Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 141836

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1136 AM MST Sat Jan 14 2017

Issued at 1135 AM MST Sat Jan 14 2017

Evaluating a complicated forecast today through tomorrow. Current
band of radar echoes stretched from South Park northeast into
Washington County are bringing light to moderate snow to the high
country then dissipating before reaching the ground across the
lower plains. The exception may be over Lincoln County where some
webcams show a dusting of snow. This band is expected to keep
moving north while dissipating, to affect the I70 mountain
corridor early this afternoon with less than two inches of snow.
Across the plains, there may be a few flurries to sprinkles.
Heading into tonight, there is vast differences in moisture
forecast between the models. It seems as the NAM is holding on to
today`s band moisture, then when a weak cyclone develops and weak
QG lift happens tonight, its producing precipitation across the
plains and urban corridor. The NAM forecast soundings have low
level moisture just below freezing, but not any colder than -8
degrees C, with no seeder feeder above at times. This would
indicate periods of freezing drizzle. However it seems as the NAM
is overdoing it with low level moisture. For now, will see only a
slight chance of a mix of rain and snow over the plains along and
south of I70 today for an hour or two. Tonight, shouldn`t seem
much around the metro areas until we head towards morning.

As for the Winter Weather Watch over the eastern plains, overall
the models seem to be slowing the system a bit as well as keeping
it further south and east. This would lower the chances of
heavier freezing rain over our forecast area, but will likely
still see freezing drizzle. But with the models still in
disagreement this close to the event, it makes it hard to make a
call on whether to upgrade or cancel the Watch. Will continue to
interrogate the models and weather situation.


.SHORT TERM...(Today and tonight)
Issued at 408 AM MST Sat Jan 14 2017

An upper level low will move across the Baja Peninsula today and
then track east over northern Mexico tonight. For Colorado, this
will result in a southwest flow aloft turning to the south
tonight. Mid and upper level moisture will increase today ahead of
the upper level low. Still enough moisture in the flow aloft for
snow showers over the mountains, mainly this morning. Any
additional snowfall is expected to be light and less than an inch.
Cloud cover will limit heating some with high topping out in the
upper 30s to lower 40s. A weak wave could bring precipitation to
the southern parts of the area before its shear apart today.
Airmass appears cool/dry enough that any precipitation that falls
should result in snow. As flow aloft turns southerly tonight
warmer air will move in just above the surface. Chances for
precipitation will be low tonight. If any occurs it may fall as
freezing rain over far eastern Colorado.

.LONG TERM...(Sunday through Friday)
Issued at 408 AM MST Sat Jan 14 2017

Models and ensemble members continue to present a similar
solution at least through Sunday morning. As of 12z/Sunday...
models show the 500 mb low just west of El Paso TX and a
moist mid-level southerly divergent flow over Colorado as of
12z/Sunday. During the day....models generally show the low
lifting north-northeastward over ern New Mexico and the western
Texas panhandle. From there model differ even more with the GFS
showing the 500 low over west Oklahoma as of 12z/Monday...while
the Canadian and SREF show it positioned over southwest Kansas
and the European model farther east over south central Kansas.
Where the low ends up tracking will be critical as to the type
and amount of precip observed across the eastern half of the CWA.
Past few days...the European model and recently the Canadian
model have been more consistent in their solutions. The GFS has
fluctuated quite a bit from run-to-run with the path and timing
of the upper low. For that reason will mean more of the European
and Canadian models this go round.

Sunday night...southeast boundary layer flow will advect relatively
mild and moist air up into northeast Colorado... overrunning a
shallow layer of sub-freezing air near the ground. Model soundings
indicate this layer of cold sfc air to be no more than a couple
thousand feet deep across Lincoln...eastern Elbert and southern
Washington counties. This is where precip will more likely be
freezing rain or a mix of freezing rain and snow form Sunday
morning through late Sunday night. North and west of the
there...with additional cooling as the upper low and trough move
closer...precip should be all snow. As for`s possible
as much as a quarter inch of ice could accumulate in Lincoln and
ern Elbert Counties by 12z/Monday...and on top of that anywhere
from 3-6 inches of snow ending by around midday Monday. Should
precip remain as all snow...could see another 2-3 inches added to
this total. For that reason...will issue a WINTER STORM WATCH for
zones 46..47 and 49. Watch will run from 13z/Sunday to 18z/Monday.

Elsewhere...the I-25 corridor...esply the greater Denver metro
area will remain under the influence of a fairly moist nely
upslope flow for most of the Sunday. Precip/snow chances should
steadily ramp up through the day with likely pops by mid to late
afternoon when parts of the metro area could see an inch or so of
accumulation. However it`s overnight with the upper low passing to
our southeast and best qg divergence/ascent will be aloft that
the I-25 corridor and foothills will see the bulk of the snowfall
with this storm. Amounts vary widely from model to model. The SREF
Plume indicates anywhere from a trace to as much as 12 inches at
DIA. At this point...will go with the lower amounts ranging from
1-4 inches by Monday morning with the greater amounts indicated on
the east and south sides of the metro area. Do not expect to see
much wind with this system. In the High country...east slope areas
will be affected most by this storm with snow totals anywhere
from 2 to 7 inches. West of the divide snow amounts generally
under 3 inches.

On Monday...the main upper low continues to track east-northeast
away from Colorado...a secondary upper low over the Desert
Southwest tracks east across New Mexico Monday night. This low
should have little to no impact on our weather except to delay
clearing overnight.

Tuesday thru Wednesday night...a dry period with gradual warming
as an upper ridge builds in from the Great Basin. By
Thursday...medium range models shows a strengthening zonal flow
advecting the next batch of moisture laden maritime air towards
Colorado. Could see the first sign of this moisture in the form
of snow showers moving into the high country by Thursday morning.
Friday into the weekend looks moist and unsettle...particularly
for the mtns and western valleys. temperatures will alos be on the
decline with the increase in clouds and cooling aloft.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 1135 AM MST Sat Jan 14 2017

Mid and high clouds will move in from the southwest through the
day. Ceilings are expected stay above 6000 feet through 00z.
Winds will remain light and probably quite variable through
tonight. A few flurries or sprinkles will be possible between
18-21z, no accumulation is expected with this.

Overnight, a weak cyclone will develop with some models hinting at
fog likely around. Low level moisture seems to increase, so some
reduction in visibility will possibly happen around the area
airports. If this happens, it may make it possible for some snow
to begin by 12z into Sunday morning. Depending on how far west
mid level warmer air moving up from the south makes it, there is a
small chance for freezing drizzle Sunday morning.

The better chance for snow will come after 18z Sunday. Still a
difficult forecast for late Sunday and Monday in regards to
snowfall. There will be a good chance for light snow during this
period. Significant snow is not expected, though there is a small
chance for up to 6 inches of snow by 18z Monday.


Winter Storm Watch from Sunday morning through Monday morning
for COZ046-047-049.



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