Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 231140

Area Forecast Discussion
National Weather Service Sioux Falls SD
540 AM CST Fri Feb 23 2018

.SHORT TERM...(Today and Tonight)
Issued at 406 AM CST Fri Feb 23 2018

Strong short wave is currently moving east of our forecast area,
with an attendant surface low over Wisconsin moving eastward with
it. High pressure will fill in behind these features today. One
challenge in the near term has been fog. Widespread fog developed
west of Interstate 29 in our SD zones, and is currently edging
eastward toward I 29 itself. This fog is in a rather dead area of
air as the wind shifts to the west. At this time, there is not
enough dense fog to issue a dense fog advisory, however will
continue to highlight the fog in a special weather statement and
message it. The fog will likely ooze east of I 29 into extreme
northwest IA and southwest MN before dissipating. Once the westerly
winds increase a little later this morning, the fog should mix out,
but the stratus layer may be tough to get rid of at least along and
north of Interstate 90. If the fog gets worse, we may have to
headline it but at this time, locations such as Huron and
Chamberlain have been improving. For highs today, hedged on the
cooler side of guidance readings with the fresh snow cover in place
over many locations, with widespread temperatures in the 20s.

Tonight, the main challenge is how far temperatures may drop during
the evening hours especially over the low lying areas. This is
compounded by whether there will be any lingering stratus or not.
For now in general, our western zones look coldest, with increasing
mid and high clouds putting the brakes on falling temperatures after
midnight. Late tonight, introduced likely pops in our far
southwest corner for the beginning of the snow event on Saturday in
our area.

.LONG TERM...(Saturday through Thursday)
Issued at 406 AM CST Fri Feb 23 2018

On Saturday, the models continue to be in pretty good agreement with
the timing and track of the surface and upper air features. At 12Z
Saturday, the upper trough is moving just east of the Rockies, with
a strong short wave taking over as it lifts into the central plains
with a subtle negative tilt. PV is strong as it moves across south
central NE to southwest IA Saturday afternoon and evening,
strengthening Saturday night as it moves into Wisconsin. Because of
this, the surface low deepens as it moves eastward. The surface low
is rather broad initially in the central plains midday Saturday, but
strengthens to near 990mb east of our forecast area. The various
models with their soundings still show quite a bit of instability
aloft across the southeast half of our CWA, with strong 700mb
frontogenesis laying across our winter storm watch location
coincident with unstable EPV* near 600mb. This layer is saturated,
and temperatures are cold aloft with -10C to -20C in a thin layer
averaging near 600-500mb. The upper QG forcing in the 500-300mb
layer still looks to track across the watch area. One huge fly in
the ointment is what will happen to snow amounts in our CWA if
convection takes over to our southeast. This is what the GFS is
trying to do and could really hammer our snow amounts downward. A
couple of days ago, the Canadian GEM was placing the heaviest QPF
further southeast where it went convective where the current GFS has
it. Now, the two models have change places with the GEM Global
placing the heaviest precip where our watch resides. The ECMWF also
has moderate amounts of QPF in our watch area with snow ratios
averaging 12:1 to 16:1 from south to north in our forecast area.
Therefore all said, our current watch location did not need altered.
Still projecting an average of a possible 6 to 7 inches of snow in
the watch area, tapering off to 2 to 3 inches in our far northwest
area. In time as we get closer to event, the 12Z cams may shed some
light if there will be any heavy banding here or there similar to
what happened last evening. Opted to not issue a warning at this
time due to the aforementioned convective potential which

There does not appear to be a lot of wind with this event as the low
does not strengthen dramatically until it gets well east of our
forecast area. The winds may get a bit breezy east of I 29 late
Saturday night but the snow will be over by then.

The next system to watch will be next Wednesday afternoon and night
where there is some consensus on another strong short wave moving
across the central plains. It looks to mainly possibly impact our
southern and eastern locations, as our west is cutoff more by a dry
northern stream polar flow. Overall concerning temperatures, they
will not be overly warm, especially considering the snow cover
around. Highs will generally be at or below normal for the next
several days.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 536 AM CST Fri Feb 23 2018

Overall, confidence is low with the 12Z TAF set due to cloud
ceilings. Stratus exists just north of Sioux City in the 300 to
800 feet AGL range, and its quite possible that the stratus could
make it to Sioux City within the next couple of hours. Otherwise
with the fresh snow cover over many areas, there is a risk that
upper end IFR to lower end MVFR ceilings may continue to develop
through the day. One exception may be closer to central SD where
clear skies to the west may eventually take over at the KHON TAF
site giving them VFR conditions this afternoon. TAFs will be
updated as needed.


SD...Winter Storm Watch from Saturday morning through Saturday
     evening for SDZ056-062-066>071.

MN...Winter Storm Watch from Saturday morning through Saturday
     evening for MNZ071-072-080-081-089-090-097-098.

IA...Winter Storm Watch from Saturday morning through Saturday
     evening for IAZ001>003-012>014-020>022-031-032.

NE...Winter Storm Watch from Saturday morning through Saturday
     evening for NEZ013-014.



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