Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 182250
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
550 PM CDT SUN SEP 18 2016

.SHORT TERM...(This evening through Monday)
Issued at 303 PM CDT Sun Sep 18 2016

Boundary is finally now moving into south central SD. This front
continues its eastward track this evening, then washes out near the
I29 corridor late evening and overnight. Breezy south-southwest
winds this afternoon ahead of the front will diminish some this
evening, with winds becoming around 5 mph overnight. While good
mixing has lower dewpoints in the western half of the region,
residual moisture and light winds across the Iowa Great Lakes region
may allow some patchy shallow fog to develop late tonight. Further
north, a weak shortwave along with mid level frontogenesis setting
up across northern South Dakota will produce some mid level clouds
primarily along the highway 14 corridor, however very dry low levels
will prevent much of this activity from producing measurable rain
showers.

The wave exits to the east along the quasi-zonal flow, leading to
slightly cooler temperatures, albeit still seasonally mild across
the region on Monday. Clouds will clear in the morning while north
to northwest winds increase to around 7 to 15 mph in the afternoon.
Highs are expected in the mid 70s to lower 80s.

.LONG TERM...(Monday night through Sunday)
Issued at 303 PM CDT Sun Sep 18 2016

Monday night into Tuesday will see a weak mid level boundary,
suffering from dry air, found mainly north of Interstate 90. The
chances for showers still appear like they will remain north of the
area so will not introduce any precipitation chances. Otherwise a
very nice period, albeit a bit breezy, with morning lows in the 50s
and highs in the upper 70s to mid 80s.

Tuesday night still posing a threat for thunderstorms but the latest
runs of the models are just a bit farther north with the warm front.
That may put the main threat for thunderstorms to the north of the
area, especially given that the latest GFS and Nam soundings look
fairly capped. This is due to a bit of a loss in moisture in the
925mb to 800mb layer. If there is a way to get the thunderstorms
going the best chance will be north of Interstate 90 and probably
east of Interstate 29. This is one area where the cap does show some
weakness. A few storms could become severe with elevated CAPE values
about 2000 J/kg to 3000 J/kg, which is backed down from the
suggested 4000 J/kg to 5000 J/kg that the Nam is showing. Much of
the summer it seems as though the Nam has been overdoing
instability. Shear also very supportive with good mainly
unidirectional speed shear which could support splitting cells if
something can develop.

Late Tuesday night into Wednesday morning an upper level wave will
move from the high plains into southeast SD. This may help to back
and regenerate the low level jet and convergence to the west which
would keep a continued threat for thunderstorms through the morning,
with a possible lull in the afternoon as the wave moves by. But by
late afternoon and evening with a surface front nearby and a
potentially very unstable atmosphere additional severe thunderstorms
could be possible. The late night and morning thunderstorm threat
would likely see better chance south of Interstate 90 but may suffer
from dry air issues in the low levels. However, if storms can fire
over the western high plains and advect into the area very dry air
below 600mb could support an enhanced threat for strong winds. All
in all, while it is too early to come up with an exact answer,
assuming models are fairly close to the expected environment Tuesday
night into Wednesday night could prove to be a busy period of
thunderstorms.

It does look like there could be a window Thursday when any
thunderstorm activity in the region would be to the south.
Temperatures would be limited with easterly flow so highs in the 70s
most likely, possibly a little cooler if clouds are more widespread.

Thursday night into Friday will see a deep upper level low pressure
work into the central Rockies which will begin to force the low to
mid level boundary back to the north. Another threat for
thunderstorms, a few again severe, will be possible. The models all
consistently place the area into the warm sector by Friday afternoon
and Friday evening, then swing the main wave onto the plains
Saturday. The main threat would likely be north of highway 14 Friday
night then if we remain in the warm sector Saturday the enhanced
low to mid level shear will likely provide the area with a threat
for a few tornadoes. A lot of timing needs to come together for
these threats so will remain mid range chances and will need to
watch the trends from run to run.

Saturday night into Sunday will see the cooler air begin to settle
into the region bringing a good chance for a BIG pattern change for
the last week of September.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 548 PM CDT Sun Sep 18 2016

Mainly VFR conditions through the TAF period. There will be a
possibility of patchy fog development through northwestern IA
late tonight, with the potential to affect KSUX.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...
LONG TERM...08
AVIATION...JM



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