Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201129
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
629 AM CDT SAT AUG 20 2016

.SHORT TERM...(Today and Tonight)
Issued at 357 AM CDT Sat Aug 20 2016

Deep trough pushing across southeast South Dakota in the wee hours,
with lift forcing along sharp mid level frontal boundary leading to
a band of light to moderate showers west of a KBKX to KYKN line at
08z.  Forcing remains strong as trough trudges eastward through
early morning, but forcing does drop off fairly quickly behind the
trough axis other than some minor wrap around brushing the far
northern CWA toward highway 14 early this morning. Experimental HRRR
doing a fine job thus far as would be expected given the strongly
forced set up, and generally was followed with only minor
modification to speed up ending initial chance for precip through
early to mid morning.  A temporary break in cloud cover will work
into mainly areas near/west of I-29 and through the lower Missouri
valley during the morning, with a fair amount of clouds lingering
for most other areas. Gradually will see surface temperatures
moderate, and with the continued cooling aloft, likely to generate a
minor amount of surface-based instability by afternoon.  With
support of at least weak cyclonic flow through the lower to mid
levels, should be able to neutralize subsidence forcing from the
cold advection.  Likely result will be development of instability
showers, initially somewhat terrain driven, but likely over a good
portion of the CWA as consensus field of 100-300 J/kg MLCAPE
develops. CAMS and coarser models alike suggest the potential, so
have continued isolated to lower scattered level mention, along with
the isolated lightning threat. While have no qualms about mentioning
isolated to lesser scattered pops mainly east of the James valley,
have some concern that a shade too great of surface instability as
will certainly mix some increasingly drier air into the western
areas this afternoon.  Breezy northwest winds are expected most
locations, with 20 to 25 knots available to a deepening mixed layer,
especially near and east of I-29.

Other than a few very early evening sprinkles, would look for clouds
and winds to decrease fairly rapidly, leaving a cool night under
surface ridge axis across the area. Lows in the mid 40s to around 50
likely, which will be the coolest night since the start of July.

.LONG TERM...(Sunday through Friday)
Issued at 357 AM CDT Sat Aug 20 2016

Upper level troughing gradually becomes replaced with a flattened
ridge Sunday and Monday. With return flow developing on Sunday, and
southerly winds increasing quite a bit on Monday, plenty of warm air
advection can be expected. Strong mixing will result in gusty winds
of 25 to 35 mph in the afternoon. The return flow will also begin to
tap into higher dewpoints in the 60s. High temperatures will be in
the mid 70s to mid 80s on Sunday, then the mid 80s to mid 90s on
Monday.

Closed low over southern British Columbia slowly tracks eastward
along the US and Canada border early next week, arriving in western
North Dakota Tuesday afternoon. Initial shortwave moves in central SD
by the evening, while a frontal boundary drops across south central
SD. Model timing is coming into better alignment, although the GFS
remains slightly faster than the other solutions. With a good amount
of instability forming, scattered showers and thunderstorms will
spread along and ahead of the front in the evening, exiting to the
east Wednesday afternoon with the advancing front.

Cooler air once again returns to the northern Plains behind the
front, with upper level northwest flow briefly settling in on
Thursday. A bit more of a question beyond Thursday, with the GFS
bringing a brief bit of flat ridging before a secondary trough
arrives. The ECMWF meanwhile hangs into the initial trough through
the end of the week. Not a great deal of confidence in the pops
during this period, particularly with the timing, but will leave the
pops from the blend as is for now.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 629 AM CDT Sat Aug 20 2016

Areas of MVFR ceilings will accompany a band of weak showers
through northwest Iowa and southwest Minnesota early this morning,
leaving behind scattered to broken VFR ceilings.  Diurnal heating
will likely develop a healthy cumulus field by midday and early
afternoon, yielding isolated showers and a few thunderstorms
through the afternoon hours. A few wind gusts to around 25 knots
are possible. Showers and stronger winds should meet a rapid end
by early evening, leaving mainly clear skies with light winds
overnight.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...
AVIATION...Chapman



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