Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 151713

Area Forecast Discussion
National Weather Service Sioux Falls SD
1213 PM CDT Tue Aug 15 2017

.SHORT TERM...(Today and Tonight)
Issued at 416 AM CDT Tue Aug 15 2017

Active forecast today with dense fog and several rounds of
convection expected.  Have issued a dense fog advisory for I-90
corridor in southwest Minnesota.  While there was some question as
to how dense fog would get with expanding high clouds across the
area, visibilities have remained low.  Do not have dense fog
advisory lasting as long as yesterday, as band of convection
associated with strong theta-e advection in the 925-850 mb layer is
expected to lift northeast through the region likely improving
visibilities as it works through.

By late this morning, the focus shifts back toward south Central
Dakota as front across Nebraska works north as a series of
disturbance track through the region. This should result in a series
of rounds of convection develop as each impulse tracks through the
region.  PWAT values are near 90th percentile creating a very
efficient atmosphere for rain production. In addition to the heavy
rain threat, CAPE values of 1500-2500 J/kg combined modest veering
wind shear profile may result in severe convective clusters. Agree
with SPC assessment that an isolated tornado threat is possible with
boundary lifting north through the area. Convection is expected to
last through the better part of the overnight Tuesday night into
Wednesday morning with ~30 knot low level jet leading to a conveyor
of moisture and instability ahead of a slow moving front that works
through the region on Wednesday morning. 0-6 km bulk shear values
are greatest with initial round of convection early this afternoon,
and gradually tail off throughout the overnight. Therefore, expect
the greatest amount of severe weather in the afternoon-early
evening, then transitioning to a hydro event tonight.  Think storms
will have some movement to them, but training could result in some
localized higher amounts of precipitation.

.LONG TERM...(Wednesday through Monday)
Issued at 416 AM CDT Tue Aug 15 2017

Wednesday will continue with ongoing showers and storms associated
with the slow-moving shortwave and frontal boundary gradually moving
east into southwest Minnesota and northwest Iowa. Convection could
re-develop in the afternoon east of the I-29; but at this time,
severe weather risk is low due to more stable air entering the area
behind the front. Cloud cover will slowly decrease from west to east
as a surface high pressure builds in bringing drier conditions
Wednesday night.

With the high pressure over the region on Thursday, a quiet and
pleasant day will be expected. Several impulses of energy aloft will
continue ejecting weak disturbances ahead of the next upper wave
approaching by the end of the week. A few spotty/skittish showers
could develop Thursday night, but better chances return Friday
afternoon and evening with the arrival of a warm front in response
of the aforementioned wave.

A weak ridge aloft will maintain the area mostly dry through the
weekend, with temperatures a few degrees above normal to even warmer
than normal on Sunday. The GFS and ECMWF bring an upper trough
riding along the zonal flow aloft by Sunday night into Monday. At
this time, showers and storms will be possible on Monday morning,
depending on the speed/track of the upper trough moving across the
area Sunday night. Showers/storms could re-redevelop on Monday
throughout the day as a strong upper wave dives south into the area
in the evening and overnight hours. Model solutions differ in
depicting the speed and track of this system. Therefore, have left
this period with isolated to scattered PoPs in the afternoon,
increasing chances towards the evening and nighttime hours.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1212 PM CDT Tue Aug 15 2017

Occasional MVFR ceilings are expected into the afternoon. Showers
and thunderstorms will increase in coverage through the afternoon
south of Interstate 90 and west of Interstate 29. By evening into
the overnight hours the chances will spread east and MVFR to
possibly IFR ceilings will increase in coverage late in the




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