Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 172035

Area Forecast Discussion
National Weather Service Sioux Falls SD
335 PM CDT WED AUG 17 2016

.SHORT TERM...(This evening through Thursday)
Issued at 335 PM CDT Wed Aug 17 2016

High pressure will drift east tonight with south to southeast flow
gradually increasing. Some fairly weak elevated warm air advection
and decreasing stability should allow for a few thunderstorms to
develop from central SD into northeast SD, but at this time not
expecting severe thunderstorms. This activity should weaken or shift
north of highway 14 by mid morning Thursday as a boundary slowly
slides southeast. From mid morning into the afternoon warm and dry
air will build in to the lower and mid levels allowing a cap to
develop, so thunderstorm potential will be greatly diminished. By
very late in the afternoon heating along and just ahead of the
incoming front will likely allow a couple of thunderstorms to
develop. While CAPE values will likely be about 1500 to 2000 J/kg
shear looks marginal. While the winds are not very strong, there is
some turning of the winds in the surface to about 2 km layer just
ahead of the front so will need to keep an eye on that. So, a severe
storm will be possible late in the afternoon. Lows tonight mild and
mainly mid 60s while Thursday will be hot with highs in the mid 90s
near the incoming front and close to 90 in southwest Minnesota and
northwest Iowa.

.LONG TERM...(Thursday night through Wednesday)
Issued at 335 PM CDT Wed Aug 17 2016

Cold front timing a little uncertain Thursday night/Friday, as main
upper trough lags well behind, so speed/movement of the low level
boundary may initially be more determined by storm interaction than
by forcing from aloft. That said, greater model consensus has the
surface boundary pushing southeast a little faster than previous
runs, with the 12Z NAM the exception on the slow side.

For Thursday night, instability/mid level lapse rates are decent
with warm low level air ahead of the boundary and some cooling aloft
as lead wave lifts northeast along the boundary. Shear profiles on
the weaker side despite increasing low level jet, especially south
of Highway 14 corridor, as stronger mid-upper level winds remain
focused farther north, but will have to watch for isolated elevated
strong-severe storms through the evening/overnight hours.

Faster trend in boundary timing should lessen potential severe
coverage for Friday, as the non-NAM solutions all point to the cold
front pushing southeast of Sioux City/Storm Lake by 18Z Friday. Some
lagging instability across northwest Iowa into the afternoon, and
shear gradually increases with approaching trough, so will still
have to watch for afternoon severe storms in our southeast, most
likely southeast of Highway 60. However, appears most of the area
would be looking at more general showers/thunderstorms as upper
trough moves into a fairly moist post-frontal air mass. Highs on
Friday remain tricky, as will be largely dependent on boundary
location/timing. With strong thermal gradient, areas northwest of
the boundary should see steady to falling temperatures in the lower
70s to upper 60s through the afternoon, while areas to the southeast
warm into lower to maybe mid 80s. Occasional showers expected to
linger north of the boundary into Friday evening, but should see
general drying trend through later Friday night/Saturday. Again
slower NAM is an exception here, holding on to a stronger/slower
upper trough which lingers some showers in western/central SD late
Friday night, and into eastern SD Saturday, as another wave drops
into the back side of the trough.

Cooler air settles into the region for the weekend, with highs in
the upper 60s-lower 70s Saturday. With faster timing of the upper
trough, could begin to see warmer air work back into western parts
of the region by Sunday. Gradual warming trend still on tap through
the early part of the week, with generally dry conditions through
Tuesday. GFS/ECMWF then showing another trough swing into the region
with some scattered storm chances Tuesday night/Wednesday.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1239 PM CDT Wed Aug 17 2016

Spotty thunderstorms possible late tonight and early Thursday
morning near KHON, otherwise VFR.


.FSD Watches/Warnings/Advisories...


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