Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 151816
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1116 AM MST Fri Dec 15 2017

.UPDATE...
Issued at 1058 AM MST Fri Dec 15 2017

No changes from the previous forecast. Winds will start to
decrease this afternoon over the higher terrain and across the
plains.

&&

.SHORT TERM...(Today and tonight)
Issued at 330 AM MST Fri Dec 15 2017

Northerly flow aloft over the state is evident on satellite, due
to the West Coast ridging and an upper trough from the Great Lakes
down to Baja. A shortwave ridge will push over the area today,
helping with mountain top stability. Westerly winds are and will
continue to downslope this morning over the eastern slopes of the
Front Range mountains and foothills as a surface trough deepens
along the Front Range urban corridor. Temperatures over the
aforementioned areas are already increasing early this morning.
Looking at forecast cross-sections, a mean state critical layer
will exist this morning, and around 18z, a reverse shear
profile will be expected. With 30kts of cross mountain flow this
morning and into the early afternoon, expect localized gusts of 50
to 60 mph in the foothills, before the pattern breaks down by mid
afternoon. Warm advection under the upper ridge and clear skies
today will allow for max temperatures in the 50s to near 60 over
the plains, 40s for the foothills and South Park, and upper 20s
to upper 30s for the mountains. Winds may be light enough over the
South Platte River Valley to keep the inversion in place, so
temperatures in this area, especially near Greeley, may not even
reach 50 degrees.

An approaching upper trough from the west tonight will increase
mid to upper level moisture, with high level clouds including a
mountain wave cloud to spread over the area. This will result in
warmer minimum temperatures along the foothills and urban
corridor. A surface low will push south after midnight, allowing
for a pre-frontal northerly push to spread down from Wyoming.

.LONG TERM...(Saturday through Thursday)
Issued at 330 AM MST Fri Dec 15 2017

Saturday will start off with mild southwest flow aloft in advance
of the next elongated upper level trough moving into the Northern
Rockies and Great Basin.  At the surface, low pressure over
southeastern Colorado will continue to sink southward, with a cold
front pushing across northeast Colorado by late morning. We
should see an early day/noontime high, with temperatures slowly
slipping through Saturday afternoon. An earlier frontal passage
could keep temperatures several degrees cooler for highs,
especially with cloud cover, so we`ll have to watch that closely.

For Saturday night, the odds of much precipitation are slipping
away as the upper level trough splits significantly, with the
majority of the energy shifting through the Desert Southwest and
into far northern sections of Mexico. Low level upslope flow
should continue across the plains/foothills, but deep moisture is
scarce considering the energy split. As a result, if we see any
precipitation Saturday night into Sunday morning, the
temperature/moisture profiles would suggest a threat of freezing
drizzle more than snow. Cross sections show coldest temps in the
moist layer just shy of the -10C threshold for ice nucleation.
We have therefore added some freezing drizzle to the forecast.
However, any precipitation amounts at this time are expected to
be quite light. In fact, almost all models runs have been trending
downward and very light with QPF over the last 24 hours.

Subsidence and stronger drying kick in Sunday afternoon into the
evening as trough axis passes by. Then we`ll return to drier and
warmer weather Monday through Wednesday. At this time, Wednesday
is still shaping up to be the warmest and possibly windiest day
in advance of a stronger trough moving across the northern
Rockies. This system will likely bring much colder temperatures
along with a chance of snow to the forecast area by Wednesday
night and Thursday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1058 AM MST Fri Dec 15 2017

There is some uncertainty about the timing of westerly winds this
afternoon at KDEN and KAPA. Speeds are expected to be below 20kts
with VFR conditions.


&&

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

$$

UPDATE...Sullivan
SHORT TERM...Kriederman
LONG TERM...Barjenbruch
AVIATION...Sullivan



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