Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 172001

Area Forecast Discussion
National Weather Service Sioux Falls SD
301 PM CDT Thu Aug 17 2017

.SHORT TERM...(This evening through Friday)
Issued at 300 PM CDT Thu Aug 17 2017

Pleasant and quiet conditions have prevailed today, with
northwesterly surface winds on the backside of the closed upper low
bringing drier airmass over the region. Skies will gradually clear
over southwest Minnesota, while clouds will be in the increase from
west to east late this evening. A weak shortwave dives from the
Northern Rockies into the Dakotas late this afternoon, with the 850
mb front approaching the Missouri River area tonight. Models are in
good agreement into bringing showers and storms mainly after 04Z
west of the James River, then spreading eastward into early Friday
morning. Highest PoP chances will fall west of the I-29 corridor
where increasing mid-level theta-e advection and strengthening low-
level jet from 35 to 40 knots will be in place.

Lingering showers and isolated storms could persist through mid-
morning, then partly sunny skies will give way to a warmer but
seasonably day with afternoon dewpoints in the lower 60s. Highs will
range from the upper 70s and 80s. Scattered showers and
thunderstorms could re-develop in the afternoon and evening hours,
mainly east of I-29. This is in response to another weak shortwave,
and the passage of an upper jet providing support for forcing by
late afternoon. Deep layer shear is lacking, but daytime heating,
and surface dewpoints will increase the instability in the
afternoon. The severe weather risk is low, but a few clusters of
strong storms will be possible producing isolated large hail.

.LONG TERM...(Friday night through Thursday)
Issued at 300 PM CDT Thu Aug 17 2017

As the 75 knot to 85 knot upper level jet max dives south on Friday
evening scattered thunderstorms will be possible along and east of
the Buffalo Ridge. CAPE values should approach 1500 to 2000 J/kg but
the shear values, especially low level, are not very impressive. The
mid and upper level shear not too bad likely running about 30 to 40
knots so suspect that a few updrafts could maintain themselves long
enough to produce hail to the size of half dollars. The freezing
level also down about 4000 feet from yesterday and closer to 10000
feet AGl which would support a better chance for hail. Enough dry
air below 700 mb that wind gusts of 60 to 65 mph would also be

A weak wave will move into the area late Saturday night into Sunday
morning and went ahead and increased pops a touch from what
initialized. While chances are still low the upper level support
couple with good warm air advection hint at a better potential. Have
seen a handful of cases this year when warm air advection is decent
a couple of days out but moisture is scarce so model qpf/pops low
but as each new model run comes in the overall forcing does not
change much but the moisture in the warm advective area increases
which then allows for a better chance for shower and thunderstorm
development. Even with additional moisture severe weather would not
be expected.

Sunday night into Tuesday morning continues to look like the
potentially busiest time frame with severe weather possible. A
couple of waves look to move along the Canadian border with some
southern stream energy possibly wrapping into the system as well.
Tough to pinpoint timing right now but both Sunday night and again
Monday night look like the better chances. Temperatures both days
should be warm and a little humid with lows in the 60s and high in
the 80s.

As we move into Wednesday and Thursday a few subtle differences in
the models. The latest ECMWF and Canadian a little more agreeable on
keeping a trough to the northeast slow moving while the GFS is
faster to exit this energy. The main effect of this slower moving
trough will be to keep high pressure across parts of Minnesota and
Iowa which would keep a cooler regime and cooler southeast flow in
place. So instead of highs from 80 to 85 along and east of
Interstate 29 they would be closer to 75 to 80. Not a big difference
but it would be noticeable. Will lean closer to the Canadian/ECMWF
flow pattern, especially given the cooler pattern we have been
locked in to.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1219 PM CDT Thu Aug 17 2017

VFR conditions are expected through the TAF period. Cloud cover
will increase late this evening as an upper shortwave brings
showers and isolated storms at all terminals through the overnight
hours. Ceilings are expected to remain at or above 9-10 kft.




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