Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 171756
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1056 AM MST Fri Nov 17 2017

.UPDATE...
Issued at 1044 AM MST Fri Nov 17 2017

Snow has moved into the higher terrain with winds continuing on
the plains. Gusts of 25 to 30 mph will continue through the
afternoon. The front is currently over the SLC area moving SE into
CO and is expected to impact the region later this evening. WV
imagery combined with lightning shows the enhanced lift ahead of
the trough that will help to increase snow rates in the mountains
from 1-2 inches per hour to possibly up to 3 for the Park and Gore
ranges. Amounts still look good as does the WSW with the highest
amounts in the areas with the best lift per wind direction and jet
placement. With the cold frontal passage around 7-8 pm through the
Urban corridor and winds switching to NW than N added some light
snow with the rain as the column could be cold enough after
frontal passage with enough ice nucleation to produce a wet mix of
rain and snow. Upped daytime temps a few degrees to account for
the continued warm and strong prefrontal SW flow. Otherwise
forecast looks good.

&&

.SHORT TERM...(Today and tonight)
Issued at 245 AM MST Fri Nov 17 2017

...A Winter storm today and tonight will impact the mountains
with snow, the plains with mainly winds...

A positively tilted upper trough over the Pacific Northwest, and
another subtropical jet stretching across California, Nevada and
Utah will combine forces for a windy winter storm today through
tonight. Strong west-southwest flow aloft has kept minimum
temperatures across most of the area quite moderated, with
readings in the 50s and 60s over the foothills, plains and even
some in the high mountain valleys.

Overall the system has slowed slightly, with not much snow into
the far northern mountains, but rain and snow has nudged into the
I-70 corridor from the southern subtropical jet. Expecting the
PCNW upper trough to start digging southeast pretty quickly, which
will be bringing much colder air. Believe we`ll start to see the
heavier bands of snow arrive over the northern mountains before
sunrise. Snowfall rates in the mountains today will be in the 1/2
to 1 inch per hour rate, with some bands having the potential to
produce up to 2 inches or more in an hour. Have decreased some of
the forecasted snowfall total over South Park as it seems they`ll
be struggling with downsloping winds almost all day. Still looking
for snowfall totals in the mountains, especially above 9000 feet
to be in the 6-12 inch range, with some spots reaching 18-24
inches. The best chance for the amounts above a foot will be over
the mountains of Rocky Mountain National Park and of the Park and
Gore ranges. The strong winds gusting to 50 mph from this system
will reduce visibilities in blowing snow.

With the strong, mostly downsloping pattern of the southwest
winds, have decreased pops over the Front Range urban corridor for
most of this morning and early afternoon, as well as increased
max temperatures as well. As northeastern Colorado becomes under a
favored location from both jets later this afternoon, with upward
QG vertical motion in the 20 to 25 mb/hr, have increased pops
across the plains with the strong cold front expected to push down
soon after. An precipitation that falls over the plains will start
as rain, with snow likely mixing in after 8 or 9 pm. Strong cold
bora type winds will follow in behind the front, likely gusting to
40 mph across the plains, and temperatures plummeting. Believe
precipitation will be hard to form just east of the foothills
along the urban corridor especially northern areas that will fight
the downsloping effects of northerly winds off the Cheyenne Ridge
as well. By midnight, strong subsidence with downward QG motions
of 35 mb/hr will have diminished most of the precipitation, except
light orographic snow possibly still falling in the mountains,
with only another inch to fall at best. Temperatures across the
area will be down in the teens and 20s, with winds slowly
decreasing.

.LONG TERM...(Saturday through Thursday)
Issued at 245 AM MST Fri Nov 17 2017

A broad flat ridge of high pressure will build over the region
over the weekend with a drying and warming trend in store for
Colorado. There will still be some breezy conditions on Saturday over
the mountains and plains in the wake of Friday nights wave and
lingering subsident airmass over the region. A weak ripple moves
through the fast northwest flow on Sunday night with some enhanced
winds for the mountains and east slopes. Cross sections show cross
barrier flow around 40kt and some mountain top stability so could
see some higher gusts in the 50-60 mph range over wind prone
areas. Airmass looks quite dry with this system so no precip
expected.

A stronger system will move over Colorado on Monday
night with the next chance for snow in the mountains and cooler
temperatures for all areas on Tuesday. This is a fast moving
system so expect any snow accumulation will be on the lighter
side in the mountains. The plains could see an isolated snow
shower but moisture is limited.

The ridging aloft rebuilds for Tuesday through Thursday with
another dry and warm period for this time of year. The GFS is
advertising the next system to slide over northern Colorado in the
Friday night time frame while the European solution maintains more
ridging and continued dry. Certainly quite a bit of varied model
solutions going into next weekend so won`t put much stock in any
one solution.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1044 AM MST Fri Nov 17 2017

Winds will continue over the field through the period with
gusts into the 32-36 mph range from 19 to 23z. Winds will
gradually switch to a NW by 23z before turning northerly with
frontal passage around 02z. Expect gusts from 32 to 34 with
frontal passage and dropping temperatures. A mixture of rain and
snow will be possible with MVFR conditions between 02 and 06z. By
08z conditions will gradually improve for BJC and DEN and 10z for
APA. VFR conditions are expected by then with winds switching
around to drainage by 14z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 5 AM MST Saturday for COZ031-033-034.

&&

$$

UPDATE...Bowen
SHORT TERM...Kriederman
LONG TERM...Entrekin
AVIATION...Bowen



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