Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 252023

Area Forecast Discussion
National Weather Service Sioux Falls SD
323 PM CDT Tue Jul 25 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 323 PM CDT Tue Jul 25 2017

Convection remains the primary focus in the short term portions of
the forecast. Warm and very humid air has lifted northward into the
CWA south of a frontal boundary situated west of Chamberlain through
Huron and north of Marshall.   This boundary will be the focal point
for convection later this afternoon and into the evening hours.

MLCAPE values continue to surge along and south of the
aforementioned boundary, approaching 3000+ J/KG at mid-afternoon,
however 700 mb temperatures this afternoon continue to show issues
with temperatures aloft. Fairly intense convergence along portions
of the boundary, combined with the likelihood of attaining convective
temperatures, should promote scattered storm growth in south central
SD and adjacent areas of NE between 3 and 5pm.   Scattered
convection will then quickly build northeast along the boundary
through 00z.

Storm organization this afternoon will be limited, with an increased
trend towards multi-cellular clusters given low effective shear
values. Prevailing flow will likely push storms slightly to the east
and north with time, but most of the activity may be cold pool
driven this evening. Anticipating several clusters of storms, with
the greatest severe weather threat in an area from Gregory north and
northeast through Huron, Mitchell, Brookings and Marshall. Storms
will likely find it difficult to turn southeast into a narrow +14C 700
mb thermal axis that will be situated across the eastern 2/3rds of
the CWA. It`s possible that storms that form in the northern
portions of the CWA could cross out of the CWA quickly this evening.

Hazards:  Large CAPE with most of the shear in the favorable hail
growth region could support golf ball or larger hail in the initial
storm development.  However, steep 0-3km lapse rates, DCAPE pushing
1800 J/KG, and generally weak low level flow should promote
downburst winds with localized 70 mph wind gusts.   There is a very
brief and limited risk for tornadoes along the Hwy 14 corridor,
mainly from Brookings to Marshall from 6 to 8pm, as LCLs are much
lower and 0-1 km shear does increase upwards of 15 knots along the
surface boundary.

The slow cell trajectory and pooling moisture will lead to a
localized heavy rainfall potential.  Feel that WPC has outlined the
area well, with a slight risk for excessive rainfall into the
overnight hours.

Overnight and into Wednesday:  WV imagery this afternoon showing
stronger shortwave beginning to pivot northeast out of the Central
Rockies. This wave will begin to move into the Plains later tonight,
and will begin to drive or enhance an area of precipitation across
Nebraska as it moves eastward.   Likely reaching the CWA by 09Z-12Z,
we should start to see an uptick in new convection moving across the
area mid-morning, rooted in or around the 700 mb layer, forced . by
increasing LLJ ahead of the aforementioned wave.  Not anticipating
widespread severe storms with this activity given very poor shear,
but initial development could lean on the strong side.

Leaning towards cooler temperatures and rain chances shifting
southeast through the day on Wednesday. 850MB isentropic downglide
commences quickly by mid-morning, and should shift rain chances away
as it does so.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 323 PM CDT Tue Jul 25 2017

Precipitation should shut down fairly quickly Thursday evening as
shortwave energy migrates to the south and east and surface high
pressure noses into the region. This should keep conditions cool and
mainly dry into the weekend.  Could be some isolated
shower/thunderstorms activity that migrates in from the west Friday
night but expect activity to remain isolated concentrated fairly
close to the mid-level shortwave energy. Similarly, on Saturday
night into Sunday morning, may have a few storms form on the nose of
weak low level jet across eastern South Dakota into southwest
Minnesota. Again, expect this activity to remain fairly isolated.

Monday appears to be fairly dry and seasonal as upper level ridge
builds across the western half of the US; however, Tuesday into
Wednesday appears to be more unsettled as low pressure system moves
from the inter-mountain west into the Northern Plains.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1210 PM CDT Tue Jul 25 2017

The risk for convection this evening is the primary aviation
concern, but still remains focused near KHON for much of the
upcoming 12 hours. With this activity, brief reductions in
visibility as well as strong wind gusts may be possible.

Storms may begin to form or move into areas from FSD to SUX after
midnight and into day break Wednesday as an upper wave moves
through. Additionally, models continue to hint at increasing
stratus pouring in from the north after midnight with MVFR
ceilings possible through Wednesday morning. Feel that any
activity in the morning should be scattered in nature.




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