Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 220530

Area Forecast Discussion
National Weather Service Sioux Falls SD
1130 PM CST Sun Jan 21 2018

Issued at 839 PM CST Sun Jan 21 2018

Very difficult forecast this evening. Models continue to trend
downward with snow totals along and north of I-90 in southeast
South Dakota and in the highway 14 corridor in southwest
Minnesota. Radar shows widespread light echoes throughout the
forecast area, however surface observations are indicating that
very little of this is actually reaching the surface, with the
exception of areas near and south of the Missouri River. Forecast
soundings support a very dry layer above the surface between 900 mb
and 750 mb which is impeding the development of precipitation.
This layer is particularly dry in the I-90 corridor, with the RAP
and HRRR indicating that saturation will be difficult for much of
the night.

Meanwhile, convection has developed north of Kansas City into the
Omaha area and is lifting to the north northeast, on a trajectory
a bit further east than previous runs. High res models and the 00z
NAM still peg northeast Iowa and northeast Nebraska with
significant snowfall late tonight into Monday, with strong
frontogenesis and instability still on track to develop in the
Blizzard warning area after midnight.

Further north and west where the Winter Storm warning was issued,
confidence is much lower. Models continue to show a very tight
gradient between the extreme snowfall areas expecting 6 inches
plus of snow, versus areas with little to no snowfall. With the
storm track trending further east, at this time, it likewise
appears that the heaviest snow (6+ inches) has also shifted east -
and will fall south and east of a line from Lake Andes to Menno
to Canton to Slayton. Sioux Falls metro remains on the edge of
this gradient, however model trends overwhelmingly have shifted
the heavier QPF to the southeast - significantly lowering
amounts. Right now the expectation is 2 to 5 inches, however it
should be noted that the last several runs of the RAP and HRRR are
producing well under an inch in Sioux Falls. Given the dry mid
levels tonight and the strongest lift/forcing developing to the
south and east, have gone ahead and downgraded the winter storm
warning in the Sioux Falls area (Minnehaha, McCook, Pipestone Co)
to an advisory.


.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 06Z TAFS through 06Z Monday night)
Issued at 1115 PM CST Sun Jan 21 2018

Many aviation impacts expected throughout the TAF period. Light to
moderate snow will continue lifting north from Nebraska into the
early morning hours, becoming moderate to heavy at times in far
southeast SD into northwest IA, and portions of southwest MN.
Rain may briefly mix with the snow in the early morning hours, in
west central Iowa before changing entirely to snow by daybreak
Monday. Snow will continue on Monday, finally tapering off from
west to east in the afternoon or 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
across the southeastern half of the region. Near zero visibility
is possible at times as well, especially for the KSUX TAF site.
Conditions will improve behind the exiting snowfall.


SD...Winter Storm Warning until 6 PM CST Monday for SDZ065-066-068.

     Blizzard Warning until midnight CST Monday night for SDZ067-

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

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

     Winter Weather Advisory until 6 AM CST Monday for SDZ052-053-

MN...Blizzard Warning until midnight CST Monday night for MNZ081-089-

     Winter Weather Advisory until 6 PM CST Monday for MNZ071-072-097.

     Winter Storm Warning until midnight CST Monday night for MNZ080-

IA...Blizzard Warning until midnight CST Monday night for IAZ001>003-

NE...Blizzard Warning until midnight CST Monday night for NEZ013-014.



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