Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 212027

Area Forecast Discussion
National Weather Service Sioux Falls SD
327 PM CDT Thu Sep 21 2017

.SHORT TERM...(This evening through Friday)
Issued at 326 PM CDT Thu Sep 21 2017

Late this afternoon and evening an isolated shower or thunderstorm
will be possible over parts of southwest MN and northwest IA on the
nose of the strong low level moisture advection, but a capping
inversion should either keep the activity at bay or very isolated.

Overall a very warm and humid forecast for many over the next 24
hours. Southerly flow will remain strong tonight into Friday and not
out of the question to possibly need a wind advisory on Friday
afternoon. Suspect that winds will average about 20 to 25 mph with
gusts of 35 to 40 mph along and east of the James River. For now
will not issue but it will be very windy regardless. With strong
southerly flow and very high dew point temperatures for this time of
year lows temperatures tonight will be in the 70 to 75 degree range
in most locations, which might set some warm record lows for
September 22nd. There will be a very small chance for showers and
thunderstorms along and north of highway 14 overnight on the edge of
the capping inversion but for now see the better chance north of

Most of the day on Friday should remain dry as a fairly strong
capping inversion will be in place. However near and just west of
the James River temperatures will likely heat out enough to allow a
storm or two to develop after about 22z. Plenty of shear and
instability to support severe storms and with fairly dry air aloft
which should support isolated supercells. For now will lean towards
the main threats as golf ball size hail and 70 mph wind gusts, but
with the potential for about 15 to 20 knots of surface to 1 km shear
an isolated tornado will also be possible.

.LONG TERM...(Friday night through Thursday)
Issued at 326 PM CDT Thu Sep 21 2017

By Friday evening the surface based thunderstorm threat may lift
north out of the area but with an increasing low level jet
additional thunderstorms will likely develop northward from
central and north central Nebraska into central SD. While a few of
these storms could be severe the main threats will be lightning
and heavy rainfall. Another night of very warm lows ranging from
the mid 50s in central SD to the lower 70s in northwest IA.

Saturday into Saturday night, especially Saturday night, will likely
be the best threat for heavy rainfall. The warm cloud depth should
be at its deepest, close to 13000 feet. This is also the time when
the right entrance region of the upper level jet will shift into the
area providing broad ascent to the area. The severe weather threat
will be minimized but the heavy rain threat will be higher. At this
time locations between the James River and Interstate 29 will have
the highest chance for heavy rain on Saturday night.

The right entrance region of the upper level jet does not move much
at all through Monday morning which means that the threat for
occasional rain and thunderstorms will remain in place and the low
level boundary will be slow to shift east. Will likely see the most
widespread threat for rain and thunderstorms as we move into Sunday
afternoon and night as the broad forcing is in place, the boundary
is near and instability is weak, allowing for more of a general rain
with isolated thunderstorm threat.

Monday afternoon into Monday night the upper level jet will finally
start shifting east taking the higher chances for rain with it.
Looks like a wet day still as rain chances continue until late
Monday night.

Tuesday through Thursday will see cool air settle in with highs in
the 60s and lows in the 40s and not much of a threat for rainfall.
Tuesday may even see a few sites struggle to reach 60 degrees.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1240 PM CDT Thu Sep 21 2017

Strong southerly winds are expected this afternoon into the
overnight hours. Low level wind shear is expected at KFSD and
KSUX. Gusty south winds are expected tomorrow, ahead of the
potential for thunderstorms west of I-29 Friday evening.




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