Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 261111
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
611 AM CDT FRI AUG 26 2016

.SHORT TERM...(Today and Tonight)
Issued at 316 AM CDT Fri Aug 26 2016

Main focus in the short-term is the rain chances later this
afternoon and overnight. The GFS was a real outlier with almost no
precipitation.  In general, followed closer to a NAM/ECMWF
compromise although confidence is lower than normal as will be
detailed below.

In the large scale, all models agree that there will be a rather
weak upper wave moving through Nebraska and into Iowa later this
afternoon and tonight. As this wave approaches, the 850 mb theta-E
gradient will also slowly move northward toward Highway 20. This
midlevel front will move northeastward through the night reaching
near I90 before the upper level wave move east toward I-35 in Iowa
and Minnesota. Stability is very limited and MLCAPEs are much less
than 1000 J/kg. So not only is severe weather not anticipated but
the limited instability will mean that thunderstorms will be rather
isolated. So most areas that receive rain will likely not even hear
a clap of thunder. The limited instability will also mean that rain
rates will be low such that flooding will also not be an issue.

There will be two areas of precipitation with the system. The first
area, which has developed in southwest SD, is expected to move east
or east northeast through the day reaching the Missouri River later
this afternoon or this evening. This rain will primarily impact
areas west of the James River and it is expected to gradually
diminish as it moves east of the Missouri in southeast SD since
development of new precipitation in Nebraska will likely focus
ascent farther to the southeast. So kept PoPs generally from 40-50
percent late this afternoon and evening with this area of rainfall.

The second area of rain will be developing around 0000 UTC in
Nebraska as the upper wave begins to interact with the 850 mb front.
From a large scale perspective, the interaction of the wave and
front should keep most of the rain in Nebraska and Iowa with only
very light rain getting into southeast SD and southwest Minnesota.
And this essentially what the GFS, Canadian GEM and ECMWF are
showing as all these models keep the heaviest precipitation closer
to I80. The NAM does show a band of heavier rainfall near the Iowa
and Minnesota border but that is an outlier of the lower-resolution
models. On the other hand both the WRF-ARW 4 km and WRF-NMM 4 km
have heavy rain moving from northeast Nebraska into southeast SD and
southwest Minnesota overnight. Given the lack of instability the
model QPF of 1-2 inches seems overdone. But given the consistency
between the two models, am reluctant to completely disregard this
solution. What seems most certain is there will be rainfall from
northeast Nebraska into extreme southeast SD, northwest Iowa and at
least south of I90 in southwest Minnesota. This is where the highest
chance of rain is forecast for overnight with the heaviest rainfall
of 0.25-0.5 inches forecast in the vicinity of Hwy 60.  However, the
potential for rain may extend farther to the northwest. So the
location of the heaviest rainfall, which could exceed 0.5 inches
overnight, remains uncertain and will likely shift as additional
information become available today.

.LONG TERM...(Saturday through Thursday)
Issued at 316 AM CDT Fri Aug 26 2016

The shortwave slides off into the Upper Mississippi Valley on
Saturday morning, with showers and isolated thunderstorms east of
Interstate 29 winding down. The main upper level low will slide
across North Dakota in the afternoon, and models suggesting that
some of the resultant precipitation could scrape our far north, so
allowed for slight pops north of Highway 14 in the afternoon.
Otherwise, the general trend will be for decreasing clouds during
the day, and it will be a touch warmer as highs climb into the mid
70s to lower 80s. Saturday night looks to be dry with lows in the
lower 60s.

The southerly flow increases on Sunday with high pressure centered
over the Mississippi Valley and a deepening surface trough over the
northern Rockies. With that will see the warming trend continue as
highs climb into the lower 80s east of Interstate 29, to near 90
over the James River Valley and south central SD.

For the remainder of the extended period, a zonal upper level flow
at the beginning of the week transitions to upper level ridging
over the central CONUS by midweek. A surface boundary settles just
to the north of our area on Monday, and then meanders across our
area into Wednesday. This boundary could be the focus for
thunderstorm activity during the period, though models are all
over the place with respect to coverage/timing of convective
activity during this time frame. What is more certain is that
temperatures will remain above normal for next week, with highs in
the 80s to lower 90s and lows in the 60s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 611 AM CDT Fri Aug 26 2016

Two primary concerns for aviation. The first is patchy valley fog
in parts of northwestern Iowa including KSUX. The fog is rather
isolated and expect that it will rapidly dissipate after sunrise.
The second concern is possibility of rain and associated MVFR and
IFR conditions. While rain is possible around HON, the atmosphere
is drier and MVFR/IFR conditions are unlikely. For KSUX and KFSD,
the chance of rain is much higher and there will be a deeper layer
of moisture coming in. If rain does occur, it is likely ceilings
will fall below 3000 ft with MVFR/IFR vsbys. At this time, it
appears there is a better chance of longer duration light rain at
KSUX so am forecasting IFR conditions after 08Z. KFSD will be on
the northwestern edge of the rain and lower clouds so only have
MVFR ceilings after 09Z. There is also the possibility of
intermittent light fog late tonight due to light winds and a moist
atmosphere.

&&

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

$$

SHORT TERM...Schumacher
LONG TERM...JM
AVIATION...Schumacher


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