Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 140939

Area Forecast Discussion
National Weather Service Sioux Falls SD
439 AM CDT Mon Aug 14 2017

.SHORT TERM...(Today and Tonight)
Issued at 438 AM CDT Mon Aug 14 2017

Dense fog has developed along the Interstate 29 corridor north of
Interstate 90 this morning.  HRRR/RAP/NAM output indicates that
dense fog will become widespread north of Interstate 90 towards
sunrise. In anticipation of the fog developing, went ahead and
issued dense fog advisory through 9 am.

Once the fog clears, should be a fairly pleasant day with light
north winds transitioning to east as surface ridge passes across
North Dakota and shifts east towards the Great Lakes Region. Model
soundings hint that stratus that will transition to cumulus across
the north may keep temperatures slightly cooler north of I-90.  With
850 hPa temperatures around 11-12 C, expect highs to be within a
degree or two of 75 degrees.

With surface high pressure shifting of to the east, modest theta-e
advection in the vicininty of a stationary boundary develops
tonight possibly resulting in a few thunderstorms along the
Missouri River Valley Monday night. Of greater significance is the
potential for fog across southwest MN and adjacent areas. Cross
over temperature in these regions is 4-7 degrees higher than
forecasted lows. Have went ahead and added fog mention in the

.LONG TERM...(Tuesday through Sunday)
Issued at 438 AM CDT Mon Aug 14 2017

The main story in the extended is the potential for showers and
thunderstorms on Tuesday, with better chances Tuesday night into

Tuesday into Wednesday Morning: A weak upper ridge will continue to
move eastward across the area, while at the surface a frontal
boundary stalls south of the CWA, keeping most of the showers and
thunderstorms along and south of the Missouri River Valley and
central Nebraska through mid to late morning.  It will not be a
complete washout, but have kept low PoP chances through mid morning,
mainly across southeast SD.

The main concern/focus will be after 21Z Tuesday as strong upper
wave approaches from central Dakotas, and the surface low and
associated boundary slowly shifts eastward. Models continue to
suggest a strengthening low level jet, along with increasing mid-
level theta-e advection across the region. Confidence in timing and
position of showers/storms remains uncertain.  The
GFS/Canadian/ECMWF bring widespread PoPs after 09z Wednesday, while
the NAM moves faster and brings PoPs after 21Z Tuesday. Leaned
towards GFS/Canadian/ECMWF for showers and thunderstorms timing. In
fact, model soundings continue to suggest a well saturated
atmosphere after 00Z Wednesday with PWATs near to slightly above 2
inches. Have raised PoPs to categorical in the overnight hours
mainly along and east of I-29. Given decent dynamic forcing with
this system, marginally strong to severe storms cannot be rule out
in the afternoon and evening hours. The risk then turns to heavy
rain as we move into the overnight hours. At this time, agreed with
WPC in keeping the area under a marginal risk of excessive rainfall.

For the remainder of the extended, as the frontal boundary moves
into the Upper Mississippi Valley Wednesday afternoon, surface high
pressure will build over the region, bringing a dry conditions
through at least Friday. Spotty showers/storms return Friday night
into Saturday as a weak shortwave shifts east across the region.
Temperatures will remain slightly below normal, but trending
towards warmer than normal temperatures into next week.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1238 AM CDT Mon Aug 14 2017

Ceilings and fog will be problematic this morning along and north
of I-90. Visibility may become less than 1 sm at times. Less
confident on ceilings with fog, which should allow the fog to
buirn off relatively quickly Monday morning. After the fog burns
off, expect relativly pleasant conditions on Monday with light
north/northeast winds. Could have some fair weather cumulus
develop during the day. Expect the greatest coverage of cumulus to
be along and north of I-90.


SD...Dense Fog Advisory until 10 AM CDT this morning for SDZ038>040-

MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ071-072-

IA...Dense Fog Advisory until 10 AM CDT this morning for IAZ001.



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