Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 170907
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
307 AM CST Sat Feb 17 2018

.SHORT TERM...(Today and Tonight)
Issued at 306 AM CST Sat Feb 17 2018

Upper level wave swinging through the central Plains early this
morning. This has brought an increase in winds to the low and mid
levels with some decent warm air advection in the 850mb to 750mb
layer. This warm air advection was allowing an area of light snow to
progress through southeast SD into southwest MN and northwest IA.
This forcing should exit the area by about 13z to 14z and a few
areas could receive a half to three quarters of an inch of snow. As
the upper wave moves across some additional light snow will be
possible late this morning into the earl afternoon over east central
SD, but amounts will be minor. After this wave passes a northwest
wind will develop with milder temperatures and highs in the 30s.

A fast moving a weak high pressure will push through during the
evening with strong southerly flow developing through the overnight
hours. This is in response to deepening low pressure to the west as a
strong jet develops to the north. Precipitation is not expected
overnight as it should develop to the north of the area. Lows will
be mild in the teens and 20s with rising temperatures after
midnight.

.LONG TERM...(Sunday through Friday)
Issued at 306 AM CST Sat Feb 17 2018

Very strong warm air advection will spread across the area on
Sunday, with the strong focus north of the area. The main
challenge is a lack of moisture in the 900mb to 700mb layer which
suggests no precipitation during the day Sunday. But, with the
southerly flow at the surface turning to the northwest through the
afternoon, high temperatures will climb into the 40s. Colder air
will cause some falling temperatures north of Interstate 90 during
the afternoon so some late morning and early afternoon highs.

Sunday night into Monday a challenging period. The first wave will
lift north of the area leaving a fairly strong thermal gradient
across the area. In the low levels the arctic air will be strong
enough that the colder air will surge fairly far south. In the mid
levels, from about 850mb to 700mb, the colder air stalls from
eastern Nebraska into north central IA as the next wave begins to
lift northeast into Nebraska. This will set up an environment that
is supportive of some mixed precipitation potential along this
thermal gradient. However, while the moisture deepens, the
saturation remains on the warm side with saturation not expected in
the ice bearing layer. This will likely keep the threat as mist or
freezing drizzle. Did lower the pops a bit and the qpf, but a couple
hundredths of light freezing drizzle for now looks possible from
Sunday night through the day Monday.

The area from southeast SD into southwest MN will be on the edge of
the deeper low level moisture so the freezing drizzle threat should
be lower. The mid levels however are still a bit on the dry side as
well as a fairly shallow dendritic layer so the threat for snowfall
should also be somewhat low, but not impossible.

Central SD to along highway 14 looks like the best chance for snow
as the mid level boundary associated with the most favorable
dendritic temperatures increases through the night into Monday. The
better chances for higher snowfall amounts does look to be to the
north and west of the area, but still appears to be a decent chance
for 2 to 4 inches of snow in this area from late Sunday night into
Monday night.

Not planning any headlines due to marginal confidence and the fact
that the snowfall will likely occur over a longer period of time.
The freezing drizzle area in northwest IA will be another location
with the potential for advisory criteria for slick conditions but
not planning on introducing right now.

Light snow chances will exit on Tuesday with cold conditions
Wednesday then a milder pattern Thursday into Saturday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1047 PM CST Fri Feb 16 2018

VFR is expected through the 06Z TAF period except for one
exception in the near term. An upper air disturbance coupled with
warm air advection aloft and a strong low level jet, will likely
bring areas of light snow in the 08Z to 10Z time frame for the
KFSD TAF site. The snowfall could lower the visibilities down into
the MVFR range, and possibly brief periods of IFR before moving
off to the east between 10Z and 12Z. At this time, it is expected
that KSUX will only receive flurries starting around 09Z but with
little in the way of visibility reduction. In addition Saturday
morning between 12Z and 18Z, areas of snow may set up toward
central South Dakota, but south of the KHON TAF location.
Otherwise, non convective low level wind shear will be prevalent
at KSUX through 09Z Saturday.

&&

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

$$

SHORT TERM...08
LONG TERM...08
AVIATION...MJ



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