Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 161939
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
239 PM CDT Fri Jun 16 2017

.SHORT TERM...(This evening through Saturday)
Issued at 239 PM CDT Fri Jun 16 2017

Main forecast concerns are the potential for severe weather late
this afternoon and evening and then the larger area of rain on
Saturday.

For this afternoon, the severe weather threat appears to confined
south of Hwy 18 in Iowa as well as far southeast South Dakota and
northeast Nebraska. There is a strong jet streak moving into eastern
South Dakota this afternoon and this will interact with a convergence
line across north central Nebraska to initiate convection. 0-6 km
shear profiles are very favorable for the development of supercells
with 0-6 km bulk shear from 40 to 50 kts. The limiting factor is
moisture. Dew points over northeastern Nebraska are in the upper 50s
and lower 60s. With southerly flow, this drier air will advect
northeast into northwestern Iowa this afternoon behind accas that
developed late this morning.  There is deeper moisture south of I80
where GOES-16 imagery shows PWs approaching 1.5 inches and surface
dew points near 70 degrees. There is high confidence convection will
develop across northeastern Nebraska. However, supercell motion will
be southeast and any storms developing near O`Neill should remain
south of Sioux City and Wakefield. The question is if there will be
additional development north of the NE/SD border as the boundary
moves toward Sioux City and if enough moisture will exist to support
strong updrafts. At this time, do expect at least a few storms to
develop - perhaps as far north as Hwy 18 near the IA/SD. However,
the limited moisture is expected to keep to most storms from
becoming severe with the greatest risk being a microburst with winds
up to 60 mph due to the evaporative cooling within the dry boundary
layer. If enough moisture can get into the Sioux City area by late
afternoon, then supercells capable of producing 2+" hail will be
possible. However, current trends in dew points as well as HRRR
indicate that this threat is low.

Convection should move east of the area prior to midnight tonight. A
strong will wave will approach from the northern Rockies late
tonight and Saturday. Instability is rather limited due to limited
moisture and much cooler temperatures. However, this wave will have
a lot of ascent associated with it. Therefore begin to increase the
chance of showers and thunderstorms by late tonight and spreading
into southwest Minnesota and northwestern Iowa by early Saturday
afternoon. There is just enough instability to keep thunder in the
forecast and expect that many areas north of I-90 will see at least
light rain. With the limited moisture generally expect rainfall
amounts to be less than 0.25". Evaporative cooling and cloud cover
will result in highs 10 to 20 degrees cooler than today - generally
be in the 70s with coolest readings in southwestern Minnesota.


.LONG TERM...(Saturday night through Friday)
Issued at 239 PM CDT Fri Jun 16 2017

Should see precipitation wane through the evening Saturday night, as
upper trough and associated cool front exit to our east. In the wake
of this system, cool air will briefly settle into the region Sunday,
with highs only in the 70s for most areas. The cooler air lingers
across the northeast into Monday, as the upper trough swings slowly
east through the Great Lakes, while a building upper ridge over the
Rockies begins to bring warmer air back into southwest parts of the
region. Various weak waves slide across the region through this
upper trough, bringing light precipitation chances to the Northern
Plains Sunday, and to a lesser degree again Monday afternoon.

A northern stream wave crosses the Canadian Rockies and flattens the
ridge through the middle of next week, which brings a transition to
quasi-zonal flow across the northern CONUS through the upcoming work
week. Vast differences among the models regarding the strength of
this initial wave, and thus in their depiction of how far south into
the Canadian Prairies/far northern CONUS it will dig by mid to late
week. Stronger solutions bring another push of cooler/more stable
air into the region by Thursday, while weaker solutions maintain
near to above normal temperatures and greater instability. With the
uncertainty in solutions at this range, did not alter broad model
blend for pops or temperatures at this point. However, did limit
thunder potential to just slight chance/isolated range, with the
exception of Wednesday when there is better consensus in the models
allowing instability to build ahead of the wave.


&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1231 PM CDT Fri Jun 16 2017

Scattered thunderstorms expected to impact areas from northeast
Nebraska into northwest Iowa during the late afternoon to early
evening today, possibly including KSUX TAF location. Brief MVFR
conditions will be possible with the thunderstorms, along with
isolated gusty winds and hail with the strongest storms.

Elsewhere, abundant mid-high level clouds will be prevalent, with
spotty high-based showers or isolated thunderstorms. Potential for
thunder at KHON/KFSD is low and will not be included in the TAF.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Schumacher
LONG TERM...JH
AVIATION...JH



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