Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 111016

Area Forecast Discussion
National Weather Service Sioux Falls SD
416 AM CST Sun Feb 11 2018

.SHORT TERM...(Today and Tonight)
Issued at 415 AM CST Sun Feb 11 2018

Establishment of return southwest gradient has produced some spotty
near -20 wind chills early this morning where below zero readings
have matched the increase in winds, mainly on the western slopes of
the Buffalo Ridge and James River valley. Have not issued an
advisory as will be a very short term and spotty issue, with
marginally cold readings. Should be a quiet day for most with
sunshine starting to fade behind an increase in mid- to high-level
cloudiness during the afternoon.  Northern stream wave digging
toward the Great Lakes will push a reinforcing arctic boundary
southward during the afternoon, approaching the I-90 corridor by
very late afternoon. Should not have a strong impact on temperatures
today with timing of the front and the balance of mixing with
boundary, allowing temps to be 5-10 degrees warmer than Saturday.
Highs should vary from mid to upper teens in deeper snow covered
areas, to some lower to mid 20s in south central SD.

Some decent agreement in solutions exists in the intermediate term,
which is somewhat notable given the initial split/confluent flow
pattern over the CONUS.  Colder air filters southward with arctic
ooze tonight, but will also be dealing with a fairly decent amount
of mid- to high-level clouds. Temps should fall below zero for all
locations on merit of cold advection alone, but the juxtaposition of
lesser clouds and deeper snow cover in parts of northwest IA could
locally allow some cooler temps in valley locations, including KSUX.
Wind chills will again fall to the familiar -10 to -20 readings
overnight into early Monday.

.LONG TERM...(Monday through Saturday)
Issued at 415 AM CST Sun Feb 11 2018

Monday will return to a chillier level as core of arctic ridge
builds into Minnesota. Plenty of clouds around, but again of the
high-level variety as low-levels dried out with influence of
ridge. Return flow could bring some lower clouds toward south
central SD late day. Highs from the upper single digits and lower
teens, another day 15 to 20 degrees below normal.

The initial southwestern portion of the wave will be kicked out by a
reinforcing wave dropping through CA on Monday, and shear through
the confluence zone in streams across the northern Plains on Monday
night. Overall, dynamics for lift forcing are generally limited to
broad warm advection/quick moving shot of channeled vorticity.
Moisture is somewhat challenged, but enough to warrant at least a
small chance level PoP and low-end QPF southeast SD through
southwest MN where forcing may be persistent enough to attain
saturation. On Tuesday, the northern branch flattens the flow and
gradually turns flow toward a more westerly direction. This will
yield a trend toward increasing sunshine, and moderating
temperatures will make a step back toward normal.

Perhaps, the most impactful weather in the intermediate range of the
forecast could occur on Tuesday night.  Strength and depth of
inversion and optimal direction of the 30 to 35 knot winds at
inversion level suggest a prime setup for downstream ridge wind
enhancement developing across southwest MN Tuesday night. While
surface gradient is not strong, should be a narrow zone of wind
gusts at times around 30-35 mph.  Depending on the curing of the
current snow, these winds could locally result in blowing snow
Tuesday night into very early Wednesday.

Widespread west to southwest flow will dominate through Wednesday,
and the atmosphere will give some love to us on Valentine`s Day in
the form of well above normal temperature. Again, have greater
concerns about the widespread snow cover, and more significant
depths from northeast Nebraska through northwest Iowa and into parts
of southwest MN.  There is no doubt that it will be warm, but have
moderated this warming by several degrees in northwest IA and
adjacent locations, while actually nudging areas around KMML up a
few degrees where snow cover is much less and the inversion has less
impact at elevation.  At this point, do not see much risk in losing
the warming potential to stratus and fog, as moisture flowing over
snow cover appears insufficient for fog development.

Pattern diverges again to a greater degree after Wednesday, mainly
dealing with splitting of next trough digging into the western U.S.
The ECMWF is slower and exhibits a greater splitting to trough,
while GFS and a subset of ensembles have some splitting, but a much
more progressive northern stream. Impact develops downstream with
intensifying jet structure and southward plunge of arctic boundary
by Wednesday night and Thursday.  Ensembles are more to the cold
side, and for now enough support that temps should be steady to
falling on Thursday, at best a small gain. Have shaded a bit more
gradient from north to south than blend to start the day, then
worked in the cooling trend through the day.  Suggestion in temps
that post frontal environment could feature a blustery period of
light snow showers or flurries, but will have to wait a bit longer
before considering introduction as blends are far to dry to
coordinate changes at this juncture.

While operational runs are all in for the warming for the weekend,
ensemble guidance suggests caution, with high deviations and a mean
which is 15-20 degrees cooler than the operational. Rising heights
and moisture trapped toward system moving through the southern
plains should keep dry through Saturday.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1035 PM CST Sat Feb 10 2018

VFR through the TAF period.




SHORT TERM...Chapman
LONG TERM...Chapman
AVIATION...05 is the U.S. government's official web portal to all federal, state and local government web resources and services.