Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 131745

Area Forecast Discussion
National Weather Service Sioux Falls SD
1245 PM CDT Sun Aug 13 2017

.SHORT TERM...(Today and Tonight)
Issued at 353 AM CDT Sun Aug 13 2017

Difficult forecast this morning as a complex of storms to the north
and south of the area.  The complex to the south is disrupting
inflow to the region, so despite a a broad area of positive 850 mb
theta-e advection in the region, am not expecting significant
development across the region like the HRRR suggests. Instead, am
favoring a drier solution, but still may see some shower activity
expand across northwest Iowa through the mid morning hours.
Thereafter, shortwave is expected to rotate around the upper level
low across southern North Dakota. Area could see some scattered
thunderstorms develop this afternoon, though the greatest
instability appears to be contained across the southern edge of
the forecast area near the Missouri River Valley. In this area,
there could be an isolated thunderstorm that produces hail to the
size of quarters, or a wind gust to 60 mph due to the dry air
throughout much of the depth of the troposphere.

Convection will wane through the overnight tonight as wave shifts
towards the east.  As skies clear across central South Dakota,
temperatures will cool into the mid 50s while further south and east
clouds will keep temperatures in the 60s.

.LONG TERM...(Monday through Saturday)
Issued at 353 AM CDT Sun Aug 13 2017

The main concern is the potential for showers and storms Tuesday
night into Wednesday as unsettled weather return.

On Monday, some scattered showers and storms may linger across
portions of northwest Iowa and southwest Minnesota through midday.
There will be a lull in precipitation for the rest of the day,
giving way to a partly cloudy and mild day. Highs will range from
the mid 70s to mid 80s. High temperatures over southwest Minnesota
will only reach the low to mid 70s.

By Monday night, southerly surface flow will become more dominant,
increasing mid-level theta-e advection, and precipitation chances
into the area through Wednesday. A surface low pressure will develop
across central/eastern Rockies and western Dakotas with a warm front
lifting northward into the Missouri River Valley by late Monday
night. Compared to previous forecast runs, models are now in better
agreement into bringing the warm front along the Missouri River by
00z, then along and south I-90 by 06z. Models begin to diverge after
06z regarding the location and extent of the frontal boundary. The
NAM is the fastest one, bringing the front along and north of
Highway 14 by 12z Tuesday, while the GFS/ECMWF/Canadian remain
slightly along and south of the I-90. Have leaned towards the
GFS/ECMWF/Canadian, with increasing showers and storms along and
ahead the warm front. The highest chances will likely occur after
06Z when the nocturnal low-level jet strengthens along the boundary,
bringing PoPs likely west of the James River.

Off and on periods of showers and storms continue on Tuesday, before
widespread rain and storms chances arrive in the evening. As the
warm front lifts northward, a strong upper wave and associated cold
front over central Dakotas will be approaching the area late in the
day. With diurnal heating, and model soundings suggesting a rich-low
level moisture; showers and storms will become likely into the
evening and overnight hours. Better PoPs chances will occur north of
I-90 and east of I-29. At this time, the severe weather risk from
Monday night into Wednesday is marginal, but will need to keep
monitoring this time frame. Decent dynamic and forcing could favor
strong to severe storms across the area.

Precipitation chances will gradually diminish from west to east on
Wednesday morning. There are differences between models from Thursday
into Saturday in terms of timing of multiple embedded shortwaves and
coverage of PoPs. Looks like the next chance of showers and storms
could be Thursday night into Friday, but due to low confidence, have
left it as the blend suggests with scattered showers/storms.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1228 PM CDT Sun Aug 13 2017

Several concerns through the next 24 hours. Isolated to Scattered
showers and thunderstorms will track across the region late
afternoon through the overnight hours. Activity will begin in
south central SD late afternoon and track southeast through the
region. A few strong to severe storms may contain hail and strong
winds. MVFR ceilings will lift to low end VFR later this
afternoon, but will likely drop back to MVFR in shower and
thunderstorm activity. Low clouds and patchy fog will become more
widespread late tonight into Monday morning.




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