Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 242007
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
307 PM CDT SAT SEP 24 2016

.SHORT TERM...(This evening through Sunday)
Issued at 307 PM CDT Sat Sep 24 2016

Well advertised large upper low will lift slowly northeastward
tonight and Sunday, with the center of the low tracking from
northwest SD, to around Lake Winnipeg during that time frame. A
subtle short wave is now discernible moving from south to north
across South Dakota this afternoon and early evening, then enters
North Dakota late this evening and overnight. Mass transport of deep
layer moisture is evident ahead of this short wave right in our
area, generally east of the James River Valley, where widespread
showers and storms will be prevalent through the early evening
hours. Areas of heavy rainfall will also occur, with rainfall
amounts averaging roughly an inch, and localized amounts of two
inches, again generally east of the James River Valley. ML CAPE is
rather thin, with values close to or less than 1000 J/Kg helping to
make this a very efficient rain maker. Not too concerned about
severe weather at this point as the rain cooled air will likely
rapidly stabilize the lower to mid levels. But would not rule out
some 40 mph wind gusts and small hail up to the size of dimes. This
was already relayed in the HWO product a while back.

As the short wave moves northward overnight, the rainfall will
rapidly wane from west to east late this afternoon and evening. Much
drier air will entrain in from the west. With a strong surge of cold
air advection and stout mixed layer winds, our south central SD
zones around Chamberlain will be very close to a wind advisory from
roughly about 09Z to 18Z Sunday. Loading up the heftier guidance
values in terms of wind speeds were used, and then manually added a
few knots to the speeds of those to match the mixed layer winds were
used. So this is something to watch for a bit later on. The rest of
the forecast area will be breezy to windy on Sunday, but wind speeds
actually subside a tad in our western zones Sunday afternoon after
the cold air advection and tight gradient subside. Sundays highs
will be much cooler when compared to this past week, with widespread
60 to 65 in store. A lot of cold air advection and a bit of
trowaling in the 295-305K layer will push stratus southward into
much of our forecast area, and the moisture gets deep enough in the
afternoon in our northeast zones to warrant a mention of light
showers.


.LONG TERM...(Sunday night through Saturday)
Issued at 307 PM CDT Sat Sep 24 2016

Upper trough shifts east into the Upper Mississippi Valley Sunday
evening, followed by a slow transition to a dominant mid-upper level
ridge over the Northern Plains by mid next week. Upper low stagnates
over the Great Lakes through Tuesday night, which prevents the upper
ridge from moving too far east. But it appears as though it should
have enough influence that daytime temperatures will shift from a
few degrees below normal Monday, to a few degrees above normal by
Tuesday. Monday should be a fairly mixy day in northwest flow, and
have nudged winds upward and dew points downward from the broader
model consensus. The drier air mass moving in for Monday should
allow for some cool nighttime lows Monday night as the surface ridge
moves into the James River Valley. Cannot rule out some readings
dipping into the 30s in some valley areas, but expect most locations
to see lows in the lower-mid 40s.

Great Lakes upper low begins to open up and slide east Wednesday,
allowing the upper ridge to build eastward through the Plains/Upper
Midwest Thursday, then into the western Great Lakes by Friday. This
will allow the temperatures to remain above normal, with highs in
the 70s anticipated through the end of the week. Models differ in
how quickly the upper ridge will move far enough east to allow for
southwest flow to develop into the region. ECMWF is slower, and thus
remains largely dry through the end of this forecast period. GFS is
faster, which results in scattered precipitation chances by Friday
and Saturday as waves lift northeast across the region. Given the
uncertainties at this range, left some slight chance pops in the
forecast for late this upcoming week.


&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1223 PM CDT Sat Sep 24 2016

18Z TAFs reflect showery conditions this afternoon with embedded
thunderstorms. Rainfall beginning to fill in across eastern
Nebraska at this time, and is heading northward. Therefore it
should affect the TAF sites for awhile this afternoon. VFR
conditions will be mixed with MVFR and possibly even short term
IFR conditions under the rainfall. Otherwise, there will be a
gradual eroding of the cloud cover from west to east through the
evening hours and into the overnight, providing VFR conditions for
a bulk of tonight. Cold air advection stratus clouds will then
move into the western half of the forecast area Sunday morning
providing MVFR cloud cover for KHON and KFSD.


&&

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

$$

SHORT TERM...MJ
LONG TERM...JH
AVIATION...MJ



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