Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 181739

Area Forecast Discussion
National Weather Service Sioux Falls SD
1239 PM CDT Mon Jun 18 2018

.SHORT TERM...(Today and Tonight)
Issued at 354 AM CDT Mon Jun 18 2018

Focus for today will be on precipitation chances, and associated
severe/heavy rain threats. As of 08Z, surface boundary is positioned
in the extreme southeast corner of our forecast area, and question
of the day is whether it will lift a bit farther north again this
afternoon, as this should be the primary focus for potential strong
to severe development this afternoon.

NAM is more aggressive in lifting the boundary northward again by
peak heating, close to a Ponca NE-Spencer IA line, while GFS/GEM
leave it near/south of a SUX-SLB line. CAM consensus is between the
two, but a little closer to northern NAM idea prior to 20Z-21Z
convective initiation. 0-6km shear south of the boundary is rather
weak, but counterbalanced by high instability, with SBCAPE values in
excess of 2000J/kg thanks to lingering surface dew points in the
lower 70s along and south of the boundary. Shear increases along and
north of the boundary, while instability gradually decreases, but
all-in-all, areas along and south of the boundary could see isolated
strong to severe storms during the mid afternoon-mid evening hours.
This is currently highlighted by the SPC Marginal Risk. Hail likely
the greatest threat, but soundings depict some mid-level drying
which could also contribute to an isolated wind threat. While
tornado potential appears low, cannot rule out isolated threat near
the main front or any organized outflow boundaries. Precipitable
water values remain quite high as well, so locally heavy rainfall
rates will also be a concern with the stronger late afternoon/early
evening storms.

Remainder of the forecast area will be much cooler today north of
the front, though still somewhat on the humid side as not able to
get a strong push of drier air into the region with front hanging up
across our south. Had previously thought western and northern areas
of the CWA would be relatively rain-free later today/tonight with
focus nearer to the front. However, 18/00Z models showing pretty
good agreement in tracking a decent mid-level wave northward into
central/southeast South Dakota later today. Timing varies a bit, but
track is similar, and should bring increasing coverage of showers
and a few thunderstorms to our western and northern areas late this
afternoon/tonight. Sounding profiles closer to moist adiabatic with
limited instability, but rainfall could be quiet efficient with some
areas possibly seeing in excess of a half inch through tonight.

.LONG TERM...(Tuesday through Sunday)
Issued at 354 AM CDT Mon Jun 18 2018

Rain associated with leading shortwave should diminish through the
day Tuesday as the wave exits, but attention then turns to what may
be a prolonged wet period for parts of the region. Long-wave trough
currently over the inter-mountain west tracks across the northern
Rockies through Tuesday night, then reintensifies as it drops into
the central Plains Wednesday through Thursday. Models showing much
better agreement on this tonight, showing the mid-upper low digging
into southeast Nebraska/northeast Kansas Wednesday night, before
slowly swinging out into the mid-Mississippi Valley through Friday.
Persistent feed of moisture around the system expected to maintain
decent rain chances across the Missouri River Valley from Tuesday
night through Wednesday night, gradually diminishing Thursday into
Friday as the system swings east. Instability through this period is
rather low, with soundings showing well-saturated moist-adiabatic
profiles, so will have limited thunder mention through the mid-week
period. GEFS Precipitable Water levels remaining at or above the
90th percentile, with values 1.5 to 2 times seasonal normals, so
again expect rather efficient rainfall with periods of moderate rain
potentially leading to rainfall amounts in excess of 2 inches in
some areas over the 2 days from Tuesday night through Thursday.

Temperatures through this period will be held below normal by the
extensive cloud cover/rain, especially south where some areas may
struggle to top 70 Wednesday and Thursday. Northern areas should be
warmer, but not excessively with readings in the mid to maybe upper

Unsettled pattern remains in place for the end of the week and
upcoming weekend, though model differences preclude pinpointing any
one period as having significantly higher rain chances at this time.
Do see some increase in daytime instability as temperatures warm
closer to seasonal normals, so better chance at seeing isolated to
scattered thunderstorms, though severe chances are uncertain.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1229 PM CDT Mon Jun 18 2018

MVFR/IFR conditions are expected through the period. Periods of
showers and storms return this evening, impacting mainly KSUX
terminal. A few storms may become strong to severe, producing strong
winds to 50 knots and large hail mainly over northwest Iowa and
far southeastern South Dakota. Low- clouds/stratus will persist
during the overnight hours, producing IFR to LIFR ceiling
conditions at all sites. Ceiling conditions will slowly improve by
Tuesday afternoon.




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