Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 200446

Area Forecast Discussion
National Weather Service Sioux Falls SD
1046 PM CST Fri Jan 19 2018

.SHORT TERM...(This evening through Saturday)
Issued at 332 PM CST Fri Jan 19 2018

Mostly cloudy skies are expected to continue through the night.  NAM
is struggling with moisture from the recent snow melt in the near
surface layer, and is once again creating widespread fog tonight.
This is not supported by the other model solutions, so am not
forecasting any low clouds/fog. With 925 hpa temps remaining a few
degrees above freezing, expect lows to be relatively mild near
seasonal averages.

Looking at one final mild day across the area on Saturday.  925 hpa
temps falling 2 to 4 C from today, and with a fair amount of clouds
across the region, do not expect temperatures to be as warm.

.LONG TERM...(Saturday night through Friday)
Issued at 332 PM CST Fri Jan 19 2018

Focus continues to be on Winter Storm that is expected to impact the
area on Sunday.  Currently the wave is positioned off the California
coast and is expected to work towards the four corner region on
Saturday.  Wave is expected to move into the plains on Sunday and
rapidly deepen.  As the wave moves into the plains, there is still
some lift that develops with limited mid-level saturation across
northwest Iowa.  Could see some light drizzle/freezing drizzle
develop in this area.  This drizzle could expand north towards the I-
90 corridor.  Expect snow to develop along 700 mb frontogenetic band
Sunday afternoon extending from Chamberlain to Brookings and up
towards Marshall MN towards 00Z.

Overnight, strong wave lifts northeast from the western plains
towards central Iowa. A strong trowal develops on the 295-300K
surface potentially wrapping into the eastern half of the forecast
area. What makes this unique is the air in the warm conveyor
appears to be unstable, with negative LIs from 925 mb and above.
This is also noted in negative EPV*. Models are really hitting
the qpf quite hard with the TROWAL, and believe this is in
response to the convective nature of the atmosphere Sunday night.

For this forecast iteration, the NAM and ECMWF shifted ever so
slightly south, while the GFS remained fairly consistent with the
placement of the heaviest band of precipitation.  Have raised Pops
and QPF amounts associated with the TROWAL.

Other concern are the winds.  Winds at 925 hpa increase to 35-40
knots Sunday night into Monday morning.  With intense snow fall
combined with strong winds expect near blizzard with whiteout
conditions possible.

System shifts east during the day on Monday, but winds remain brisk.
Currently headlines highlight snowband well, and have just added a
few counties on the southern edge. Have also extended the headlines
across the eastern half of the forecast area in time for the
potential of blowing snow Monday afternoon.

Behind this system, temperatures don`t drop appreciably through the
week.  Potential for another winter system moving through next
Friday - Saturday.  This system looks to be a wave moving through in
northwest flow, and doesn`t appear to be as strong as the system
Sunday into Monday.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1043 PM CST Fri Jan 19 2018

Mainly VFR conditions are expected through the TAF period. Will
need to watch for the potential for some shallow MVFR or IFR fog
developing late tonight with light winds expected along with
increased shallow moisture from snow melt. Not enough confidence
to place in TAFs at this point, but will continue to monitor
potential. Late in the TAF period mid clouds will increase, with
some MVFR ceilings and visibility nearing the Missouri Valley
after 00z.


SD...Winter Storm Watch from Sunday morning through Monday morning
     for SDZ050-053-054-058>060-063>065-068.

     Winter Storm Watch from Sunday afternoon through Monday morning
     for SDZ040-055-056-061-062-066-067-069>071.

MN...Winter Storm Watch from Sunday evening through Monday afternoon
     for MNZ071-072-080-081-089-090-097-098.

IA...Winter Storm Watch from Sunday afternoon through Monday morning
     for IAZ020-031.

     Winter Storm Watch from Sunday evening through Monday afternoon
     for IAZ001>003-012>014-021.

NE...Winter Storm Watch from Sunday afternoon through Monday morning
     for NEZ013-014.



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