Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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709
FXUS65 KBOU 211719
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1019 AM MST Sat Jan 21 2017

.UPDATE...
Issued at 1012 AM MST Sat Jan 21 2017

No significant changes needed although may need to add some low
pops over nern CO for this evening as wk gq ascent combines with
decent lapse rates and moisture to produce at least a slight
chance of -shsn.

UPDATE Issued at 651 AM MST Sat Jan 21 2017

Widespread fog over southern Lincoln County has been persistant.
Have issued a Dense Fog Advisory to cover the visibility less
than zero, likely impacting traffic. A finger of the dense fog is
visible stretching into northern Lincoln county, but isn`t
widespread, so chose not to add zone 46 into the advisory. Some hi
resolution models show the fog heading further north and west into
Washington, Adams and Arapahoe counties, but the 11u-3.9u IR
satellite has yet to show any of this extension trend. Expect this
fog will dissipate by late morning, if not before.

&&

.SHORT TERM...(Today and tonight)
Issued at 320 AM MST Sat Jan 21 2017

An upper trough stretching from the Pacific Northwest southeast
into Arizona and Mexico will be transitioning across the area
today. Currently clouds and showers are starting to increase
across western Colorado with only high clouds streaming across
the forecast area. A few areas of fog across Middle Park as well
as east of the Palmer Divide are impacting visibility, but are not
expected to last much into the morning. Moisture will be
increasing over the mountains today with light snow likely by mid
afternoon. A surface trough should keep the lower foothills and
plains dry in downsloping flow. A vertically stacked low will
develop over the TX/OK panhandles by early this evening. Surface
winds will turn northerly in response, in which drier air will
move in. There may be a brief period of semi upslope flow, though
mostly over the Palmer Divide for a few possible snow showers this
evening, though amounts will be little to none. Some models
showing possible showers over the Denver area, however with winds
turning more downslope through the evening and downward QG motion,
decided against following them. Snow will decrease over the
mountains by late evening, with 1 to 4 inches expected. The
subsidence on the backside of the trough and strong northwesterly
flow aloft will increase surface winds over the mountains tonight,
gusts up to 45 mph will be possible. This may produce blowing snow
conditions overnight.

.LONG TERM...(Sunday through Friday)
Issued at 320 AM MST Sat Jan 21 2017

High pressure ridge aloft will transition across Colorado on
Sunday with drying conditions over the mountains and lingering
gusty winds over the mountains and east slopes. Cross sections
showing cross barrier flow increasing to 30-35kt through Sunday
morning so could see some gusts from 30-50 mph through midday
Sunday. Dry with warming trend across lower elevations for Sunday
and Monday given increasing downslope flow and warm advection
pattern.

Moisture increases again over the mountains Sunday night ahead of
next Pacific storm system. Expect snow to spread back over the
mountains during that time and continue through Monday. Snowfall
amounts will generally be light on Sunday night with low lapse
rates under warm advection pattern. Mountain top flow will be
southwest so will generally favor zone 31 and some mountain
aspects of zone 34. Appears heavier snow possible by Monday
afternoon through Monday evening over all northern mountains with
cold air advection resulting in steepening lapse rates and some
upward forcing. At some point may need some hilites/advisory types
for mountains on Monday and into Monday night.

Main concern revolves around track of the storm system which moves
over the Rocky mountains Monday night and Tuesday. Model consensus
is to intensify the upper low mainly North and East of Colorado,
generally over Nebraska which would usually result in drier,
downslope flow over Eastern Colorado. Latest 06z NAM as the storm
system a bit further south and a fairly decent cold front through
much of Eastern Colorado by Tuesday morning and some deepening
upslope flow. The GFS is much drier with this solution with the
upper levels still further North. Given the strong influence of
160kt jet with this system and QG height tendency fields showing
best height falls across New Mexico would not rule out a further
south solution. Time will tell. Therefore it would still be
prudent to carry at least some light qpf across the plains on
Tuesday. Will need to add some qpf as latest ensemble model
guidance showed zero.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1012 AM MST Sat Jan 21 2017

Light winds will be in place this aftn with variable directions.
For this evening ceilings may lower to around 8000 ft with a
slight chc of -shsn between 02z and 06z based on latest data.
After 06z ceilings should rise abov 12000 ft. Winds early this
evening will become more wnw and then transition back to wsw after
06z.

&&

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

$$

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



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