Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 160915
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
415 AM CDT Sat Sep 16 2017

.SHORT TERM...(Today and Tonight)
Issued at 415 AM CDT Sat Sep 16 2017

Unseasonably cool and deep trough in the northern Rockies this
morning. How cold? Cold enough for snowfall at elevation over parts
of WY and MT. Closer to home, displacement of the warm and
summer-like air has occurred thanks to a leading wave pushing
northeast into southern MN/northwest IA, continuing to support a
gradually weakening low-level jet and some convection. The front has
been a bit quicker than expected - along a KMWM to KSUX line at 08z.
Back to the west closer to the main trough and jet pushing toward
southwest SD, another enhancement to elevated convection over the
last several hours in the western high plains which has been subtly
shifting eastward toward central portions of the state. Evolution of
the precipitation threat is the main focus on the short term, with
secondary attention to the much colder temperatures in place through
tonight.

Have been far from impressed how almost all guidance has been
handling both ongoing precipitation as well as how they forecast
short term evolution, but some of the short term, high-res forecasts
in WRF-ARW/HRRR may be starting to pick up on the right trends.
Should continue to see decrease in coverage of existing convection
over northwest Iowa early this morning with weakening and veering of
low-level jet, but even despite a dropoff in deeper lift forcing
through the morning, there is a persistence to elevated layer of
theta-e advection and weak elevated instability in the post frontal
environment even through the morning which will spread northeast
across eastern SD/southwest MN/northwest IA. Thunder will certainly
be far less common than showers in this pattern through the morning.
 Will also see the more organized area of showers and thunderstorms
to the west slide northeast to mainly impact areas west of the James
River valley through mid morning, again potentially dragging a tail
of precipitation somewhat further eastward through the morning.

By afternoon, there looks to be another increase in deeper lift
forcing as the jet begins to push more strongly into the central
Dakotas, and this is likely to enhance the coverage of the post-
frontal showers somewhat more. Again, some minor and thin elevated
instability will continue across southeast SD, with gradually higher
amounts heading toward the southeast where could perhaps be close to
1000 J/kg toward Spencer through Ida Grove areas near the frontal
location. At this point, the frontal boundary looks to be completely
through the CWA by 20z, if not even a touch sooner, which reduces
the effective shear quite a bit and decreases the impact of a deeper
convective layer. At this point, the severe weather threat for the
area is likely quite low in just a small sliver of northwest Iowa
toward KSLB, and basically none in other locations.

Temps tricky today around the location of the frontal boundary and
how quickly it clears the area. Those areas mired in the stratus and
in continued cold advection today will likely struggle to recover
more than a few degrees after continuing to drop into the mid
morning hours.  Some clearing late day could boost areas west of the
James River, but not by much in the cold air mass. Tonight, will see
progression of clearing, and large ridge sliding through the
Missouri River valley to start to brake the cold advection winds a
bit. Could see a few upper 30s in the James Valley, but generally
40s across the area, warmest in stronger gradient and later
clearing toward SW MN and IA Great Lakes.

.LONG TERM...(Sunday through Friday)
Issued at 415 AM CDT Sat Sep 16 2017

Medium and extended range guidance continues to have differences,
but at least have trended to show a few potential impact points in
the forecast.

We`ll start the medium range out on Sunday with a fairly nice, but
cooler finish to the weekend. Temperatures may only reach the upper
60s, and we`ll begin to see increasing mid-level clouds by late
Sunday.

Southwest flow aloft will allow a series of minor shortwave troughs
to move through the region by Monday.  It doesn`t look like we`ll
have any widespread heavy rains, but scattered showers with
occasional thunder will be possible. The greatest chances will be
south of I-90.

Deep troughing begins to move into the Pacific NW on Monday, with a
strong jet nosing into the Plains by Tuesday.  Energy rounding the
trough will force a frontal boundary eastward during the day,
establishing the potential for strong to severe convection by the
afternoon and evening hours along the front.  Still considerable
uncertainty regarding the convective details, including moisture
return, but some potential for a high shear and low instability
setup with a progressive linear squall line into the evening in the
Dakotas.

Model consensus pushes very dry air into the area for Wednesday as
high pressure races southeast behind the boundary.

Concerns begin to grow for the end of the week as the longwave
pattern would turn favorable for a rapid advection of warm air and
moisture back northward on Thursday and Thursday night.   Again, yet
another strong upper jet and upper wave pivots around the longwave
trough by Friday, shoving a frontal boundary into the area.  Still
plenty of time to for things to change, but certainly increased
severe weather threats appear to be headed into the Plains and
Midwest by the end of next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1042 PM CDT Fri Sep 15 2017

A boundary will continue to push across the area tonight into
Saturday with winds transitioning to northwesterly behind the
front. Showers and thunderstorms are expected to develop along and
ahead of this boundary during the overnight hours. Also, MVFR/IFR
stratus will fill in behind the boundary, spreading eastward
across the area tonight into Saturday.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...JM



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