Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 221708
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1108 AM CST Wed Feb 22 2017

.SHORT TERM...(Today and Tonight)
Issued at 441 AM CST Wed Feb 22 2017

There is still time to enjoy one more day of lingering warm weather,
although it will not be as warm as Tuesday, and more significant
changes are fairly close at hand. A weak wave will be traversing
north of the area through mid afternoon, with decent mid-level
frontogenesis brushing the northern CWA around KHON toward KBKX.
Airmass below remains fairly dry, so at best an isolated sprinkle to
shower threat through mid-morning.  A weak surface wave will slip
past the area later this morning as well, and this will pull a
frontal zone southeast across the area with a shift to some gustier
northwest winds.  Temps aloft remain warm enough through the
Missouri river corridor that temps will climb well into the 60s and
perhaps to a couple of record highs. To the north toward the highway
14 corridor, likely only reaching the mid 50s, and those temps due
to increasing cold advection during the afternoon should actually
occur earlier in the day and begin to tail off somewhat as advection
outpaces mixing and see an increase in some lower cloudiness as
well. The I-90 zone will be in between, with mainly upper 50s to
around 60 with a steady to slow fall initiating by early to mid
afternoon.

The cooler air will begin to slide southward with more forcefulness
tonight as the broad entrance of anticyclonic jet streak works
across the area as jet core moves into Minnesota. A blanket of lower
clouds will drop across the area from the north with strengthening
north to northeast low level flow. Cooling overnight will be
significant enough that as the developing precipitation in strong
frontogenetic zone toward south central South Dakota evolves by late
tonight, there should only be a brief period of anything other than
snow, and that is only if the wet bulb process is not as efficient
as it could be.

.LONG TERM...(Thursday through Tuesday)
Issued at 441 AM CST Wed Feb 22 2017

Focus for this forecast period in the medium range, as a significant
winter storm system impacts the region late this week. Still some
spread among the various model solutions, although they do continue
to show some convergence toward a common solution, certainly more so
than was seen a few days ago. ECMWF remains on the southern end of
the solutions, with GFS/Canadian slightly north and fairly similar
to each other. NAM remains on the northern fringe, and a bit of an
outlier when compared to other deterministic and ensemble solutions.

Thursday will begin with strong isentropic lift developing along
295K surface into south central SD, in advance of a strengthening
mid-upper level trough sliding east across the Rockies. ECMWF is a
little slower to saturate than GFS/Canadian, but all show a rapid
uptick in saturation/lift after 18Z Thursday, with precipitation
likely throughout the area by late afternoon commute time. Will be
some precip type issues at the onset, especially south of I-90 and
east of the James River Valley, where midday surface temperatures
could be close to 40 degrees. Northern and western portions of the
forecast area more likely to see temperatures remain in the lower
30s throughout the day, closer to raw model guidance which seems
more representative with anticipated precip development than a blend
which includes MOS guidance values. Given rapid increase in lift and
saturation, especially in our far west, have bumped the start time
of the Winter Storm Watch forward to 18Z Thursday in areas west of
the James River.

By late Thursday afternoon, strong band of mid level frontogenesis
develops across the forecast area, and differences in placement of
this seem to most greatly influence northern bias of the NAM QPF and
snowfall when compared to other models. GFS/Canadian maintain the
strongest mid level front support near or even a little south of the
I-90 corridor into early Friday evening, while the NAM lifts this
closer to Highway 14. With preference for GFS/Canadian, will keep
highest pops/QPF focused along/south of I-90 for Thursday afternoon
and evening, though location of the heaviest snowfall will again be
impacted low level temperature profiles.

Later Thursday night through Friday, generally expect the system to
continue strengthening as the upper level and low level low centers
shift slowly into western Iowa. Through this time, the ECMWF becomes
a bit of an outlier on the fast side, with less strengthening and
thus a more progressive shift toward the Mississippi Valley than the
GFS/Canadian. Have continued to side more with the slower solutions
at this time, which keeps moderate to heavy snowfall across much of
the area into Friday morning, and for locations near/east of I-29
through Friday afternoon and possibly into the evening hours. Along
with the falling snow, the strengthening system will bring increased
northeast-north winds through this period, with a potential for wind
gusts of 35-40 mph at times Friday into Friday evening. Given the
delayed exit of snowfall and potential for strong winds into the
evening hours, have extended the Winter Storm Watch through Friday
evening for our MN/IA counties, along with Dakota County, Nebraska.
Cannot rule out blizzard conditions developing, especially in open
areas Friday, but with lingering uncertainty given the latest ECMWF
timing, will stick with Winter Storm Watch for now.

The trend toward a stronger/slower system in the latest GFS/Canadian
has resulted in higher precipitation totals throughout the area,
especially potential snowfall maxima within the highest part of the
band, which currently favors a line from south central SD, through
Sioux Falls, into southwest MN. Consensus points to a fairly strong
potential for snowfall in excess of a foot in these areas. While
higher QPF axis is actually farther south, amounts to the southeast
of this line will be initially limited by warmer temps and rainfall.
Areas to the northwest toward Huron/Brookings are expected to be
north of the strongest frontogenetic forcing, which should likewise
limit amounts. Snowfall in excess of 6 inches currently looks quite
possible, though if mid level forcing is strong enough, could see a
sharper cutoff to amounts along the north edge of the heavier band,
as has been seen in previous events this season. As with any potent
system such as this, though, further refinements will surely be made
in the later forecasts.

With focus on the medium range winter storm, little change has been
made to the forecast through the weekend and early next week. The
expected widespread snowfall will surely keep temperatures closer to
normal than we`ve seen in recent days. Could see a weak system or
two track across the region early-mid next week, though little
agreement among the models on timing/location of these waves at this
range, so nothing more than chance pops are warranted at this time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1104 AM CST Wed Feb 22 2017

MVFR and IFR ceilings across North Dakota are poised to settle
south into the area tonight. Expect widespread low end MVFR to IFR
conditions late tonight into early Thursday morning.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from Thursday evening through Friday
     afternoon for SDZ038>040-053>056-059>062-065>071.

     Winter Storm Watch from Thursday afternoon through Friday
     afternoon for SDZ050-052-057-058-063-064.

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

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

NE...Winter Storm Watch from Thursday evening through Friday
     afternoon for NEZ013.

     Winter Storm Watch from Thursday evening through Friday evening
     for NEZ014.

&&

$$

SHORT TERM...Chapman
LONG TERM...JH
AVIATION...08



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