Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 181126
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
626 AM CDT Wed Apr 18 2018

.SHORT TERM...(Today and Tonight)
Issued at 425 AM CDT Wed Apr 18 2018

Complex forecast over the next 12 hours, as a compact shortwave
tracks east from northeast Nebraska across northern Iowa today.
High-res models all pointing to potential for a band of heavier
snow around the north side of the wave, but the question remains
exactly where this band will end up. 00Z models trended south with
the system, with decent consensus in placing the higher QPF/Snow
near and mainly south of I-90, with much lighter amounts in our
far northern CWA along the Highway 14 corridor. This solution
makes sense given relatively dry north-northeast low level flow
which could limit snowfall farther north. Made changes to our
headlines based on this idea, upgrading a portion of northwest
Iowa to a Winter Storm Warning where intense snowfall rates could
quickly produce an area of 6+ inches of snowfall. Also cancelled
the Winter Weather Advisory for the Highway 14 corridor, where the
model consensus suggested amounts would remain around 2 inches or
less.

Concern is that very strong lift early this morning could quickly
overcome the dry air, and in fact, 06Z NAM and more recent runs
of the HRRR/RAP have shifted the axis of heavy snow a county or
two northward again during the late morning-early afternoon, while
also lingering the chance of snow later into the afternoon along a
persistent low level boundary across the northeast half of the
forecast area. Will not jump on these changes now, but day shift
will certainly have to watch trends and adjust if necessary.

Snow should exit the area early-mid evening as the wave moves east
and subsidence takes over across the area.

.LONG TERM...(Thursday through Tuesday)
Issued at 425 AM CDT Wed Apr 18 2018

Surface high pressure and upper level ridge will bring quiet
weather to the region Thursday, while our next upper level system
tracks through the southern Rockies. Could see some very light
precipitation extend into the Northern Plains by Friday as the
upper low slides east into the southern/central Plains. However,
with the strongest lift well south of the region, and abundant dry
air in place as the surface high drifts east toward the Great
Lakes, precipitation (mainly light rain) will have a tough time
progressing east later Friday/Friday night. GFS hangs on to some
very light QPF into Saturday morning, but other models maintain
dry conditions Saturday and Sunday, which seems reasonable given
the position of the upper support well to our south. Temperatures
through the weekend are expected to gradually warm, though will
still be below normal for this time in April, hampered by any
lingering snow cover.

Northern stream wave and an associated cool front push into the
northern Plains late Monday, and could see some decent warming
ahead of the boundary on Monday. Did not make significant changes
to the consensus blend yet, as differences in timing could impact
the potential warming. But if current timing verifies, could see
widespread highs in the 60s on Monday. Scattered showers possible
along the boundary Monday night into Tuesday as it tracks across
the region, with temperatures Tuesday falling back below normal,
though still in the 50s for most areas.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 626 AM CDT Wed Apr 18 2018

Widespread low MVFR-IFR ceilings with IFR-LIFR visibility in snow
expected to start the period this morning. Snow will persist into
the afternoon, gradually ending with visibility improving during
the afternoon, though ceilings will largely remain MVFR-locally
IFR into the evening. The snow will exit southwest Minnesota and
northwest Iowa this evening, with VFR conditions expected to
prevail by the end of the period most areas. Some models do hint
at fog developing after 06Z, especially west of I-29. However,
confidence is not high enough to include at this time.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until 4 PM CDT this afternoon for
     SDZ053>056-060>062-066-067.

     Winter Weather Advisory until 1 PM CDT this afternoon for SDZ050-
     052-057>059-063>065-068>071.

MN...Winter Weather Advisory until 7 PM CDT this evening for MNZ080-
     081-089-090-097-098.

IA...Winter Weather Advisory until 4 PM CDT this afternoon for IAZ020-
     032.

     Winter Weather Advisory until 1 PM CDT this afternoon for IAZ031.

     Winter Storm Warning until 7 PM CDT this evening for IAZ002-003-
     013-014-021-022.

     Winter Weather Advisory until 7 PM CDT this evening for IAZ001-
     012.

NE...Winter Weather Advisory until 1 PM CDT this afternoon for NEZ013-
     014.

&&

$$

SHORT TERM...JH
LONG TERM...JH
AVIATION...JH



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