Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 201134
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
634 AM CDT Thu Apr 20 2017

.SHORT TERM...(Today and Tonight)
Issued at 331 AM CDT Thu Apr 20 2017

Main forecast issues for today and tonight are persistence of low
clouds today and then potential for fog and frost tonight.

Latest satellite fog imagery shows that the stratus extends east of
a line from Hettinger, ND to Valentine, NE. The clouds are
generally below 1000 ft and low level flow is basically parallel to
the edge of the low clouds with 925 mb winds 10 kts or less. This
will mean only a very slow advection of clouds today. In addition,
there is only weak cold air advection around 850 mb so cooling will
only slowly increase boundary layer lapse rates to help mix out
clouds. With April sun, there will also be some surface heating
which will also help to slowly increase lapse rates but the impact
of both the weak cold advection and surface warming may be to just
slowly raise ceilings rather than break up the stratus - except near
the edge of stratus where it may be thinner and mixing will be more
effective. Therefore expect the clearing to only slowly move north
and east through the day with some breaks in the Missouri Valley by
noon and perhaps across most of southeast South Dakota and
northwestern Iowa by early evening. Southwest Minnesota looks to
remain cloudy during the day. Tonight, as the ridge move over, there
will be stronger subsidence below 700 mb and expect that will erode
most of the stratus. With the ample cloud cover, lowered highs into
the lower 50s in southwest Minnesota, mid 50s in most of northwest
Iowa and east central South Dakota and upper 50s to lower 60s in the
Missouri Valley where there will be more sun today.

Overnight, winds will go light. There will still be a lot of
boundary layer moisture as the daytime stratus will prevent this
moisture from mixing out. With clear skies and light winds, expect
temperatures to fall into the 30s in most areas with mid 30s
basically along and north of I90 and in the Iowa Great Lakes. This
will mean two things - first as temperatures cool, expect at least
patchy fog to develop after midnight. Also where it falls into the
mid 30s, there is enough near surface moisture that there patchy
frost is possible - at least in sheltered areas. At this point,
think that fog will not be widespread enough to prevent temperature
at the ground from reaching freezing allow frost to develop.
If fog is more widespread and dense then that may keep temperature
near the ground above freezing and prevent frost from forming.

.LONG TERM...(Friday through Wednesday)
Issued at 331 AM CDT Thu Apr 20 2017

Ridging surface and aloft will allow for a nice end to the work
week, with clear to partly cloudy skies and light east-northeast
winds across the region on Friday. Temperatures will warm a few
degrees further than today, with highs in the mid 50s to around 60.
The warming trend will continue through the weekend, as the upper
ridge is undercut by a southern stream storm system tracking through
the central/southern plains. Abundant sunshine and slight low level
warming will help temperatures climb a few degrees warmer Saturday,
despite limited mixing within lingering surface ridge.

Thermal ridge shifts eastward Sunday as the low level ridge axis
slides east and deeper south-southwesterly flow develops across the
Plains. Position of surface ridge will limit any low level moisture
return, and any cloud cover from a weak northern stream wave should
remain north of the forecast area. Given expected sunshine and more
favorable low level mixing, have nudged temperatures toward warmer
available guidance for Sunday, with highs in the upper 60s to lower
70s across the area.

Unlike last night, seeing less agreement among the models in
timing/strength of various waves set to track across the northern
Plains during the latter part of this forecast period. All show some
semblance of a mid-upper trough affecting the region Monday/Monday
night, but disagree on southern extent of precipitation chances.
GFS/ECMWF focus greater rain chances farther north, largely north
of Highway 14 corridor during the day Monday, while Canadian expands
rain southward much closer to I-90. Will carry chance pops south of
Highway 14, mainly in the afternoon/evening as weak instability may
support spotty development south of the main wave, but confidence is
generally low regarding precip chances during this period. Even less
agreement is seen among the models as we move toward the middle of
next week, and for this reason made little change to blended model
pops, which spread another chance of precipitation across the region
Tuesday night and Wednesday.

As far as temperatures, Monday will remain on the warmer side of
normal. Low level jet will hold Sunday night lows in the upper 40s
to possibly lower 50s, with highs on Monday still in the 60s. Could
be on the windy side Monday as well, with gusty south winds ahead of
a cold front which pushes into eastern South Dakota throughout the
afternoon. Fortunately, slightly cooler temperatures than Sunday,
along with slightly higher dew points, should hold fire danger in
the moderate to high category. Cooler air returns for Tuesday and
Wednesday, and current low level model temperatures suggest going
highs in the mid 50s-lower 60s may be a bit too warm. However, given
aforementioned uncertainty in how this pattern will evolve toward
the middle of next week, have not made any changes to blended model
temperatures at this point.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 633 AM CDT Thu Apr 20 2017

Low clouds will be the primary issue for aviation the next 24 h.
MVFR with embedded IFR ceilings extend across all of eastern South
Dakota, western Minnesota and western Iowa. As northwest winds
increase this morning and drier air moves in, ceilings are
expected to slowly rise at all 3 TAF sites - becoming MVFR by mid
morning and VFR by early this afternoon in Sioux City and late
this afternoon or early evening in Huron and Sioux Falls. Any low
clouds will dissipate shortly after sunset. However, light winds
and clear skies may result in some fog after midnight. This fog
could be dense enough to produce IFR visibilities late tonight
but it will also be fairly patchy. Confidence is highest around
HON to include MVFR visibilities later tonight but have not
included fog in KFSD or KSUX TAF at this time.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Schumacher
LONG TERM...JH
AVIATION...Schumacher



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