Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 140856

Area Forecast Discussion
National Weather Service Sioux Falls SD
356 AM CDT Thu Sep 14 2017

.SHORT TERM...(Today and Tonight)
Issued at 356 AM CDT Thu Sep 14 2017

Shortwave trough moving northeast through the Dakotas will bring a
narrow corridor of moisture into the western CWA this morning. These
light showers should struggle as they move eastward and forcing
moves away. Nevertheless, will keep a low PoP to the James river
through mid-morning.   Further south, strong isentropic lift on the
315K sfc across eastern Nebraska should promote mid-level clouds and
a few showers that should stay south of the CWA.

Today: Model consensus brings a sfc trough slowly southeast through
the day, perhaps stalling it along a Lake Andes to Brookings line by
this afternoon and bisecting the CWA by the evening. The northeast
advection of an EML should prevent much in the way of convection
later this afternoon.  Temperatures will again climb nearly 10 or
more degrees above normal.  Have bumped winds across NW Iowa and
adjacent areas of NE/SD/MN this afternoon with mixing up to 800 mb
supportive of downward momentum transfer of wind gusts 25-30 knots.
Only other thing to watch would be for potential of wildfire smoke
to linger in behind the frontal boundary over central SD this
afternoon and evening. HRRR simulations do support some smoke moving
SE, but no real confidence on whether or not it will be thick enough
to impact visibilities.

Tonight: Have backed off on PoPs during the first half of the
overnight hours, and introduced a low end PoP by midnight as models
begin showing a warm advection signature within the 700:600mb layer.
Whether or not moisture is sufficient is in question, but could see
a decent amount of virga or very light showers focused primarily
along or southeast of the surface boundary from Lake Andes to
Marshall and southeastward.

.LONG TERM...(Friday through Wednesday)
Issued at 356 AM CDT Thu Sep 14 2017

Warm and breezy conditions are expected on Friday.  Both the NAM and
the GFS hint at early morning elevated convection along and east of
the I-29 corridor, while the GEM and ECMWF remain dry. Left guidance
pops as is, which keep the highest pops in the west in the vicinity
of a leading shortwave lifting northeast through the region.

Increasing winds ahead of the front on Friday will aid in
destabilizing the atmosphere with strong low level moisture
transport with flow extending from the Gulf of Mexico northward.
Fairly stout cap will likely keep the bulk of the day dry, but as
strong wave pivots into Central South Dakota towards 00Z expect the
cap to break and thunderstorms to develop along the front. Good
veering profiles, with 0-6 km bulk shear values near 40 knots. With
steep environmental lapse rates in the hail growth zone, expect
stronger storms to be capable of producing large hail.  In addition,
environmental winds are fairly strong, so potential for a severe wind
gust or two is also possible.

Front is expected to stall Friday night near highway 60 in northwest
Iowa into southwest Minnesota.  North of the front, strong cold air
advection and stratus is expected to prevail with a fair amount of
stratus.  East of the front, expected good heating leading to a
strong thermal contrast across the area on Saturday. Less mid-level
forcing on Saturday afternoon into Saturday night near the front, so
the mid range chance pops look reasonable for now.  Weaker shear
profiles on Saturday, so think the severe weather threat is much
more conditional, if something can even develop.

Cooler conditions are expected on Sunday with surface ridge building
into the area. This doesn`t last for long though as breezy southerly
flow develops early next week and continues through much of the
week. Pattern is relatively slow to evolve as Jose works north along
the eastern seaboard keeping the plains in a windy pattern.  A
series of short waves break off elongated trough across the west,
creating the chance for scattered rounds of convection. Not the best
model agreement on timing of waves, and for now have left blend as


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1040 PM CDT Wed Sep 13 2017

In the near term for the rest of tonight, will continue to monitor
the possibility for MVFR visibility due to smoke at KHON. MVFR
visibility reduced by smoke still resides north of this forecast
area in northern SD, but so far has not oozed southward. The
current expectation is it will not. Also, the chances of the
isolated TSRA currently in central SD making it eastward to KHON
late tonight are not strong enough to warrant a mention in the TAF
site itself. To the south, mentioned some MVFR fog at KSUX late
tonight and very early Thursday. Surface dew point depressions are
small once again in the Missouri River valley, so cannot rule
fog. Did not want to mention IFR fog, or worse LIFR fog, because
any fog formation will be very shallow.




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