Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 210942

Area Forecast Discussion
National Weather Service Sioux Falls SD
342 AM CST Sun Jan 21 2018

.SHORT TERM...(Today and Tonight)
Issued at 334 AM CST Sun Jan 21 2018

Amplified trough with base over desert southwest is evident on water
vapor imagery. Lee cyclogenesis will commence as the main vort
lobe rotates through the trough and a closed 500 mb low ejects
into the TX/OK Panhandle/SW KS through the morning/early afternoon
hours. Meanwhile, mid level frontogenesis has led to a band of
light radar returns across our far northern coverage area early
this morning, although dry air is largely limiting anything
reaching the ground at this point. This should begin to change
through the morning as a band of snow develops across our
northwestern counties from around Chamberlain to De Smet.
Additionally, as moisture continues to saturate the lower levels
below the ice growth zone, expect drizzle or freezing drizzle to
become possible across our NW Iowa counties.

Upper level wave continues to lift northeast with the surface low
moving into NE KS by late evening/overnight and SE IA by Monday
afternoon. Aloft, a jet streak will extend from TX to IA/IL with our
region benefiting from the added lift from this favorable jet
dynamic. This is also noted in solid Div Q, most focused over our
southeast half counties. Also expect to see some convective
enhancement over the previously mentioned southeast half of our FA
with models advertising favorable lapse rates, occasional negative
EPV* above the frontogenetic forcing and even some elevated CAPE
being flagged. Forecast soundings are also rather impressive at
times with omega in the dendritic growth zone. Finally, the best
TROWAL signature rests over this area, further enhancing confidence
of enhanced snowfall rates in this region. Would note be surprised
to see some thundersnow which further complicates snow amounts
with its inherent localized nature. It is also worth noting that
this corridor south of Sioux Falls to near Sioux City is also
painted in the GEFS Extreme Forecast Index (EFI) for
precipitation. Will still need to watch for a period of icing
across our far southeastern counties near Storm Lake/Ida Grove as
the dry slot swings near. As of now, any icing amounts look
fairly light.

Winds will also be a big concern with this tightly wound system.
Good pressure packing will be focused across SE SD, SW MN and NW IA
with 925 mb winds in the 35-45 kt range. This should translate to
surface winds of 20 to 30 mph with gusts up near 35-40 mph at times.
This will no doubt cause issues with blowing and drifting snow and
reduced visibility.

Impacts: This will clearly be an impactful winter storm for the
region. With the help of some convective enhancement, expect to
see some snowfall amounts near or over a foot from south of Sioux
Falls to near or just north of Sioux City. With this in mind, a
very sharp gradient in snow amounts is also expected on either
side of this heavier band. The likelihood of snow totals varying
greatly over small distances can not be emphasized enough and any
further wobbles in the storm track could lead to further changes.
This uncertainty is further shown in a very wide spread in the
ensemble QPF and snow totals. Strong northerly winds will lead to
significant blowing and drifting with blizzard or near blizzard
conditions at times from this evening into Monday. Travel will be
dangerous and not recommended for many areas. Because of the
expected heavy snowfall rates at times, combined with the wind,
have issued a Blizzard Warning for far southeast SD, northwest IA
and portions of southwest MN. Winter Storm Warnings and Advisories
buffer this region where either lower snow totals are expected or
somewhat lighter winds.

.LONG TERM...(Monday through Saturday)
Issued at 334 AM CST Sun Jan 21 2018

No changes were made to the extended forecast with relatively quiet
weather expected through the rest of the work week. High
temperatures will gradually warm through the week although will be
impacted by any snow cover that occurs. Light precipitation is
possible by next weekend.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1052 PM CST Sat Jan 20 2018

Several concerns through the next 24 hour with a potent winter
storm expected to arrive on Sunday. Light rain and snow will
spread across south central SD into east central SD late tonight
into Sunday afternoon. Additionally, areas in northwest Iowa and
northeast Nebraska could see patchy drizzle develop Sunday
afternoon during the daytime on Sunday. Expect MVFR and IFR
ceilings and visibility to develop in these area during the
drizzle and rain/snow mix, however much of southwest MN into
extreme southeast SD may remain VFR through 00z. Snow will become
widespread across the area late in the afternoon Sunday into
Sunday evening, however areas of northwest Iowa and northeast
Nebraska will likely see a mix of rain, freezing rain, and snow in
the evening hours. North winds are also expected to increase to 15
to 30 kt Saturday afternoon and evening. Widespread MVFR to IFR
conditions are likely in the evening, with LIFR possible at times.


SD...Winter Storm Warning from 6 PM this evening to 6 PM CST Monday
     for SDZ061-062-065-066-068.

     Blizzard Warning from 9 PM this evening to midnight CST Monday
     night for SDZ067-069>071.

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

     Winter Weather Advisory from 6 PM this evening to 6 PM CST
     Monday for SDZ040-054>056-060.

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

MN...Blizzard Warning from 9 PM this evening to midnight CST Monday
     night for MNZ081-089-090.

     Winter Weather Advisory from 6 PM this evening to 6 PM CST
     Monday for MNZ071-072.

     Winter Storm Warning from 6 PM this evening to midnight CST
     Monday night for MNZ080-097-098.

IA...Blizzard Warning from 9 PM this evening to midnight CST Monday
     night for IAZ001>003-012>014-020>022-031-032.

NE...Blizzard Warning from 9 PM this evening to midnight CST Monday
     night for NEZ013-014.



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