Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 172245 CCA
AFDBOU

Area Forecast Discussion...CORRECTED
National Weather Service Denver/Boulder CO
345 PM MST Sat Feb 17 2018

.SHORT TERM...(This evening through Sunday)
Issued at 324 PM MST Sat Feb 17 2018

Strong westerly flow aloft is going to continue over the state
tonight with a region of strong flow at 700 mb impinging on the
high country overnight. As the flow increases, winds over the
ridges and down the east slopes of the foothills will strengthen.
The High Wind Warning that has been in effect will continue
through tonight. With 700 mb winds expected to increase to 60 to
70 knots, winds at the normally windy locations could top 80 MPH
overnight. The strongest winds are forecast to be in place from
03z thru 15z, so an extended period of windy conditions will be
possible. The airmass over the state is currently dry, so no other
sensible weather is expected overnight. The dry conditions should
continue through much of tomorrow, before mid-level moisture
begins increasing over the northern mountains late in the
afternoon. On the plains, the dry westerly flow will bring a very
warm day with highs into the mid 60s across the plains. Over the
eastern Palmer Divide, the warm temperatures will drop humidities
down to the lower or mid teens. This may lead to elevated fire
danger levels. A Red Flag Warning will be issued for Sunday
afternoon.

.LONG TERM...(Sunday night through Saturday)
Issued at 324 PM MST Sat Feb 17 2018

Main forecast concerns shift to winter weather and a return to
snow and very cold temperatures.

Sunday night and Monday, a broad deep trough carves into the Great
Basin. Southwest flow aloft will gradually moisten over the
forecast area as some moisture is drawn up through Arizona. This
can be seen in the 700-500 mb specific humidity plots with values
of 2 g/kg advecting into the state. Our forecast area will remain
on the edge of the better moisture plume, but zone 31 near Rabbit
Ears Pass could get in on some heavier snowfall starting late in
the evening or overnight Sunday night, as this area is less
sheltered by southwest flow. There is potential for a heavier snow
band to set up in this area due to the mid level frontogenesis
and location of the upper level jet through Monday, so we`ve
issued a Winter Storm Watch for Rabbit Ears Pass area starting
late Sunday evening and continuing through Monday and Monday
night. Confidence is only marginal at this point, however, as if
moisture band does not make it to Zone 31 tomorrow night the
duration of the storm could be limited to just Monday into Monday
evening, reducing snow accumulations. We do expect that band of
snow to gradually push eastward across the rest of the mountains
Monday and Monday night as the upper trough shifts eastward during
this time frame. At this point, accumulations would most likely
support an eventual Winter Weather Advisory with 4-10 inches of
snow over most of the mountains, but up to 15" in Zone 31 as long
as the band develops there Sunday night.

On the plains, a shallow but strong cold front will back across
the area from the northeast. The shallow nature of the cold air
will once again allow for potential for freezing drizzle starting
Sunday night. This can be diagnosed by the latest model soundings
and cross sections showing limited potential for ice nucleation
and considerable supercooled liquid water (i.e. temps warmer than
-10C in the moist layer). The cold air does get deeper during the
 day on Monday, so that should help freezing drizzle eventually
 turnover to snow. Best snow accumulation should be Monday night
 when main jet energy shifts across the plains and moisture
 deepens. At this time, expecting this to be mainly a light snow
 vent with 1-4" across the lower elevations.

Temperatures will be much colder starting Monday. Reduced high
temperatures a few more degrees as we see little hope for warming
from Sunday night`s lows given stratus deck and light freezing
drizzle or flurries through the day. Single digit lows are likely
across the plains Monday night, and then barely reach the teens on
Tuesday. Drier air moves in Tuesday with lingering light snow
decreasing and coming to an end.

Drier weather will develop by Wednesday and Thursday with
moderating temperatures. But readings should still stay below
normal with general troughiness over the state. Then next weather
system could bring a return of just some light snow by Friday and
Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 324 PM MST Sat Feb 17 2018

No ceiling or visibility problems tonight or Sunday. However,
gusty winds in the foothills will spread out onto the adjacent
plains. Primary impact will be at KBJC where westerly wind gusts
could approach 40 knots at times overnight. No other aviation
impacts.

&&

.FIRE WEATHER...
Issued at 324 PM MST Sat Feb 17 2018

With the dry and warm conditions expected tomorrow, will upgrade
the Fire Weather Watch on the eastern Palmer Divide to a Red Flag
Warning. Will not include Douglas County and western Elbert County
(zone 41) in the warning since satellite imagery earlier today
showed that a good portion of the zone still had a few inches of
snow on the ground. Soil moisture conditions in this area are
probably sufficient to limit any fire activity tomorrow. However,
further east, the dormant vegetation is probably dry enough that
dangerous fire behavior would be possible. Humidity values will
bottom out in the mid teens, with gusts well into the 20s and
afternoon temperatures in the mid and upper 60s.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 11 AM to 5 PM MST Sunday for COZ246-247.

High Wind Warning until 8 AM MST Sunday for COZ033>036.

Winter Storm Watch from Sunday evening through late Monday night
for COZ031.

&&

$$

SHORT TERM...Dankers
LONG TERM...Barjenbruch
AVIATION...Dankers
FIRE WEATHER...Dankers



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