Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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617
FXUS65 KBOU 131034
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
334 AM MST Wed Dec 13 2017

.SHORT TERM...(Today and tonight)y
Issued at 325 AM MST Wed Dec 13 2017

A brief change to winter is on the way for Colorado tonight with
the main impact of light snowfall over the mountains and adjacent
plains for tonight.

Initially, there is a weak wave that is dropping southeast into
the central plains early this morning with a batch of high level
clouds and a weak frontal system. This system will drop
temperatures across lower elevations of 10 to 15 degrees with
readings in the lower to mid 50s this afternoon. The gusty winds
that have been occurring overnight in the mountains and foothills
will subside towards daybreak as the weak frontal system pushes
across the area and weakens the surface gradient.

A stronger system now seen over the Pacific Northwest will drop
southeast into Colorado this evening. There is some modest amounts
of QG ascent along with a moist, shallow upslope flow developing.
Local snow model yielding 2-4 inches in the mountains and
foothills and this looks reasonable given latest QPF from models.
The system is coming in stronger than previous forecast so will
be increasing snow chances and amounts over the urban corridor and
especially sections south and west of Denver. Over the I-25
corridor, expect 1-2 inches of snow, higher amounts west and south
of Denver and over the Palmer Divide. This is a rather fast
moving system so snowfall window around 4-6 hours and will begin
to decrease towards daybreak on Thursday. Further east over the
northeast plains, impacts will be less as region is further away
from the strongest QG lift and moisture.

.LONG TERM...(Thursday through Tuesday)
Issued at 325 AM MST Wed Dec 13 2017

Light snow ongoing early Thursday morning, especially along and
west of the I-25 corridor, will decrease from the north - with
Larimer and Weld counties probably clearing out already. Even
though only another quarter to half inch is expected over the
urban corridor and surround plains, cold morning temperatures
along with the overnight snow may create some impact to Thursday
morning`s commute, with road temperatures getting colder. The
storm looks to linger for longer than previously forecast, with
isolated showers lasting over the plains through the afternoon.
Went ahead and lowered high temperatures assuming clouds will hang
around. The plains should remain in the upper 30s to lower 40s.
The high country will be clearing from north to south as well,
with another half inch up to 2 inches of additional accumulation,
for a storm total of 2 to 6 inches. The bigger impact will be the
strong northerly winds that push across the eastern plains during
the day, with speeds of 30 to 50 mph peaking in the early
afternoon. This will make travel difficult for light and high
profile vehicles across east to west oriented roads.

Thursday night will feature clearing skies, cold temperatures and
weakening winds. The exception will be along the foothills where
a deepening surface trough will produce some gusty warming
downslope winds. Friday will feature upper ridging, warm
advection and clear skies to allow for temperatures to warm above
normal again.

A positively tilted open trough will drop out of the PacNW Friday
night. The ECMWF and Canadian have similar solutions, faster than
the GFS which solution ends up with a cutoff low southwest of the
Four Corners. Every model pushes snow into the mountains Saturday
morning, spreading over the plains Saturday night possibly
lasting through Sunday. This system looks quite similar to
tonight`s and Thursday morning`s system, but likely with more
upslope flow, which would mean a better chance and possibly
higher amounts of snow for the urban corridor. Temperatures
Saturday will depend on the speed of the system, I went with the
warmer solution of the GFS since the EC trended slower with its
latest run. Then look for a colder and possibly snowy Sunday.

As the trough moves east of the area, a weaker ridge will be over
the west coast, placing Colorado under a dry northwesterly flow
aloft Monday and Tuesday. A pattern change is present in all the
models with a longwave trough to start pushing in from the Gulf of
Alaska. This will flatten the West coast ridge and turn flow
westerly over the state. The rest of the week should start to see
colder temperatures and a more active pattern.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 325 AM MST Wed Dec 13 2017

A batch of high level cloudiness moving across terminals through
mid morning hours. There will be a brief reflection of the front
in the wind fields between 11-15z as winds shift east and
southeast for a brief time. A stronger system will drop into the
area by later this evening with low clouds and light snow
developing between 05-07z and then a period of MVFR/IFR and light
snow from 07-11z and accumulations of 1-2 inches at
terminals...highest at BJC/APA.

&&

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

$$

SHORT TERM...Entrekin
LONG TERM...Kriederman
AVIATION...Entrekin



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