Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 170359 CCA
AFDBOU

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

.UPDATE...
Issued at 850 PM MST Sat Dec 16 2017

Circulation around a mid-level cyclone now passing over the Texas
panhandle will continues to deepen/strengthen the east-northeast/
upslope flow covering eastern Colorado. This moistening upslope flow
has produced a solid deck of stratus, with its leading edge cover
passing through Ft.Morgan about an hour ago. At its present forward
speed, and based on HiRes models, should see this low cloud deck
(ceilings AOB 1500 ft AGL) reaching Greeley area within the next 30
minutes or so and not too much longer after that (about 04z) in Ft.
Collins. Also expect to see these clouds to move into the Denver
metro area about 05z. The initial surge of moisture appears quite
shallow, although model cross sections and soundings show this
saturated layer deepening after midnight. Once clouds thicken
sufficiently, areas of light freezing drizzle and snow flurries are
expected to form, probably first along the Wyoming border and up
against the northern Front Range foothills. Ice accretion rates
should remain less than 0.05 inch per hour, and any very light snow
accumulation should be confined mainly to the northern foothills and
Cheyenne Ridge area in Larimer County according to the latest model
guidance.

Our forecast grids have been adjusted to speed up the arrival of
these low clouds and the formation of light pcpn on the plains. Also
added patchy fog on the plains after midnight, but almost left it
out of the forecast as temp/dewpoint spreads appear just borderline
for fog.

&&

.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 00Z TAFS through 00Z Sunday evening)
Issued at 850 PM MST Sat Dec 16 2017

VFR CIGs and VSBYS at Denver area terminals as of 0345z/Sun are
likely to go MVFR between 0430 and 0530Z this evening with
arrival of a thick deck of low clouds from the northeast. Should
see CIGS quickly lower to around 1500-2000 ft agl once this deck
moves in on northeast winds of 10-15kts. Then, IFR conditions
after midnight, with a slight chance of freezing drizzle and light
snow with ceilings possibly lowering below 500 ft AGL. These ILS
conditions are expected to linger through 12z, with only gradual
improvement through the early daylight hours on Sunday. Snow
accumulation is not anticipated, although some areas may see a
light coating of ice as a result of the freezing drizzle. Should
see a return to VFR conditions after 16Z Sunday.

&&

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

$$

UPDATE...Baker
SHORT TERM...Gimmestad
LONG TERM...Hanson
AVIATION...Baker



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