Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 132154

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

.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

.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 243 PM MST Wed Dec 13 2017

light easterly winds are continuing at DIA. Models keep
this direction going this evening. At about 05Z-06Z, a front with
northerly flow behind it. After 04Z ceilings will begin to lower
with BKN010 by 06Z. There should be snow as well later tonight,
after 06Z will go with with 1 1/2SM -SN after 06Z.  Conditions
will begin to improve after 15Z Thursday morning.




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