Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 152137

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

.SHORT TERM...(This evening through Wednesday)
Issued at 419 PM CDT Tue Aug 15 2017

Showers and thunderstorms rapidly developed early this afternoon in
northern and central Nebraska near the 850mb boundary. While CAPE
values are likely 1500 to 2000 J/kg shear is a limiting factor which
will keep updrafts from persisting too long. Suspect that the
activity in Nebraska will move to the east and southeast if it
strengthens into the late afternoon. Will need to watch for isolated
small updrafts in parts of northwest IA where nickel to quarter
sized hail will be possible.

The next area of interest will be central SD where one piece of
energy will move through this evening, then another later tonight,
generally from about 8z through 15z. This second piece of energy may
prove to be the more interesting as the first might lack some of the
better moisture and instability. Initially it appears that the
activity in central and northern Nebraska will keep the deeper
moisture and instability to the south but if this complex shifts
east and/or weakens this evening some of the deeper moisture and
instability might sneak north and west into central and southeast
SD. While the severe threat would likely be very isolated some heavy
rain will be possible. This is a time when diffluent flow will
increase across the boundary and slow storm motion will be more
likely. The threat for some locations to see 2 to 4 inches is still

The threat for rainfall will be pretty slow to exit on Wednesday as
the upper level trough of low pressure is just moving east of
Interstate 29 by about 21z.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 419 PM CDT Tue Aug 15 2017

Overall, coming out of the active shorter term has some fairly
decent model agreement on exit of trough and transition through weak
ridging through Thursday evening.  Some lingering showers and
thunderstorms mainly east of I-29 and mainly early evening, then
should end as weak high pressure ushers in a short drier period and
seasonable temps to the area. Next shot at thunderstorms will start
later Thursday night as warm advection ahead of sharper northwest
flow wave concentrates through areas west of I-29 late. This trough
will dig across the area during Friday and Friday evening, with
dynamic support at least strong enough for scattered thunderstorms.
Will have to continue to monitor the potential for isolated severe
storms given some fairly steep mid-level lapse rates resulting in a
potentially thicker CAPE profile, along with a strongly veered
profile from south/southwest to northwest supporting storm
organization along the frontal boundary.

System pushes through again fairly quickly during the evening,
leading into another period of transition to ridging aloft over the
weekend. Temperatures will work toward near to slightly above normal
Saturday. Flattening of the flow will result from wave crashing
along the Canadian border through Sunday, which looks to eventually
drop a frontal boundary southward across the area in the Sunday to
Monday time frame.  Consistency in forcing has been a bit lacking,
but a few solutions indicate potential for an organized convective
development around Sunday or Sunday night on southern edge of
stronger westerly flow aloft.  Should be able to squeeze out a
warmer day on Sunday with stronger south to southwest gradient.

Still no more than cautiously optimistic on sky cover for eclipse
day, with the exit of any convection likely during the morning.
There would at least be potential for some post-convective
cloudiness to linger, but still too early to diagnose how much or
how long. Likely to see a minor impact on decreasing temperatures
and wind around the eclipse time, but mean forecast error at this
point precludes more than a minor tweak to hourly grids.

Could see a little enhancement to precip chances Monday night south
of I-90, and especially toward the Missouri River valley, with
another wave rotating along the southern edge of the westerlies, but
this could be overdone given the likely southward shove to
baroclinicity with Sunday night system.


.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




LONG TERM...Chapman
AVIATION...08 is the U.S. government's official web portal to all federal, state and local government web resources and services.