Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 171143

Area Forecast Discussion
National Weather Service Sioux Falls SD
543 AM CST Sun Dec 17 2017

.SHORT TERM...(Today and Tonight)
Issued at 358 AM CST Sun Dec 17 2017

Decided to issue a dense fog advisory for the morning hours for
southwest Minnesota and much of northwest Iowa. Stratus developed on
the edge of our eastern zones and has since expanded to the west as
the low level flow continues out of the northeast. Therefore am
concerned that areas of dense fog will continue to creep closer to
Interstate 29 early this morning. We will monitor for westward
development of fog for this possibility, and may have to expand the
advisory for locations along I 29.

Weak winds today will make it difficult for the fog to lift and
dissipate. Have raised cloud cover, and have it gradually eroding
from the west as surface ridge moves east and westerly flow begins
to develop. With weak winds throughout the day in northwest Iowa,
have lowered highs across the east.

.LONG TERM...(Monday through Saturday)
Issued at 358 AM CST Sun Dec 17 2017

Warmer air continues to work back into the region tonight into
Monday with 925 hpa temperatures warming nearly 10 degrees C.  This
should allow temperatures to once again warm to well above normal

Cold front works through the region early Tuesday morning knocking
temperatures back a bit.

Main story in the forecast continues to focus on system that moves
through the region on Thursday.  ECMWF has trended back to the north
with this run, and advertises a slower progression compared to the
transient GFS solution.  The Canadian falls somewhere in between the
two solutions.  At this point, precipitation is still a question,
but there are some very strong jet dynamics.  Initially, forecast
area is in the left exit region of jet streak intensifying over the
Central Plains.  However, as this shifts east, second intensifying
jet streak works down the backside of the upper trough and once
again puts the forecast area in a favored position.  WPC prefers the
ECMWF forecast, and has extended pops with this system into Friday
morning across the eastern CWA.  Considered doing the same, and
after discussion with surrounding offices, held off for now.

Much cooler air work in behind the cold front on Thursday.  Have
cooled temperatures next weekend with the idea that fresh snow cover
will limit the amount of potential mixing.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 538 AM CST Sun Dec 17 2017

TAFs are quite a challenge for the morning hours and confidence is
not overly high. LIFR stratus, and in some cases VLIFR stratus has
developed over extreme northwest IA and southwest MN east of the
Interstate 29 corridor. With northeast winds just off the surface,
these conditions have been inching closer to KFSD and KSUX however
thick high clouds now currently mask the stratus on the GOES 16 IR
imagery. KSUX has recently received IFR ceilings, so am suspecting
that the stratus is beginning to encroach into that site. Will
monitor for further lowering, especially with regards to
visibility. KFSD is a challenge because it has been between
stratus fields all night. Again, will monitor for changes in the
near term, but overall KFSD looks to have at least MVFR visibility
possibly followed by MVFR ceilings later this morning. At KHON,
many guidance values point to IFR or LIFR conditions there.
However their low level flow is actually from the west and
northwest where no stratus exists. Therefore did not wish to bring
in any low ceilings at KHON for now, and may have to raise their
visibility after sunrise. Overall conditions should improve this
afternoon and into this evening. The NAM shows low stratus again
late tonight but was not backed up by other models.


MN...Dense Fog Advisory until noon CST today for MNZ071-072-080-081-

IA...Dense Fog Advisory until noon CST today for IAZ001>003-012>014-



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