Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 121712
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1212 PM CDT Thu Oct 12 2017

.SHORT TERM...(Today and Tonight)
Issued at 330 AM CDT Thu Oct 12 2017

The main forecast concern for today are temperatures ahead of a
cold front that will bring cooler air in over night.

There were not significant differences between models other than
the warm bias in the GFS for low level temperatures. Current
satellite picture shows stratus between Hwy 71 and I-35 in
Minnesota and Iowa. With southerly flow at 925 mb, these clouds
are not expected to move too much farther to the west early this
morning. After sunrise these clouds will move to the east. With
925 mb winds from 20-30 kts aloft, full sun and a relatively dry
air mass, expect to mix to around 3000 ft and this should bring
some of the stronger winds to the surface with gusts up to 30 mph
this afternoon east of the James River. With the mixing and sun
have raised highs into the lower to mid 70s west of I29 with upper
60s in the east. Overnight clouds will slowly increase after
midnight. There are some hints of weak frontogenesis from
southwestern South Dakota into south central SD after midnight. It
will be dry below the cloud deck so did not add precipitation
chances. Lows will be in the upper 30s to upper 40s.

.LONG TERM...(Friday through Wednesday)
Issued at 330 AM CDT Thu Oct 12 2017

For Friday through Wednesday, the focus of the discussion will be
rain chances through Saturday night.

The cold front that moves through tonight will slow near the Iowa
and Missouri border late Friday night and then will waver across
southern Iowa and southeastern Nebraska through Saturday. A weak
upper level wave will move across the eastern Plains on Friday.
The dynamic forcing for ascent is very limited with some weak
frontogenesis around 700 mb from far northeast Nebraska toward
Storm Lake. The combination of the front and wave could bring a
few sprinkles or light showers south of a YKN to FSD to MWM line
with any measurable precipitation really confined to northwestern
Iowa. This will not be an all day rain but just a few small showers
across the area with the best chance from late morning into mid
afternoon. The main impact of this will be clouds and cooler air
which will keep highs in the upper 50s to lower 60s.

As mentioned above, the front will linger across far southern Iowa
Friday night so cannot completely rule out a shower around SLB and
SUX, but most of the night should be dry. Lows will be in the
upper 30s to upper 40s. On Saturday, the long wave trough will
move across the Dakotas reaching western Minnesota and Iowa by 00Z
Sunday. The NAM and Canadian are slower with this system than the
ECMWF and GFS. The GFS ensemble favors a quicker motion as well
and have sided with the EC/GFS for this system. The deeper
moisture will remain well south of the area. However, the flow
will turn to the southeast around 850 mb and this will allow
limited moisture to return to the area. In addition, the approach
of the long wave trough will be accompanied by a 700 mb boundary
as it moves east. With moisture present, the lift associated with
these features should be able to produce a few showers across the
area with the best chance being Saturday afternoon northeast of a
line from Huron to Sioux City. For southeastern South Dakota and
southwestern Minnesota, this a scenario where there is high
confidence most areas will see light rain but amounts will be less
than a tenth of an inch. In other words this will not be an all
day rain just a few light showers. Of course, with cool air and
clouds, highs will only be in the lower to mid 50s. In
northwestern Iowa, there will be more moisture and there is even
elevated CAPE around 200 J/kg so an isolated thunderstorm is
possible around Storm lake. Rainfall may last a little longer in
northwestern Iowa such that 0.1-0.2 inches of rain are possible.

The wave will move east on Saturday night. The GFS and ECMWF show
any precipitation ending before sunset even in northwestern Iowa.
The Canadian and NAM are slower. With uncertainty on how fast the
wave will be did keep a chance of rain into Saturday night
although trends are favoring the rain ending sooner. With clearing
skies and cooler air moving in, lows will range from the mid 30s
around Huron to lower 40s in northwestern Iowa.

For the extended, there was general agreement between the GFS,
ECMWF and GFS ensemble through Wednesday. Only the Canadian was
markedly different showing a significant trough deepening west of
the Great Lakes by Wednesday. No member of the GFS ensemble shows
this solution although many show a much weaker trough moving
southeastward into the Great Lakes. So have a slight chance of
showers Wednesday afternoon east of the Buffalo Ridge. However, at
this time, dry weather looks more likely. Otherwise, expect a
warming trend into early next week. Temperatures will be below
normal Sunday and warm to above normal by Tuesday. There are
signals that highs could even warm into the 70s sometime next
week. However, we will wait until there is higher confidence
before increasing highs to be 10+ degrees above normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1210 PM CDT Thu Oct 12 2017

IFR ceilings will diminish in northwest IA between 19-20z. After
this VFR through the period. Small chance for fog in nortwhest IA
on Friday morning, but chances are low.

&&

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

$$

SHORT TERM...Schumacher
LONG TERM...Schumacher
AVIATION...08



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