Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 212341
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
541 PM CST Sun Jan 21 2018

.SHORT TERM...(This evening through Monday night)
Issued at 323 PM CST Sun Jan 21 2018

Very unclear forecast tonight as strong deepening upper level low
moves from the intermountain west into the Central Plains tonight,
and into the upper Midwest on Monday.  In the short term, stationary
frontogenetic band of light snow across the north along highway 14
is expected to weaken by 00Z as a secondary frontogenetic band
develops from Tyndall to Sioux Falls over towards Worthington MN.
Along this line, could see light snow through the evening hours.
However, dry northeast fetch in the near surface layer will limit
the potential of this band, especially the further east you go along
this band.

12Z model trends are to shift the heavy snow band south slightly.
This is because the atmosphere appears to be less capped across
southern Nebraska into southern Iowa allowing convection to develop
further to the south than previously expected. Short term hi-res
models suggest convection will develop in the 00-02Z time range
along and south of I-80 lifting north through the evening hours. The
trends in the evening convection will be critical in producing an
accurate forecast!! After 06Z, these storms are expected to lift
north into the forecast area.  Expect moderate to heavy snow to
first develop along the Missouri River, and developing into a
significant TROWAL across far southeastern South Dakota into
northwest Iowa.  With the unstable nature of the atmosphere, expect
snowfall rates to exceed 1"/hr rates in the band.

Precipitation type for our far southeast near Storm Lake is VERY
difficult with pronounced warm nose between 900-800 mb.  Further
aloft, saturation is a question through the overnight hours, making
precipitation type a real concern through 12Z.  Am not expecting
more than a few hundredths of precipitation till after 12z, when
dynamics significantly increase and the thermodynamic profile cools
and saturates leading to snow.  Snow could be quite heavy through
the morning hours, and will keep blizzard warning in place as this
will be the prevailing precipitation type.

Winds still look to be strong as low pressure system deepens tonight
into Monday.  Pressure falls around 1mb/hr coupled with increasing
winds at 925 hpa to 35-45 knots are expected to lead to strong winds
across the forecast areas, but especially east of the James River
Valley after 06Z tonight into Monday.

As mentioned by the previous forecaster, there will be a very
clearly defined northern edge to the TROWAL precipitation, but where
exactly this occurs still remains unclear.  Have lowered snow
amounts on the northern edge of the band (mainly within the winter
storm warning), but have still kept 12-15" in the heart of the band,
where very intense snowfall can be expected.

Evening crew will need to monitor convective trends closely, and
adjust the forecast accordingly.

.LONG TERM...(Tuesday through Sunday)
Issued at 323 PM CST Sun Jan 21 2018

Have not adjusted the forecast beyond the storm.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 532 PM CST Sun Jan 21 2018

Many aviation impacts expected throughout the TAF period. Light to
moderate snow will begin lifting northeast from Nebraska this
evening, becoming moderate to heavy at times in far southeast SD
into east central SD, northwest IA, and southwest MN. Rain may
briefly mix with the snow this evening to the west of I-29. In
northwest Iowa, rain will be possible this evening, gradually
mixing with snow and changing entirely to snow by daybreak Monday.
Snow will continue on Monday, finally tapering off from west to
east by the evening. Northerly winds will increase tonight,
causing further visibility reductions in snowfall. Expect
conditions to rapidly deteriorate in snowfall, with IFR and LIFR
conditions likely for much of the TAF period. Near zero
visibility is possible at times as well, especially for the KSUX
TAF site.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning until 6 PM CST Monday for SDZ061-062-065-
     066-068.

     Blizzard Warning from midnight tonight to midnight CST Monday
     night for SDZ067-069>071.

     Winter Storm Warning until noon CST Monday for SDZ050-063-064.

     Winter Weather Advisory until 6 PM CST Monday for SDZ040-054>056-
     060.

     Winter Weather Advisory until 6 AM CST Monday for SDZ052-053-
     057>059.

MN...Blizzard Warning from midnight tonight to midnight CST Monday
     night for MNZ081-089-090.

     Winter Weather Advisory until 6 PM CST Monday for MNZ071-072.

     Winter Storm Warning until midnight CST Monday night for MNZ080-
     097-098.

IA...Blizzard Warning from midnight tonight to midnight CST Monday
     night for IAZ001>003-012>014-020>022-031-032.

NE...Blizzard Warning from midnight tonight to midnight CST Monday
     night for NEZ013-014.

&&

$$

SHORT TERM...BT
LONG TERM...BT
AVIATION...



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