Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 091825
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1125 AM MST Fri Dec 9 2016

.UPDATE...
Issued at 1107 AM MST Fri Dec 9 2016

There is still some snow falling along and just east of the
divide in the high mountains. This is especailly the case from
I-70 northward. Will leave the advisory going in zones 31 and 33
through late afternoon. Temperatures have warmed into the 40s over
the greater Denver area and the Palmer Ridge. Will change some of
the sky cover grids with way less cloudiness over the east. Will
lower pops over the high mountains south of I-70. Will warm
temperatures a bit over some of the plains and foothills. All this
is based on real data.

&&

.SHORT TERM...(Today and tonight)
Issued at 510 AM MST Fri Dec 9 2016

Snow continues to pile up in the high country. We earlier upgraded
the advisories to warnings. Snowfall as seen on the snowstakes
and Snotel data is now reaching the 5-10 inch range. Upstream
satellite and radar returns still show quite a bit of activity in
northern Utah and southeast Idaho, so moderate to heavy snowfall
will continue early this morning before decreasing later today.
Some convective elements were noted which is very rare in a warm
advection regime as the airmass is generally too stable, but
upstream raobs do show at least a neutral stability profile above
700 mb, and even some potential instability so not a surprise
based on the real data. Precipitable water values were also
abnormally high with readings around 0.40 inch, all making for
efficient orographic snowfall.

The general idea of snow decreasing first in the I-70 corridor
and then gradually lifting to the Wyoming border by this afternoon
still looks good as upper level jet max and best moisture plume
eventually lift northward. Flow does turn a bit more
west/southwest too, so focus would shift toward the mountains
north of Rabbit Ears Pass. Will have the warning for the I-70
corridor through the mountains til noon and farther north
including Rabbit Ears and Rocky Mountain National Park til 5 PM.

Meanwhile, downslope and mild air has spread down through almost
all of the foothills with gusty winds. The plains remain in the
arctic airmass but given the shallow nature of inversion we should
be able to finally break out of the cold along the Front Range.
Warmest temps likely from the foothills into the Denver metro area
and Palmer Divide where inversion is now only about 1000 feet
deep. Greeley and points eastward will see a modified warmup today
as inversion is deeper in the lower elevations on the plains.

Mountain snow should continue to decrease tonight to just
scattered light snows as moisture decreases. A modified arctic
airmass on the eastern plains may slosh back a little overnight so
will go with the slightly colder guidance.

.LONG TERM...(Saturday through Thursday)
Issued at 510 AM MST Fri Dec 9 2016

An active weather pattern for the mountains is going to do good
things for the state`s snowpack for the next week. Strong zonal
flow aloft is forecast to continue across the entire country
through the next week with at least one significant short wave and
another disturbance embedded in the flow pattern. And at the end
of next week, a significant upper trough is expected to carve
itself out over the Pacific Northwest and then move into the Rocky
Mountain region. Meanwhile, at lower levels, a few surges of
Arctic air are expected to accompany the upper level disturbances.

The GFS and ECMWF have similar ideas through their runs, but
differ on the details at the latter time periods. In general, it
appears that a strong upper jet will drive a short wave across
Colorado from Saturday afternoon through Sunday. This is expected
to produce a period of moderate to heavy snowfall in the
mountains. The weather system will contain a healthy amount of
Pacific moisture and may produce snow amounts up to a foot of snow
between Saturday Night and Sunday. Snowfall amounts will probably
satisfy warning criteria, but with current highlights already in
effect today, will wait to issue any watches or warnings for
Saturday Night and Sunday. On the plains, the presence of the
upper level jet may be enough to bring some showers out over the
lower elevations. Temperatures will be warm enough for rain
Saturday afternoon, but then cool down and change over to snow in
evening and overnight.

Another embedded short wave is forecast to cross over the state
late Tuesday. More accumulating snow will fall in the mountains
due to the ongoing orographic flow pattern and Pacific moisture.
An Arctic front will also move in from the north Tuesday
afternoon, dropping temperatures on the plains some 10 to 15
degrees from Monday`s highs. Cold air is forecast to linger over
the region through Wednesday, keeping high temperatures on the
plains below the freezing mark.

The final feature, at the end of the forecast period is both the
GFS and ECMWF producing a significant upper trough over the
Pacific Northwest next Saturday. This will also have a strong
Arctic front associated with it. We will be keeping an eye on
the next few runs of the medium range models to see if they hang
on to the idea of developing this system and sweeeping it across
the country.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1107 AM MST Fri Dec 9 2016

Although there is a pretty weak low level pressure gradient over
Colorado there is enough to keep a downsloping component to the
winds at DIA. Models insist on a southeasterly component by early
afternoon, so will keep that in the TAF. With the winds and
warming temperatures, the airmass has mixed out and there should
be no lower clouds/ceiling issues.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 5 PM MST this afternoon for COZ031-
033.

&&

$$

UPDATE...RJK
SHORT TERM...Barjenbruch
LONG TERM...Dankers
AVIATION...RJK



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