Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 051744
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1044 AM MST Mon Dec 5 2016

.UPDATE...
Issued at 1045 AM MST Mon Dec 5 2016

Winds in the foothills failed to materialize this morning and
don`t see any mechanism that would lead to an increase in winds
this morning so have cancelled the high wind warning. A few
snow showers have developed along and east of the Continental
Divide that may produce a quick shot of snow for elevations above
7,000 feet but given the strong downslope component of the flow do
not expect much accumulation in those areas. Temperatures across
the Plains and urban corridor look on track today topping out
around 50 degrees with some sun and plenty of westerly low-level
flow. Today will be the warmest day of the week before the weather
turns much colder tomorrow.

&&

.SHORT TERM...(Today and tonight)
Issued at 335 AM MST Mon Dec 5 2016

Radar imagery has indicated an uptick in light snow over the
higher mountains over the past hour as next ripple and jet streak
moved into Northwest Colorado. This is a fast moving system has
already some drying noted over southwest Wyoming and Northern
Utah. Main concern this morning is the high wind potential over
the foothills. Simulated imagery showing increased subsidence and
drying between 12-15z over the Front Range. Meanwhile forecast
soundings showing increased stability and strong mountain top
inversion developing by 15z. This will help amplify the mountain
wave and bring stronger winds down through the foothills. Cross
sections showing cross barrier flow increasing to 50-60kt from
NAM/RAP model data. The only negative factor for high winds is the
upstream moisture along and west of the divide which may work to
de-amplify the wave. However...feel there is enough subsidence
moving in to bring high winds to wind prone areas in the
foothills through the morning hours.

Moisture in the mountains becomes more shallow by afternoon and
even more so tonight. have trended to lowered pops for tonight
with the brunt of the snow accumulations during the daytime hours.
Looks like a 2-5 inch snowfall for the mountains today along with
strong and gusty winds over mountain passes with blow snow.

Across lower elevations...one more mild day given the downslope
flow this morning with highs reached by early afternoon. Should
again see some 50s noted over the urban corridor with the enhanced
downslope flow with strong surface gradient across Colorado. No
precip expected over lower elevations with the downslope flow and
dry low levels. Initial surface cold front expected by late
afternoon/early evening hours.

.LONG TERM...(Tuesday through Sunday)
Issued at 335 AM MST Mon Dec 5 2016

Tuesday morning will begin fairly dry with only a slight chance of
light snow in the high mountains as QG motion is downward still.
As the next piece of energy moves towards the area in the
afternoon and a surface low forms over southwestern Colorado,
westerly 700 mb level flow turns southerly over the Palmer Divide
and into the Denver area. This should shut off any potential for
precipitation due to the drying effects. The better moisture to
the north should be increasing snowfall over the mountains and
potentially over the northern foothills and urban corridor. A
secondary cold push, increasing easterly upslope flow and a
bullseye of moderate upward QG motion will be over the forecast
area Tuesday evening - when snowfall rates are expected to be the
highest. Moderate to heavy snow will likely occur over the
mountains during the evening with favored flow and cold advection.
Nighttime travel may be difficult in the high country. The QG
decreases quite a bit after midnight, however model soundings over
the plains show instability increasing whereas there is a slight
stable layer during the evening - so snowfall rates could increase
or stay about the same after midnight. Model trends have been
interesting though, with the latest NAM decreasing snowfall
amounts, significantly over the Denver area. Whereas the GFS has
increased. Temperatures should only warm into the mid 20s Tuesday,
so really it will depend on the surface to 700mb wind direction
and whether the upper dynamics will overcome any drying.
Temperatures are expected to drop into the single digits over the
plains overnight and below zero in the mountains to increase snow-
to-liquid ratios.

The upper trough will push east Wednesday, but upslope flow and
cold temperatures will squeeze out all remaining liquid out of the
system. Highs will likely be in the single digits to teens across
the area.  Should see snow coming to an end in the late morning
to early afternoon. Right now, thinking will be snowfall amounts
of 5 to upwards of 15 inches for the mountains, with highest
values over the Front Range mountains, especially over Larimer and
Jackson counties. Thinking 1 to 5 inches over the plains with the
higher values towards the northern urban corridor and lower
values toward Limon and areas south. Confidence in the numbers
however is low due to the conflicting model trends, and dependence
on how far those southerly 700 mb winds will push and how long it
will last.

As the system departs, clearing skies will allow lows overnight
to drop below zero across the area Wednesday night and Thursday
morning, with areas in the mountains seeing close to 20 below in
some places. Upper ridging will build in Thursday though high
surface pressure will slow much warming.

Flow aloft will become zonal over the area through the week with a
series of shortwaves and Pacific moisture expected to keep snow in
the forecast over the mountains each day. Temperatures will be
warming though back to seasonal averages. A more significant
shortwave on Friday night into Saturday will be more accumulating
snow to the mountains and may bring some precipitation back to
the plains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1045 AM MST Mon Dec 5 2016

Strong and gusty westerly winds today should continue through the
afternoon at the metro area terminals today. Expecting a wind
shift to around 320 degrees after 22Z with the arrival of the
first shot of cold air at KDEN. After several hours of strong
northwest winds, the flow should weaken overnight after 06Z at
KDEN and after 04Z at KAPA. At KBJC wind should be 10 kts or less
after about 01Z. Not expecting any ceiling or visibility
restrictions through 00Z Wednesday (early Tuesday evening) but
conditions could start to deteriorate not long after 00Z at the
three terminals with the arrival of snow and lower clouds
resulting from the low-level upslope flow and approach of the
mid-level trough axis.

&&

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

$$

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


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