Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 201720
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1020 AM MST Sat Jan 20 2018

.UPDATE...
Issued at 1020 AM MST Sat Jan 20 2018

Latest model data coming in is slowing the incoming storm down
slightly as it crosses southern Colorado early Sunday, especially
the NAM, as well as weakening it ever so slightly. This may
translate into slowing down the arrival of snow by an hour or two,
not enough to make drastic changes to the ongoing forecast.
Forecast soundings still show favorable profiles for freezing
drizzle late this evening into a couple hours past midnight for
the northeastern plains including the urban corridor. Both the GFS
and NAM have come down slightly with their QPF totals from the
06z, but with the amount of QG vertical motion expected over the
area early tomorrow morning through the afternoon, do not want to
make huge changes to the snow amounts expected. Still expecting a
total of 6 to 12 inches with locally higher amounts in the
mountains, with 4 to 7 inches for the urban corridor and eastern
plains - except a band with 6 to 10 inches stretching from the
Palmer Divide out to the northeastern corner of the state. Strong
northerly winds Sunday will likely cause difficult driving
conditions in blowing snow. Visibility will likely be reduced to
1/2 to 1/4 mile at times. Continue to monitor the latest forecasts
and storm impacts, and adjust travel plans if possible.


&&

.SHORT TERM...(Today and tonight)
Issued at 410 AM MST Sat Jan 20 2018

The strong Pacific storm system set to move across the Central
Rockies later this weekend is currently moving onto the California
coast. The upper level trough will move across the Desert
Southwest and Great Basin today and into the Central Rockies late
tonight. As it moves into the Central Rockies, a closed low forms
near the Four Corners 12Z Sunday.

Surface low pressure will deepen over southeast Colorado today
causing winds to turn northeasterly north of I-70 across northeast
Colorado. This will usher in cooler air today. Highs will range
from the lower 40s north to lower 60s over southern Lincoln
county. Only chance for snow today will be over far north central
Colorado.

The northeast winds will increase this evening as the surface low
continues to strengthen. Low levels cool and moisten up this
evening, ahead of the main storm system moving in. It appears a
period of freezing drizzle is setting up late this evening and
after midnight. There should be a few hours of freezing drizzle
before the snow arrives. Strong QG ascent ahead of the upper level
trough will move into west Colorado this evening and shift
eastward through the night. This will produce a band of snow,
which is expected to move over the north mountains late tonight
and onto the Front Range towards 12Z.

Biggest change to the forecast through tonight was to added
freezing drizzle for late this evening through 12Z for northeast
Colorado. This should produce slippery conditions prior to the
snowfall.

.LONG TERM...(Sunday through Friday)
Issued at 410 AM MST Sat Jan 20 2018

Models continue to be a little slower with this weekend`s system,
which results in more snow, especially close to the mountains. The
slower speed not only means a longer period of snow, but allows
the low level winds on the plains to stay northeast longer. This
should allow for some damming of the cold air near the Front
Range, enhancing the snowfall a little more. Not really much depth
to any upslope against the foothills, so no big enhancement there.
Still expecting a bit more of a downslope component Sunday
afternoon, but at first it will be the NNW wind that dries out
Larimer and Weld counties, while northerly flow into the Palmer
Divide, and maybe a little turning into the foothills W/SW of
Denver, will continue through the afternoon. Model QPF is pretty
uniform at around half an inch for Denver, with a bit more to the
south and southeast, and less due to an earlier end with the wind
picking up in Greeley/Fort Collins. This forecast adds another
inch or two, especially in the Denver area which could do very
well for about 6 hours mid morning through early afternoon.

Winds will take a little while to wind up, but should be going
pretty strong in the afternoon, with 40-50 mph gusts over the
eastern part of the plains, and at least 30 mph near the
mountains. Enough for some drifting, and with plenty of snowfall
we could have blizzard conditions in the areas of stronger winds.
Confidence not high enough for a blizzard warning yet, but we will
be mentioning that possibility for the plains Sunday afternoon.

There is fairly quick drying Sunday evening, though a shallow
moisture layer will linger over the mountains into Monday. There`s
a brief window of opportunity for mountain wave enhancement for
the Front Range foothills Monday morning, but the flow shouldn`t
be strong enough for a high wind threat. On the plains there will
still be some wind mixing down Monday, but probably only limited
drifting snow without much impact.

Northwest flow will continue through midweek with and modest
warming will be further limited by the snow cover. By Thursday,
flow should go southwest and could be increasing ahead of another
strong trough. This could bring a milder day as some of the low
level cold air gets flushed out, especially near the mountains.
There`s reasonable agreement on the presence of another strong
trough Friday or Saturday, but lots of variety in the details.
About a quarter of the GFS ensemble members have a strong closed
low moving south of us which could give us another snow like this
weekends, about Friday night into Saturday. The rest either have a
sharp open wave, or are focused further north. We will introduce
low PoPs again on Friday, though the better chance is probably
Friday night or Saturday if we do have a stronger/slower system.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 1020 AM MST Sat Jan 20 2018

Winds have turned northeast at all but KAPA and areas south, these
should switch soon. This will usher in cooler air and increase
low level moisture. Low clouds with ceilings of 1000 to 3000 feet
are expected to form around 03Z. Clouds will continue to lower
through this evening. Eventually, freezing drizzle is expected to
form around 04-06Z with ceilings falling below 1000 feet. Freezing
drizzle will change to snow between 09Z and 12Z. Snow will then
persist through most of Sunday. The heaviest snow is expected to
occur Sunday morning and into the early afternoon. Total snowfall
for the upcoming storm through Sunday evening for the Denver
airports is expected to be in the 5 to 8 inch range.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from midnight tonight to 8 PM MST Sunday
for COZ046-048>051.

Winter Storm Warning from 6 PM this evening to noon MST Sunday
for COZ031-033>036.

Winter Weather Advisory from 6 PM this evening to noon MST
Sunday for COZ030-032.

Winter Storm Warning from midnight tonight to 5 PM MST Sunday
for COZ039>042-044-045.

Winter Storm Warning from midnight tonight to 2 PM MST Sunday
for COZ038-043.

&&

$$

UPDATE...Kriederman
SHORT TERM...Meier
LONG TERM...Gimmestad
AVIATION...Kriederman



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