Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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902
FXUS63 KFSD 220439
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1039 PM CST Sat Jan 21 2017

.SHORT TERM...(This evening through Sunday)
Issued at 257 PM CST Sat Jan 21 2017

Stratus is slowly eroding along the Missouri River Valley, but
clearing will really struggle to make more significant headway
across the area as flow turns westerly and northwesterly tonight.
While the winds may keep things a bit more mixed resulting in
limited amounts of dense fog, still could see fog develop along and
east of the I-29 corridor.  Fog will be most dense across far
eastern edge of the forecast area, but formation may be disrupted by
disturbance moving through after 06z.

Weak wave is expected to move through southwest Minnesota early
Sunday morning.  Unfortunately, there is a lack of saturation in the
dendritic layer, so may see some freezing drizzle develop in the
Marshall- Windom vicinity.  Have kept mention in the HWO, and
future shifts will need to keep an eye on this area for the
potential for a small winter weather advisory.

On Sunday, we`ll still stratus again try to erode across the west as
northwest flow becomes more established as low pressure moves east
across the southern plains.  Will be a real struggle to clear along
and east of the James.  As such, will see limited diurnal range
again tomorrow.

.LONG TERM...(Sunday night through Saturday)
Issued at 257 PM CST Sat Jan 21 2017

The primary story in the medium and extended portions of the
forecast is with the snow chances Tuesday into Wednesday.  There
remains some uncertainty on the exact track of the upper wave moving
into the Plains by Tuesday.  The GFS remains a northern outlier as
compared to the ECMWF/GEM which both suggest this storm wraps up
faster and digs a bit deeper. Still considerable variance noted in
the ensembles, but a majority of the solutions support the EC/GEM,
and have trended the forecast towards the WPC consensus of EC/UKMET
blend.  That said, have raised PoPs for Tuesday through Tuesday
night up significantly over blended guidance and back towards the
previous forecast. Temperatures in the far southern CWA may approach
36-38 degrees, but the column above fully supports snow.  Have
pushed the snow mention further south, but with warmer ground
conditions, will keep snow amounts Tuesday afternoon slightly lower
along the HWY 20 corridor. Favoring a slightly southern track also
forced modifications to the SLR grids to force higher snow
accumulations further south. Generally tried to utilize a 10-12:1
ratio where the highest likelihood of a wet heavy snow may fall.

While there will likely be more concentrated focus on QPF over the
next 24-48 hours, this continues to look like a widespread 3 to 6"
snowfall, with some potential for isolated higher amounts over the
12 hour period. Given the increasing winds, can`t say a winter storm
watch won`t be needed, but at this point, still too much uncertainty
on where.

Depending on the speed and intensity of this system as it continues
to develop over the Great Lakes, we may continue to have an impact
into Wednesday, especially through mid-day as light snow continues.
The other concern we`ll have will be with blowing snow Tuesday night
into Wednesday. Low level winds upwards of 40 knots will develop on
the backside of this system, translating into 20 to 25 mph sustained
winds with gusts in the 30-35 mph range.  With those types of winds
and fresh snow, blowing snow may be a concern through Wednesday.
Have introduced at least patchy blowing snow wording through
Wednesday.

Rather quiet weather expected after this storm into the weekend. A
return of northwest flow aloft and fresh snow will drop temperatures
back to seasonal normals in the 20s. There is an outside risk of
light snow lingering behind on the western flank of this upper
trough Thursday and Friday, but uncertainty too high to include at
this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1036 PM CST Sat Jan 21 2017

IFR conditions along and north of Interstate 90 will sink south
and southeast through the next several hours. LIFR conditions
appear as though they will be more isolated and possibly confined
to locations along the southern and western fringes of the stratus
field. Conditions will again improve along the southern edges and
advect northward during the day Sunday but many locations will
remain MVFR or IFR through the period.

&&

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

$$

SHORT TERM...BT
LONG TERM...Dux
AVIATION...08



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