Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201945

Area Forecast Discussion
National Weather Service Sioux Falls SD
245 PM CDT Sun Aug 20 2017

.SHORT TERM...(This evening through Monday)
Issued at 244 PM CDT Sun Aug 20 2017

Quite a bit going on the next 24 to 36 hours including a chance for
thunderstorms tonight, the eclipse tomorrow with a chance for
thunderstorms during morning, then another chance for thunderstorms
on Monday late afternoon and evening.

Initially a trough of low pressure was moving east along the
Canadian border while a southern jet was increasing ahead of a
southern wave in New Mexico. This should bring enhanced lift and
increased low to mid level warm air advection to the area today. The
most likely location will be southern SD into northwest IA from
about 2z through 9z. However the latest soundings from much of the
area east of the James River Valley suggest a small to no cap
lifting a parcel from about 850-800mb so will need to watch the
trends on this. If storms can ignite from this level CAPE values
will likely be around 2000 J/kg with some directional and speed
shear in the 2-6km layer, running about 30-35 knots. This would
support a decent threat for severe thunderstorms with hail to ping
pong ball and wind gusts to 65 mph the main threats.

Later tonight towards Monday morning the right entrance region of
the northern jet sags into northern SD and with a decent mid level
thermal boundary through parts of central SD showers and
thunderstorms may become a higher threat. Will need to increase the
chances in central SD late tonight and early Monday morning, then
swing that threat into east central and southeast SD late morning
into early afternoon.

Monday afternoon becomes complicated for two reasons. One, we may
have ongoing convection which will change the shear and instability
profiles around the area. The other is if there is little to no
activity in the morning and we look to peak heating/surface based
convective initiation what affect will the eclipse and the cooler
temperatures from about 1200 pm through 200 pm do to that
potential? For now, with the strong combination of the incoming
southern stream wave and the northern stream wave still believe
that thunderstorms will develop earlier in the morning and we will
have ongoing activity through the morning into the afternoon.

.LONG TERM...(Monday night through Sunday)
Issued at 244 PM CDT Sun Aug 20 2017

Any convection that forms Monday afternoon will head to the
southeast by the evening hours, finally clearing the CWA late Monday
evening. Considerable uncertainty on the degree of destabilization
lingering into the evening hours, which would impact our severe
weather risks. Given level of moisture advecting into the area,
locally heavy rain could be a possibility.

Through the middle and end of the week, quiet conditions are
expected as high pressure dominates much of the central US.
Temperatures will continue to remain below normal through the end of
the week.

Mid-level flow begins to flatten again by the end of the week,
bringing a slight warmup by next weekend, with increased rain
chances for next weekend.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1215 PM CDT Sun Aug 20 2017

Showers and thunderstorms will become possible late this evening
and continue into the overnight hours. The better chance will be
far southern SD into northwest IA. Additional chance for showers
and thunderstorms will occur very late tonight into Monday morning
from central SD into east central SD. Outside of the thunderstorms
VFR conditions are expected, although there will be a chance for
some MVFR ceilings in northwest IA on Monday morning.




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