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FXUS63 KFSD 090438

Area Forecast Discussion
National Weather Service Sioux Falls SD
1038 PM CST Thu Dec 8 2016

.SHORT TERM...(This evening through Friday)
Issued at 310 PM CST Thu Dec 8 2016

Looking at a fairly quiet period through tonight and Friday, with
focus on the cold temperatures in place as a weak surface ridge
moves across the area. A few lingering flurries east of the Buffalo
Ridge in southwest Minnesota are expected to diminish through the
mid-late afternoon, with mostly clear skies and decreasing winds
dominating over the rest of the forecast area. Despite the lack of
snow cover across the region, the mostly clear skies and light winds
within the surface ridge should allow for overnight low temperatures
near the colder end of the guidance spectrum with most areas near to
a few degrees below zero.

Will see an increase in clouds from the southwest Friday as a subtle
wave slides through the mid-upper level flow. Moisture somewhat on
the sparse side with this weak wave, but could squeeze out some very
light snow in areas west of the James River during the late morning
and afternoon. The cloud cover and persistent cold airmass will
again lead to highs only in the teens throughout the region.

.LONG TERM...(Friday night through Thursday)
Issued at 310 PM CST Thu Dec 8 2016

Lingering moisture and some weak lift and frontogenesis with the
Friday wave will probably bring flurries to much of the area along
and east of Interstate 29 in the evening, as the system slips east
of the area. Temperatures will be a little less cold with lows in
the 2 to 12 above zero range. 

The Saturday snow system will be one of the two the main weather
features of the week. In fact, by giving us snow cover, it will help
lead to colder surface temperatures, contributing to dangerous wind
chills when arctic air arrives by midweek.

A rapidly moving but strong short wave will act on a very tight
thermal gradient aloft Saturday, producing efficient isentropic
lift. The NAM continues to be the obvious outlier with its far
northern track. The other models, including the GFS, Canadian, and
EC, are all fairly close. Together these models suggest a band of
snow of decent intensity developing over the area from the
southwest. While the heaviest snow should not last real long, it
could be intense for a short time at any place. The GFS shows deep
frontogenesis from just above H850 to H600, helping to produce a
band of deep lift. This lift is likely to produce 4 to 5 inches of
snow east of the James River in southeast SD and southwest MN, and
into the northern edge of northwest IA. Heavier accumulations than
that seem quite possible if the band slows for a while, because of
its potential intensity. The strongest lift indicate by the GFS
includes the heart of the dendritic zone. 

Easterly/southeasterly winds will become fairly stiff as the system
arrives, but at this time are not expected to be strong enough to
produce significant blowing snow. In any event, pretty low
visibilities are likely for a short time just from snowfall

There are still some uncertainties in track and speed, and expected
accumulations, for now, fall a little short of winter storm
criteria, so a watch will not be issued at this time. We have noted
that the model projections of the heaviest snow, not including the
outlier NAM, have shifted a little south from last night's runs,
from southwest MN closer to the Interstate 90 area from Sioux
Falls east. Further changes are quite possible. 

In the midst of the snow, Saturday is likely to be the warmest day
of the week, with highs in the mid teens to mid 20s. 

The snow should decrease rather quickly Saturday evening. Another
fast moving wave may bring light snow accumulations Sunday into
Sunday evening, mainly to the area south of Interstate 90.

Monday should be a mostly dry day, until an arctic front arrives by
early evening. Areas of snow Monday night should be light. Tuesday
through Thursday will be dominated by arctic high pressure
approaching slowly, but remaining to the west as it becomes anchored
all the way up into western Canada. This should produce mostly dry
weather, perhaps with an occasional flurry, which will not be
included for now.  However, as mentioned above, wind chills will
nose dive, and the expected winds, snow cover and colder
temperatures seem likely to drive wind chills down to that 20 below
or colder zone at times.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1035 PM CST Thu Dec 8 2016

VFR conditions expected through the TAF period. Northwesterly
winds will become light and variable overnight. Otherwise, clouds
will increase from the southwest early on Friday morning with
some light snow or flurries over portions of the James River
Valley, though impacts would be minimal. &&



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