Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 162013
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
113 PM MST Sat Dec 16 2017

.SHORT TERM...(This evening through Sunday)
Issued at 1247 PM MST Sat Dec 16 2017

Upper trough will drag across Colorado later tonight into
Sunday. There will continue to be a layer of dry air beneath the
high clouds, with low level cold/moist air dropping into eastern
Colorado from areas further north later tonight. Upstream low
level air is pretty dry this afternoon, but there has been some
snow falling into the air which will be coming south, and it is
becoming saturated. The combination of upslope winds and some lift
from above will likely produce areas of freezing drizzle late
tonight. We will need to monitor for something more significant,
but given the limited moisture depth and light winds, it seems
like the idea of just a small amount of drizzle is right.

On Sunday the forcing for lift will pass in the morning with
surface winds becoming southerly. This should help dry out the
low levels in the Denver area, though there will still be some
higher clouds. We will likely see the sun return in the afternoon.

Forecast temperatures tonight are above guidance and look good
based on the upstream readings, with lots of temperatures around
freezing and saturating airmass in central and northern Wyoming.

I nudged temperatures up a little for Sunday afternoon in
accordance with latest guidance. This looks good provided that
there will be some sun, with southerly surface winds on the plains
and some warming aloft redeveloping.

.LONG TERM...(Sunday night through Saturday)
Issued at 1247 PM MST Sat Dec 16 2017

First half of the week is quiet with surface ridging bringing
moderate temperatures and no precipitation. 500 mb flow begins a
change during this period. Our familiar ridge over the eastern
Pacific and northwest flow undergoes a change as low pressure drops
south along the Canadian Pacific coast, and into the intermountain
west. ECMWF and GFS in good agreement bringing center of the low to
the Four Corners region Thursday morning. Meanwhile a northern
stream trough moves southeast out of the Canadian Rockies and across
the Canadian plains. A surface front associated with the upper flow
will move south across Colorado early Thursday, turning temperatures
sharply colder. A narrow band of moisture will move along the
surface front and upper trough, and snow is forecast to move in to
the northern mountains after midnight Wednesday night and spread
south and east across the forecast area through Thursday morning.
Good chances for accumulating snowfall during this period. Mixed in
some higher pops to get to likely category in the 06-18z timeframe,
based on 100 kt 300 mb jet sagging across Colorado and associated
strong vertical ascent.

Temperatures will begin the week 10-15 degrees above normal, and
fall to that much below normal for the latter half of the week. Low
level flow remains northerly, with a reiforcing shot of cold air set
to arrive on Saturday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1247 PM MST Sat Dec 16 2017

MVFR and IFR ceilings will develop around 06z with areas of light
freezing drizzle. There may be localized LIFR conditions for a few
hours. The drizzle should end by 16z Sunday with slowly improving
ceilings in the morning. VFR conditions are expected to prevail
after 19z, with the low clouds breading up. There may still be a
need for instrument approaches to KDEN in the afternoon from
lingering areas of low clouds.

&&

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

$$

SHORT TERM...Gimmestad
LONG TERM...Hanson
AVIATION...Gimmestad


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