Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 181609
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
909 AM MST Mon Dec 18 2017

.UPDATE...
Issued at 900 AM MST Mon Dec 18 2017

Only minor adjustments planned for the grids today. Some high
cloudiness from the northwest will increase across the region
today. Otherwise dry, mild and stable.

&&

.SHORT TERM...(Today and tonight)
Issued at 440 AM MST Mon Dec 18 2017

Dry and subsident airmass over the area will remain in place
today. Temperatures will warm considerably with 700 mb temps
warming to around +1C along the Front Range, and downslope
component/sunshine to aid warming. We should see most plains
locations reach the mid to upper 50s, with lower 50s around Fort
Collins and Greeley where the cold pool is deeper. There is some
shallow moisture trapped in the high country, so expect a bit more
cloud cover there, while most of the cirrus should stay thinner
over the plains barring significant mountain wave enhancement.

With regard to winds, there is a mean state critical layer (flow
reversal aloft) which will stay in place until upper level speed
max passes later this morning. However, cross mountain component
is only around 25 knots. Peak gusts may be able to reach 40-50 mph
in some of the wind prone areas of the higher foothills this
morning, but then gradually relax as shear profile becomes less
favorable for significant mountain wave deflection today. Gusty
winds will be more likely to spread across the northern border
area with improved mixing there this afternoon.

Dry conditions will remain in place tonight. Temperatures will be
a few degrees above normal.

.LONG TERM...(Tuesday through Sunday)
Issued at 440 AM MST Mon Dec 18 2017

Medium range models continue to advertise a big change in the
weather pattern, starting Thursday morning, that is going to
bring much colder weather and snow to northern Colorado in time
for the holiday weekend. Tuesday and Wednesday are still going to
be somewhere between mild and warm, with highs in the upper 50s
and lower 60s across the plains. The warm weather will be courtesy
of moderate westerly flow aloft under a weak upper level ridge.
Surface pressure gradients won`t be particularly strong, so the
mild temperatures will be accompanied by weak winds.

Everything starts changing on Thursday with the arrival of a
surface cold front and an upper level storm system. The surface
cold front will drop across the plains Thursday morning, sometime
between and 6 AM and noon. Upslope flow behind the front and
dynamic forcing with the upper level storm will combine to produce
snowfall over the mountains and along the front range urban
corridor. Present indications are that the upper level storm
system will not hang around very long, which will limit the
amounts of snow that fall. By Thursday evening the upper trough is
forecast to be moving over western Kansas with flow aloft
transitioning to downslope northwesterlies. Temperatures through
the day will remain in the upper 20s or lower 30s as the snow
falls. Mountain areas should see a few inches of snow, while the
urban corridor and plains only receive an inch or two. Friday will
remain cool, but dry. Then Saturday is expected to bring another
surge of Canadian air with temperatures dropping into the teens or
cooler. Quasi-geostrophic diagnostics show that Saturday`s weather
system is going to be stronger than Thursday`s, but the airmass is
going to be colder with limited amounts of moisture. This will
probably result in a dry type of snow with light accumulations
again. Sunday morning could be the coldest of the week, with MOS
guidances indicating sub-zero readings at KDEN. For now, our
forecast is holding Sunday morning`s low just above zero. Sunday
and Monday should be dry, but if the ECMWF is right, Christmas Day
is going to be a cold one. The GFS shows some moderation in
temperatures by Christmas Day. Will have to wait and see which of
the solutions is right. Do not see any need for highlights at this
time, but messaging is certainly going to be talking about the
cold temperatures that are on their way.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 900 AM MST Mon Dec 18 2017

VFR conditions will persist through Tuesday. South/southwest winds
this morning may become light/variable through this afternoon,
with a bias toward northeasterly 20Z-23Z. KBJC, expect light and
variable winds today. Normal drainage winds will redevelop 00Z-
02Z tonight.
&&

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

$$

UPDATE...Cooper
SHORT TERM...Barjenbruch
LONG TERM...Dankers
AVIATION...Cooper



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