Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 121152
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
552 AM CST Thu Jan 12 2017

.SHORT TERM...(Today and Tonight)
Issued at 348 AM CST Thu Jan 12 2017

While tendency for large scale cyclonic zonal flow will remain
across the area, confluent streams and various impulses in these
streams will continue to impact weather across the area.  Weak
trough in the southern branch now shearing away southeast of the
area this morning with a few lingering flurries coming to an end,
but stubborn clouds are getting ready to begin to decrease along a
KHON/K9V9/KICR line.  Trend will continue today with flow turning
briefly west to southwest ahead of sharp arctic branch trough
sliding along the international border. Some burly pressure rises of
from 6-8 hPa/3h behind the arctic front currently crashing through
northwest North Dakota.  Winds also have been gusting 30 to 45 knots
behind the front, with temps in the teens below zero.  This front is
forecast to reach the far northern portions of the area along the
highway 14 corridor around 18z, and plowing through the remainder of
the area during the afternoon.  All guidance indicates that the
strongest pressure rises and wind gradient will start to peel more
toward the east/northeast of the FSD CWA by this afternoon, but hard
to ignore even the 3-4hPa/3h rises and 25 to 35 knots of wind in the
upper half of the mixed layer, as well as the current observations
of the extreme cold and wind behind the front. Current snowcover is
not likely to improve the airmass as the front moves southeast. RAP
may be the only guidance that is actually acknowledging the
potential for the falling temps and dewpoint readings behind the
boundary this afternoon along with the gusty wind threat. Still
looks to be potential for gusts 25 to 30 knots for at least a couple
or three hours, and would be enough to at least end up with some
pockets of blowing snow across east central SD and southwest MN by
midday and through mid afternoon.  There should even be enough of an
increase in westerly winds ahead of the front to pick up some patchy
blowing snow late morning near the Buffalo ridge with some localized
wind enhancement.

Arctic high will settle across the area tonight, the core glancing
through Minnesota. Potentially will need a wind chill advisory,
perhaps as early as very late afternoon, which will cover areas with
expected wind chills from 20 below to 30 below zero. However, winds
will be dropping off this evening as core of arctic high settles
toward SW MN later tonight. Also have complicating factor of
increase in high level clouds overnight from the south, which will
likely steady out or allow temps to rise later in the night where
coldest in east central SD into southwest MN.

The increase in clouds a result of another minor impulse in the
southern branch of the flow. Another area of concentrated warm
advection will develop after midnight. However, lift and increase in
moisture aloft will have a tremendous battle with the very dry
arctic ridge to the northeast, so will keep precipitation threat
limited to mainly the James valley and west very late at night, and
pops only at chance, as could easily remain just flurries.

.LONG TERM...(Friday through Wednesday)
Issued at 348 AM CST Thu Jan 12 2017

With primarily quiet conditions expected into and through the
upcoming weekend, the main focus in the medium and extended forecast
will be on a potential winter storm arriving early next week and
growing risks of impacts.

Friday: After a bitterly cold morning, the only additional concern
on Friday will be with another weak shortwave sliding eastward
across the Dakotas during the daytime hours. Like we`ve seen the
past days, there may be another narrow corridor of light snow that
races northeastward by the aftn hours. Any snow amounts should stay
on the light side (less than 1") thanks to influence of dry air
intrusion into the system.

This Weekend: As flow splits and a large trough dips into the
Southwestern US, high pressure will drift through the Northern
Plains on Saturday and Sunday. With warmer air aloft spreading over
the region, temperatures will climb near normal on Saturday and
slightly above normal on Sunday.

Monday-Tuesday:  The biggest story of the upcoming 7 days will be
the potential for a winter storm early next week.  While the general
model consensus for days has been to pull the upper trough in the
Southwest US into the Midwest early next week, deterministic model
trends over the past 24 hours have started pulling deepening upper
low further north and west following the idea of the ensembles over
the past two days.

With that said, the GFS, ECMWF, and GEM are all in a bit of
disagreement regarding the degree of warm nose lifting into the
region as this system approaches on Monday.  With it`s further west
track, the ECMWF is considerably warmer aloft than the other models
and could produce a much more widespread risk of a wintry mix.
GEM/GFS are slightly further east, but also have a much sharper
degree of dynamical cooling as the upper trough passes on Monday,
limiting the potential for mixed precipitation to the onset of
the event and focusing longer over parts of NW Iowa.

From an ensemble perspective, GEFS ensembles continue to lean
support towards the warmer EC solution, suggesting the 00Z
operational GFS is a bit of an outlier. Ensemble probability and
plume plots also suggest fairly high probabilities of FZRA, sleet,
and lower probabilities of snow during the onset late Sunday night
which is very plausible given the synoptic ridging riding ahead of
this deepening trough pulling warmer air aloft north and locking
in colder air at the surface.

Still days away, similar analogs to this event (using CIPS analog
guidance) also support an increased risk of freezing rain/sleet, and
eventual transition to moderate snow.  While there is obviously
considerable uncertainty regarding the track of this system which
will have significant impacts on location and type of precipitation,
have tried to push forecast towards a higher risk of mixed
precipitation.   All this said, this is a system that we`re going to
have to keep a close eye on.

Wednesday-Thursday: Only minor change made to the forecast was to
slightly lower high temperatures behind this system given
expectations of new wintry precipitation in the area. The general
forecast trends could signal near to above normal temperatures in
the region.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 551 AM CST Thu Jan 12 2017

VFR cloudiness will pull out this morning, with a period of mainly
clear skies until the approach of a strong arctic front which will
likely be accompanied by at least a FEW-SCT layer around 2kft.
Winds will gust at times to 25 to 35 knots for a couple hours
behind the boundary.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...Chapman



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