Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 140157
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
657 PM MST Wed Dec 13 2017

.UPDATE...
Issued at 645 PM MST Wed Dec 13 2017

Satellite showing a trough in the northwest flow aloft diving
southeast over eastern Idaho and western Wyoming. The trough will
continue to move southeast and lift ahead of the trough will
spread over most of Colorado tonight. Some echos are starting to
show up on radar. The lower levels of the airmass are still dry,
so most of this is likely virga. As the airmass moistens from the
top down, snow is expected to increase this evening over the
mountains and spread onto the Front Range late tonight.

The biggest change to the forecast was to add an inch or two of
snow to Park county where the HRRR is showing more than the other
models. Expected southeast surface winds should support slightly
higher snowfall totals here. Otherwise, only other changes were to
line up with the current trends.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 243 PM MST Wed Dec 13 2017

North and northwesterly surface winds are covering the plains
right now. The radars and satellite pictures upstream show some
precipitation to our northwest in Wyoming, Idaho and Utah. The
water vapor pictures show the upper trough pushing southeastward
over eastern Idaho. Models show this trough to move across the CWA
tonight. Strong northerly flow aloft moves into Colorado on
Saturday. The is a shot of upward vertical velocity on the QG
Omega fields this and overnight, with weak downward motion on
Thursday. Models show northerly low level winds this evening. They
become northwesterly out on the eastern plains after 06z, but
stay almost due north over the western plains. For moisture, cross
sections have deep moisture over the CWA from about 06Z tonight
through 15Z Thursday morning. The mountains have pretty decent
moisture this evening already. On Thursday, moisture decreases
quite a bit, but there is still some over the mountains and
plains in the afternoon. The QPF fields on the NAM/GFS/ECMWF are
in some agreement concerning precipitation amounts for the CWA
later tonight into Thursday morning. Even the snowfall amount
agree pretty well. Will leave the current, "likely" pops going
tonight into Thursday and will also keep accumulations light. No
highlights, but at least we should get a bit of moisture. For
temperatures, Thursday highs look to be 3-6 C cooler than this
afternoon`s.

.LONG TERM...(Thursday night through Wednesday)
Issued at 243 PM MST Wed Dec 13 2017

By Thursday evening the upper trough that brought snow to the region
will move SE. Increasing subsidence behind it will help to clear out
the snow and the evening hours with overnight lows dipping into the
lower 20s on the plains. Directly along the foothills however,
models show an increasing lee side low that will help to keep lows
slightly warmer due to downsloping winds. Friday will see increased
ridging with clearing skies and highs reaching back into the mid to
upper 50s.

Over the Pacific NW a positively tilted trough will move eastward to
bring the region another round of snow. Models are still in
disagreement with the path and timing of the system with the GFS
creating a cut off low but moving the main branch of energy over CA
and the Gulf. The EC has a similar feature but separates it with an
open wave over the four corners. This in combination with a surface
cold front Saturday afternoon will bring increased chances of snow
to the mountains and foothills early Sunday. Increased upslope and
cooler temperatures behind the frontal boundary will play a large
part in how the amounts will shape up so will resist from giving any
numbers here. Will keep a high chance in the mountains with a chance
along the urban corridor and a slight chance further east.
Temperatures will drop slightly on Saturday with highs in the 40s
dipping into the teens and low 20s overnight into Sunday. With snow
and cloudy conditions temperatures will not recover with highs only
getting slightly above freezing.

With the western ridge breaking down NW flow will start to turn more
westerly and help to create a more active pattern.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 645 PM MST Wed Dec 13 2017

East/northeast winds 6-8 kts expected through 04z with prevailing
VFR conditions, ocnl ILS conditions with cigs around 050 kft agl.
MVFR cig/vsby restrictions developing 05z-15z period with
north/northwest winds 8-14 kts following fropa. Localized IFR
cigs/vsbys possible under any developing snow bands. At this time
expect light snow with a chance of brief moderate snowfall. Snow
showers will diminish after 15z with ILS cigs until 17z then VFR
returning by late Thursday morning/afternoon.

&&

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

$$

UPDATE...Meier
SHORT TERM...RJK
LONG TERM...Bowen
AVIATION...Cooper



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