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fxus65 kbou 121146 

Area forecast discussion
National Weather Service Denver/Boulder Colorado
446 am MST Thu Dec 12 2019

Short term...(today and tonight)
issued at 405 am MST Thu Dec 12 2019

The problem of the day this time around is the developing winter
storm that is expected to move into the mountains by tonight which
will then continue for several periods. Ahead of that, today
should be a mild day as downsloping westerly flow continues.
Moisture aloft will be increasing through the day, leading to
mountain wave clouds over the plains and generally increasing
clouds in the mountains. Snowfall is expected to begin in the
mountains during the afternoon, but accumulation rates are not
expected to increase until later tonight. When the storm arrives,
abundant Pacific moisture is going to be moving over the state,
being driven by a 130+ knot jet stream aloft. Strong orographic
effects should produce light to moderate snowfall over west-facing
exposures through tonight and Friday morning. Gusty winds are also
expected due to the fast flow at mid and upper levels. Blowing and
drifting snow could be a problem over the higher elevations
through the duration of the storm. Will be upgrading the Winter
Storm Watch to a Winter Storm Warning for tonight and Friday. The
rest of the Winter Storm Watch will be left as is because of the
uncertainty over how long the storm may continue. (See below) snow
accumulations with the initial surge of moisture are expected to
be 12 to 18 inches by Friday afternoon. Continuing strong
downslope effects in the Lee of the mountains will keep the plains

Long term...(friday through wednesday)
issued at 430 am MST Thu Dec 12 2019

Water vapor imagery shows a strong jet stream moving ashore over
Oregon this morning with a plume of moisture that extends as far
west as Hawaii's longitude. This jet and associated moisture plume
will be responsible for the prolonged snow event across
Colorado's mountains that lasts through this weekend. Global
models predict this jet will enter Colorado Friday morning with
maximum 300-200 mb winds of up to 180 knots. During this time, our
cwa's mountains will be under the left exit region of the jet
which will provide lift along with a quick moving shortwave
trough. This will enhance snowfall rates to between one to two
inches per hour Friday morning and create hazardous travel
conditions. By Friday afternoon, the jet will move directly
overhead with slight ridging building in behind the departing
shortwave trough. This will decrease snowfall rates to around a
half inch an hour. It is important to note that while snow rates
will decrease Friday afternoon, the strong winds, gusting up to
60 mph at times, will create blowing snow and poor travel
conditions. It is for this reason that the Winter Storm Warning
GOES through Friday evening with a Winter Storm Watch continuing
immediately after the warning ends.

A secondary plume of moisture will arrive ahead of a shortwave
trough Friday night. Snowfall rates will increase to one to two
inches in the mountains with hazardous travel conditions. These
snowfall rates will continue through the day Saturday and into
Saturday evening before tapering off. The jet stream moves farther
south to southern Colorado Saturday night which leads to the lull
in snowfall. Models still show considerable differences for
Sunday across our County Warning Area but it appears a trough will deepen over The
Four Corners region during the day. As this system moves
eastward-northeastward, it will produce a swath of moderate
snowfall. This swath will likely be farther south than the
previous two systems and will hit the central and southern
mountains harder than the northern mountains. By the time the snow
ends Sunday night, we expect significant snowfall amounts
especially on west facing mountain slopes. There was extensive
discussion on the timing of warnings and watches for this storm as
it will come in three waves. We determined that highlighting the
first wave would be ideal at this time while signaling there is
some uncertainty with the second and third waves. From Monday
through Wednesday, a ridge will build over the western US keeping
Colorado dry.

As for the plains and urban corridor, the westerly, downslope flow
will keep conditions dry on Friday and Friday night. Strong gusts
will be the biggest issue as they could reach up to 45 mph during
the day. We may even see some loosely attached Christmas
decorations taken for a ride if a gust gets a hold of them.

The second system will have better qg ascent aloft over the urban
corridor and plains which will lead to a chance of snow. The
European model (ecmwf) is the most bullish about snow amounts in the Denver area
while other models keep the snow farther north along the Cheyenne
Ridge. Since systems tend to come in slightly farther south and
deeper than models predict, I trended the forecast toward the
European model (ecmwf) and increased pops over much of the urban corridor and
plains on Saturday.

The third system will have the best qg and frontogenesis aloft
out of the three. This system appears it will hit southeastern
Colorado the hardest but areas from Denver southward may still see
light to moderate snow amounts. Confidence in the track and
intensity of this system is still rather low but it will need to
be watched closely for those traveling on Sunday. Monday through
Wednesday will see dry conditions with moderating temperatures.


Aviation...(for the 12z tafs through 12z Friday morning)
issued at 405 am MST Thu Dec 12 2019

Westerly winds at times will be about the only aviation impact
through the next 24 hours, and most of the winds should be west of
the I-25 corridor. Gusts up to 30 knots will be possible at times
at kbjc. Winds at kden should be out of the southwest to west at
speeds less than 15 knots. Mid and upper clouds will be
increasing through today and tonight, but ceilings should remain
above 6000 feet above ground level. Mountains to the west will become obscured in
clouds and snowfall. From this afternoon through tonight.


Bou watches/warnings/advisories...
Winter Storm Warning from 5 PM this afternoon to 5 PM MST Friday
for coz031-033-034.

Winter Storm Watch from Friday afternoon through late Saturday
night for coz031-033-034.



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