temperatures/PoPs look on track today. The strong west winds have
already eroded the stratus east of the mountains and despite
strong sun and downslope flow, temperatures will remain in the
mid 30s across the Plains with deep cold air aloft moving across
the state on the back-side of the trough. Temps at 700 mb are -14
to -18 degC and at 500 mb they are around
-34 degC. That translates to teens above 8000 feet and single
 digits for the ski areas. West and northwest winds will bring
 wind chills across the high country to around -10 to -20 degF
 through this afternoon. In terms of wind impacts east of the
 mountains, the strongest pressure gradient from W to E across
 Colorado holds on through early afternoon. Surface obs have been
 showing 40-60 mph gusts from Nederland south to the I-70
 corridor, especially Gilpin, Clear Creek, and west Jefferson
 Counties. Expect the strongest gusts to diminish by early
 afternoon but strong sustained winds will continue across the
 high country, east slopes and all of the Plains through sunset.
 We expect sustained northwest winds around 30-35 mph with gusts
 to 50 mph downstream of the Cheyenne Ridge across Northern
 Larimer and Weld Counties. Elsewhere across the Plains winds will
 be 25-35 mph sustained with gusts to 40 mph. Did not need to
 update the wind forecasts as this is all nicely handled, though
 we did increase the start time of the strongest winds since they
 are already occurring across the I-25 metro corridor. The only
 real change to the short term forecast is to remove the mention
 of patchy blowing snow from along I-25. Area webcams are not
 showing much unless you are right along the foothills from CO-93
 Golden to Boulder and along US 287 in Larimer County.

UPDATE Issued at 647 AM MST Sat Feb 24 2018

Last attempt of a healthy snow band over the far eastern plains
is rather weak and disorganized. Current surface observations show
only light snow or flurries, and this snow will be exiting in the
next 2 hours with little if any additional accumulation. Winter
Weather Advisory for the northeast corner has been cancelled.

Patchy fog is eroding to the east along the Front Range, as gusty
downslope winds are just now beginning to kick in. We`ve had some
reports of blowing snow and poor visibility already along Highway
93 between Boulder and Golden.

&&

.SHORT TERM...(Today and tonight)
Issued at 421 AM MST Sat Feb 24 2018

Latest satellite imagery shows the last shortwave organizing just
to the east of Denver in the pre-dawn hours. This should bring
one more period of snow to the northeastern corner of the state
early this morning. Opted to cut back the last of the advisories
to the far eastern portions as little if any snow had fallen in
most of Logan and Morgan counties. Could see another quick inch in
the bands that develop early this morning from Julesburg to Akron
and points eastward, before this weather system exits the state.
Behind that, partial breaks in the stratus deck will lead to
patchy fog into early morning.

Concern will then shift to strong, gusty downslope winds in the
post trough environment. Strong subsidence builds over the area
while 700 mb winds increase to 50 knots by mid morning. Surface
pressure gradient also tightens with GJT-DEN MSL difference
increasing to 10-12 mb by 18Z, so in this Bora pattern the strong
winds will spread across all of the plains. The strongest should
be in/next to the foothills where we could approach high wind
criteria of peak wind gusts in wind prone areas reaching 65-75
mph. Will have to watch gradient and wind trends closely this
morning, but main message will be for strong, gusty winds
spreading across the forecast area. Areas that saw a few inches
of snow should see some blowing snow for a few hours today.

In the mountains, lingering snow this morning will decrease
through the afternoon as moisture depth becomes quite shallow in
this drier, subsident airmass. The next upper level wave will
reach the mountains tonight with increasing mid level moisture.
Good instability and decent orographics (strong 35-45 knot flow
but a bit too westerly) would be favorable for light to moderate
snow accumulations. Models looked a bit low for QPF as the airmass
is unstable, so we adjusted those up slightly. Overall, look for
2-6" snow through Sunday morning, with locally higher amounts
around Rabbit Ears Pass where westerly flow is more favorable for
orographic enhancement.

.LONG TERM...(Sunday through Friday)
Issued at 421 AM MST Sat Feb 24 2018

Snowfall in the mountains will continue for a few hours Sunday
morning as the upper trough over the state begins moving eastward.
By afternoon, heights will be rising over the state as an upper
ridge begins building in. After a chilly start to the day, warm
advection with the developing ridge will bring warmer afternoon
temperatures, especially on the plains. Gusty winds will continue
over the mountains as westerly flow aloft will be around 50 to 60
mph.

Through the first half of the upcoming week, mid-range models all
show a flat ridge/westerly flow pattern across Colorado as an
upper trough intensifies along the west coast. Models show most of
the energy with this driving southward and eventually cutting off
over southern California on Tuesday. As the energy remains over
the western U.S., Colorado will be left dry with a gradual warming
trend through Tuesday evening. On Wednesday, the GFS and Canadian
models show the western upper trough shearing out and ejecting
across Colorado while the ECMWF is slower with a more cut-off
circulation. Either way, with falling heights and increasing
moisture moving across, the chance for snow in the mountains will
increase. After a 12 to 18 hour period of snow in the mountains,
more ridging is expected by Thursday and Friday with a return of
dry and mild weather for the end of the week. Friday and Saturday
look to be the warmest days of the lot as warm southwest flow
aloft will be well entrenched over the desert southwest. no other
precipitation is expected before next Sunday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 953 AM MST Sat Feb 24 2018

VFR and no precipitation through Sunday. Main impact will be the
strong west winds which are already occurring across the metro
area terminals. Still expecting the strongest winds at BJC with
40 kt gusts. DEN could see gusts to 35 kts through about 22Z from
the 260 to 290 direction. APA could see gusts to 30 kts. Expect
the wind direction to gradually become more WNW after 20Z. Winds
should decrease at DEN and APA after 23Z and turn to drainage
after 04Z around 10-12 kts. Winds may stay up at BJC through 02Z
before going to WSW around 10 kts or less. By late morning Sunday
winds should turn to west again with vertical mixing, but should
be much lighter than today, as of now expecting west winds 10-15
kts or so by midday Sunday.


&&

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

$$

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



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