Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 182046
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
346 PM CDT Sat Mar 18 2017

.SHORT TERM...(This evening through Sunday)
Issued at 346 PM CDT Sat Mar 18 2017

Surface high pressure will drift east into the western Great Lakes
overnight.  Return flow will develop, and become quite strong just
above the surface.  Low level jet of 35-45 knots will feed north
across the plains into southern Canada keeping conditions quite mild
tonight. Will likely see non-diurnal temps as the low level jet
increases.

Will see a mild day across the region on Sunday. Stout southerly
flow can be expected through the morning hours on Sunday ahead of a
cold front that will move through the area during the day.  This
will result in a fairly significant thermal gradient across the
region.  With the strong winds, will see an elevated fire danger,
especially along the Missouri River Valley where cured grasses have
dried some over the last few days.


.LONG TERM...(Sunday night through Saturday)
Issued at 346 PM CDT Sat Mar 18 2017

Sunday night is marked by a short wave passage across southern
Canada, which begins a long process of filtering chilly high
pressure southward into the northern plains through the middle of
the week. On Monday, a mid level frontal based near 700mb nudges
southward providing plenty of mid level cloud cover. However the low
levels are rather dry in the lowest 2km so it is very doubtful
whether any measurable rainfall will occur. Would not rule out a few
sprinkles here and there falling out of the mid deck, but did not
include that possibility in the forecast yet as it is difficult to
delineate what precise areas this may occur over. Although much
cooler than Sunday, highs will still be a little above normal on
Monday with a lot of readings in the 50s.

On Monday night, the aforementioned high pressure system in south
central Canada begins to strengthen further, likely anticyclogenesis
coming off the Canadian Rockies due to the northwest flow aloft. The
question mark for low temperatures is how much the mid clouds will
clear out from Monday, if any at all. Right now the profiles still
show a lot of moisture to contend with in the mid levels suggesting
that cloud cover should linger. Therefore opted to blend in some
fairly mild bias corrected ECMWF and GEM global temperatures into
the lows to warm them, especially for areas south of I 90. Then on
Tuesday, a short wave moves across the area denoted by upper QG
forcing. This is further enhanced by mid level forcing from 700-
500mb moving over northern Nebraska through the day, again denoted
by QG forcing. The frontogenetic forcing is pretty strong in our MO
River valley zones from 750-650mb near a mid level deformation zone.
Therefore high pops are warranted, at least in the morning, for some
light measurable rain or snowfall. If it does snow, with lows
projected in the lower to mid 30s in that area, its doubtful whether
much of it will stick, especially this time of year. So
accumulations should be very minimal in our southern zones, well
less than an inch. What is bothersome about all of this is the north
to northeast fetch of low level air coming down from south central
Canada. Witness last Friday night and Saturday a week ago when the
low level dry air took over and basically nothing happened. One
could state a scenario where this could happen again. Therefore for
now, really tightened up the POP gradient on the northern side
Tuesday morning compared to what superblend was giving, as we are
not convinced much of anything will happen north of the MO River
Valley zones.

As the high moves off to the east, the surface flow will once again
turn to the south and will become rather brisk by Wednesday. There
continues to be a lot of discrepancy between the various
deterministic models in what exactly will happen precip wise on
Wednesday, so confidence is low. The ECMWF is the most aggressive in
showing precip, likely a cold rain as warm air advection and theta
returns to the area right along the spine of the upper ridge axis.
The reason for this is because the ECMWF shows a discernible short
wave moving over the ridge. The GEM Global and GFS really do not
show much of a short wave, and thus show only very light precip
chances. Therefore for most locations, only chance pops are
warranted at this time for light rain or snow. One thing that is
more certain is that superblend was on the warm side of many
guidance readings. Therefore trended superblend down a category for
what should be highs a little below normals for this time of year.

The next system to watch will be late this week. Thursday is denoted
by warm air advection precip, then the system itself moves through
Thursday night and Friday. The ECMWF has a solution which is the
furthest south when compared to the GFS and GEM Global. The GFS is
the furthest north and quickest, with the GEM Global in between.
Ensembles are clustered around their deterministic solutions at this
time. Blending is the way to go this far out as its too early to
latch onto one scenario. There is a risk of thunderstorms in our
southern zones Thursday afternoon and night, even if following the
further south ECMWF, as the ECMWF places the 850mb warm front not
too far from I 90. Otherwise at this time given the thermal
profiles, it appears that much of the accumulating precip should
fall as rain.


&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1224 PM CDT Sat Mar 18 2017

VFR conditions through the period. Surface ridge will scoot east
this afternoon, with southeasterly winds increasing. Flow
increases aloft overnight tonight, leading to low level wind
shear. The threat will abate on Sunday morning as breezy
conditions develop at the surface.


&&

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

$$

SHORT TERM...BT
LONG TERM...MJ
AVIATION...BT



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