Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 172356
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
556 PM CST Sun Dec 17 2017

.SHORT TERM...(This evening through Monday)
Issued at 301 PM CST Sun Dec 17 2017

A strong jet streak will work into the Northern Plains late tonight
into Monday which will allow a westerly component to the low level
flow. This should support partly cloudy skies from late tonight into
Monday. With this drying will come a lower chance for fog, but some
patchy fog will be possible in mainly northwest IA during the
evening and early overnight hours before winds turn to the west.
Lows will be mild in the mid 20s. Went just a touch warmer on Monday
given dry ground and westerly component, so looking at mid 50s in
south central SD to mid 40s in southwest MN.

.LONG TERM...(Monday night through Sunday)
Issued at 301 PM CST Sun Dec 17 2017

While some cooler air will settle south Monday night,
temperatures will still be well above normal with the colder air
bottled up in ND and northern MN. Lows in the mid to upper 20s and
highs in the 40s.

The system forecast for Wednesday into Thursday still on track.
Latest models hinting at a little slower wave which in turn is
allowing a bit of a trowal to develop Thursday afternoon into early
Friday morning.

The first potential for precipitation will come on Wednesday as a
fairly strong right entrance region of the upper level jet moves
into the Northern Plains. This swings through some fairly strong low
to mid level warm air advection but due to a lack of favorable
temperatures for precipitation production, somewhat dry air in the
mid levels and nothing convective expected at this time it appears
that this band of strong waa will move through without producing
precipitation. There should be a band of precipitation to the north
but the threat is not close enough to include any chance for
precipitation at this time.

By Wednesday night, especially after midnight, colder air will begin
to filter into central SD which will bring a threat for
precipitation, which should be snow. Not impossible for some brief
mixed precipitation but air is still pretty dry in the warm sector
aloft so expect saturation to bring down temperatures and support
snow.

During the day on Thursday a strong southern stream jet will eject
onto the Central Plains and indications are for a better potential
for phasing of the northern and southern jets. The main effect this
appears to have is an elongated band of warm advective precipitation
from Wisconsin into southern MN and west into our area where a bit
of a trowal develops. This suggests a better chance for
accumulating snowfall and an increased potential for a stronger
southern band which, because of the trowal/slower speed, is a bit
farther north this time around.

So overall, still far from any ability to add detail but confidence
increasing that there will be an impactful winter storm across the
Northern and Central Plains east into Iowa, Wisconsin and Illinois.

After the system exits early Friday cold air will be in place
through the weekend. Yesterday the GFS hinted at a shot of even
colder air around Sunday and now the other models also grabbing in
to this with timing variations. So, will keep temperatures very cold
Friday into Sunday and continue to steer lower than guidance.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 550 PM CST Sun Dec 17 2017

Tonight will bring more chances for patchy fog for much of the
region. However, a light southwesterly wind gradient combined with
a less favorable direction for upslope flow will mean fog will be
more isolated. The area most likely to see visibilities drop is
northwest IA, where winds will be close to calm overnight. Any fog
is expected to dissipate with daytime heating, roughly clearing
out around 18/1500z. Southwest and west winds will gust in the 15
to 20 kt range during the day Monday.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...08
LONG TERM...08
AVIATION...VandenBoogart


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