Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 221656

Area Forecast Discussion
National Weather Service Sioux Falls SD
1056 AM CST Mon Jan 22 2018

.SHORT TERM...(Today and Tonight)
Issued at 257 AM CST Mon Jan 22 2018

A very impactful winter storm continues to move towards the Tri-
State area early this morning and a blizzard is expected! Currently,
an elongated band of snow developed by 700:600 frontogenesis
continues to produce fairly healthy snowfall rates over the
northwestern half of the CWA. Short term models have really
struggled with this layer, but recent HRRR/RAP runs now at least
seem to have a better grasp. As the upper low over central Kansas
begins to pivot northeast guidance suggests this band should begin
to weaken as precipitation begins to focus along stronger 850:750
frontal zone over NW Iowa.

Already seeing release of elevated instability over eastern Nebraska
and western Iowa with several lightning strikes in newly formed
clusters of convection. With upper divergence spreading overhead
by daybreak, we should begin to see a massive expansion of
precipitation on the radar as a very intense frontal boundary
begins to develop. Negative EPV above the intensifying
frontogenesis band between 12Z and 16Z suggests the potential for
an isolated lightning strike and may push snowfall rates upwards
of 2-3" per hour over NW Iowa. As this instability slides eastward
by mid-morning, a significant TROWAL will develop with a
channeled heavy snowband within continuing well into the afternoon.

Winds are steadily increasing tonight and will continue to push the
40 mph mark as snow falls through the day.  Integrating high-res QPF
into the blended data suggests potential for a narrow corridor of 12
to 18" of snow across northwest Iowa. Honestly, given the forcing,
it wouldn`t surprise me to see a very narrow channel of snow closer
to (or even above) the 18" mark.

Combining this snow with ongoing winds will make travel impossible
and extremely dangerous over NW Iowa and adjacent areas of SD/NE/MN
today. The worst conditions will align themselves from Sioux City to

The western and northern edge of this trowal will produce an
extremely sharp cutoff.  Have again tried to narrow down the snow
amounts, but grid resolution and uncertainties on where this edge
develops still forces somewhat broad accumulations. There is also
uncertainty in the extreme SERN portions of the CWA where soundings
suggest the loss of ice crystals by mid-afternoon due to the dry
slot rotating into the storm.

Snow will only very slowly end this afternoon and evening in the
eastern zones.

As far as advisories/warnings are concerned, not anticipating any
changes at this point. As daybreak arrives and early cancellation
may be possible over the western zones in the Missouri river
valley. However, with winds increasing, blowing snow will be an
issue in many areas.

.LONG TERM...(Tuesday through Sunday)
Issued at 257 AM CST Mon Jan 22 2018

Minimal changes in the medium and extended portions of the forecast
due to the ongoing storm.  The longwave pattern continues to show
fairly quiet weather conditions, absent of below normal
temperatures.  Temperatures will be driven by deep snowcover, but
given ongoing challenges have not made any changes.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1052 AM CST Mon Jan 22 2018

Widespread IFR conditions with LIFR over far southeast SD,
northwest IA and southwest MN in heavy snow and blowing snow.
Conditions will slowly improve this evening from west to east,
especially with reduced visibilityin snow and blowing snow, but
IFR ceilings may linger through the overnight. For now looking at
the TAF sites breaking out from about 6-9z but confidence is only
moderate in this happening.


SD...Winter Storm Warning until 6 PM CST this evening for SDZ065-066-

     Blizzard Warning until midnight CST tonight for SDZ067-069>071.

     Winter Weather Advisory until 6 PM CST this evening for SDZ056-

     Winter Storm Warning until noon CST today for SDZ063-064.

MN...Blizzard Warning until midnight CST tonight for MNZ081-089-090.

     Winter Weather Advisory until 6 PM CST this evening for MNZ072-

     Winter Storm Warning until midnight CST tonight for MNZ080-098.

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

NE...Blizzard Warning until midnight CST tonight for NEZ013-014.



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