Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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101
FXUS65 KBOU 121728
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1028 AM MST Tue Dec 12 2017

.UPDATE...
Issued at 1028 AM MST Tue Dec 12 2017

No updates needed to today`s forecast given the strong upstream
ridge and very dry conditions throughout the atmospheric column.
High temperatures look on track as the latest HRRR runs showing
mid 60s across the urban corridor to near 60 further east. Latest
satellite imagery shows almost no clouds upstream and with low
moisture don`t expect many to form today, so will keep the mostly clear
skies in the grids and we also shouldn`t have much trouble
reaching the high temps today. Winds will stay down this afternoon
most areas other than at and above treeline along the Continental
Divide.

Watching the potential for mountain snow late Wednesday night
into Thursday with a strong jet and weak trough passage. Models
are agreeing on chances for accumulating snow. Details in this
afternoon`s forecast package.

&&

.SHORT TERM...(Today and tonight)
Issued at 340 AM MST Tue Dec 12 2017

High amplitude ridge of high pressure covers much of the Western
U.S. centered over the Great Basin this morning. Satellite imagery
showing nearly cloud free over all of Colorado and surrounding
states,  and looks like this will continue to be the case for
much of today. Temperatures will be warmer today with readings up
into the 60s across the plains as 700mb temperatures climb to
around +4C. Water vapor imagery showing a wave rotating across
the top of the ridge which will drop down into the northern high
plains later tonight. The result will be a weak surge of high
pressure into the northeast plains by early Wed morning with some
uptick of winds over the plains and a bit of increase in mid- hi
level clouds.

.LONG TERM...(Wednesday through Monday)
Issued at 340 AM MST Tue Dec 12 2017

The passing shortwave moving into the Great Plains will push
cooler air into the forecast area Wednesday along with strong
winds for the plains. Highs should be 5 to 10 degrees cooler than
today`s readings, and should keep humidities wetter than criteria
for reduced fire danger. However, north to northwesterly winds
will be blowing at 25 to 40 mph, with highest speeds over the
northeastern plains, which may keep some concern over the dry
grasses. Another disturbance from a jet diving south over the
northern Rockies will bring snow to the mountains by that
evening. Weak upward QG motion and surface winds turning upslope
near the foothills will allow for a slight chance of precipitation
along the urban corridor, mainly west of I-25. The mountains1
should see around 1 to 4 inches of snow, with the lower foothills
and western urban corridor possibly seeing up to a quarter inch
or less. Both the latest GFS (06z) and ECMWF (00z) runs have
slowed their progression, and have become a little more bullish
with the precipitation across the plains Wednesday night and into
Thursday morning. For now, have increased PoPs slightly Wednesday
night, with PoPs hanging on to the mountains Thursday morning.

On Thursday, the jet will continue pushing into the state, with
110 kts estimated overhead. High surface pressure to the north
with moderate downward QG vertical motion will culminate into
even stronger winds for the plains, 35 to 50 mph. The colder
temperatures and relatively higher humidity will spread over the 1
forecast area to limit the fire danger under the strong winds.
Highs will likely only be in the 40s over the plains and in the
20s to low 30s for the mountains, much closer to average for this
time of year than we`ve seen since last Thursday. A surface trough
will dig in quickly along the Front Range urban corridor Thursday
night as an upper ridge pushes in from the west. This will likely
produce warm downslope winds Thursday night into Friday morning
along the east slopes of the Front Range mountains, foothills and
along wind prone areas along the western urban corridor.

Friday will quickly warm up ahead of the next weather system,
with readings 10 to 15 degrees warmer than Thursday. An upper
trough will move in over the PacNW to weaken the strong persistent
West Coast Ridge and push over the state to bring snow to the
mountains likely as early as Saturday morning. The GFS, EC and
Canadian models have all slowed the progression of this system as
well as dug more energy to the south, but with widely varying
positions of that southern energy. The GFS even slightly resembles
a Four Corner Low, while the Canadian puts it off the coast of
southern California. Have slightly increased PoPs with this
system, but with such large differences in energy location, the
change was little.

This system seems to be the beginning of a pattern change where
the West Coast ridge really starts to break down with stronger and
deeper troughs pushing into the west coast. Northerly flow Sunday
behind the trough will likely lead to a cool day, possibly
continuing into Monday with the slowing of the model solutions.
Will leave the slight warming and drying pattern for Tuesday that
the average of the models produced for now - but with a changing
weather pattern, more adjustments will be needed.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1028 AM MST Tue Dec 12 2017

VFR conditions through Wednesday and no precipitation expected.
Winds should not be an issue at the TAF sites in terms of speed,
with diurnal winds 10 kt or less through Wednesday morning. By
midday Wednesday a cold front will impact the metro area terminals
with a wind shift to north around 18Z and include gradually
lowering CIGs, though at this time they should remain VFR.

&&

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

$$

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



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