Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 231723
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1223 PM CDT Wed May 23 2018

.SHORT TERM...(Today and Tonight)
Issued at 339 AM CDT Wed May 23 2018

A couple areas of thunderstorms in the CWA early this morning. First
is a line along the elevated warm front lifting through southwest
Minnesota/northern Iowa, and the second the eastern edge of a line
extending through western/central SD. Latest hi-res guidance
suggests the eastern line lifts northeast out of the CWA prior to
daybreak, though will have to monitor for weaker/more scattered
secondary line developing near the same location. Early focus will
thus be on the western area where there is modest consensus on some
scattered activity affecting locations from the mid-James Valley
westward into the mid-late morning hours.

Afternoon/early evening hours largely look dry, though a few hi-res
solutions indicate potential for isolated storm development in our
far west/north. With the heat/humidity continuing to build into the
area, certainly begin to see the increase in instability that could
support isolated development. Confidence in this is low given lack
of any additional trigger, so will keep pops sub-mentionable for
most areas prior to 24/00Z, but certainly worth watching throughout
the day. Speaking of that heat, low level temps support widespread
mid-upper 80s for highs today, possibly pushing 90 in a few areas
near/west of I-29.

For tonight, shortwave projected to lift into central South Dakota
late evening into the overnight, and main question will be how far
east any potential storms could spread prior to losing support.
Along with the shortwave, area of warm advection remains focused
west of I-29, so will keep our pops concentrated in this area as
well. Difficult to argue with the marginal wind/hail outlook from
SPC given the degree of instability, though warm layer getting
fairly deep, mid-level lapse rates are not overly steep and shear
also pretty weak, so would not expect much more than marginally
severe hail at best.

.LONG TERM...(Thursday through Tuesday)
Issued at 339 AM CDT Wed May 23 2018

Wednesday night activity looks to diminish by 12Z Thursday as the
low level jet weakens and warm advection largely lifts north of the
area. Attention will then turn to stronger wave swinging out of the
western Dakotas by mid-late Thursday afternoon. Surface low and warm
front remain well north of the area, but dry line approaches our far
western counties by 22Z-00Z, which along with upper support should
focus convective chances in our western CWA Thursday afternoon.
Possible we could see activity in the warm sector farther east, but
anticipate this would be more isolated and will keep pops in the
slight chance range east of Brookings-Lake Andes until after 25/00Z.

Would expect scattered activity to spread east across the forecast
area through the evening/overnight as the wave continues to lift
northeast. Somewhat steeper mid-level lapse rates supportive of
isolated hail threat, and projected DCAPE values in excess of
1000J/kg at times suggest potential for damaging winds as well.
Overall deep layer shear unimpressive, though, so would favor pulsy
multi-cell development over supercells. Deep warm layer along with
precipitable water values 1.3-1.5"/99th percentile also suggest
potential for locally heavy rain.

Overnight activity should be exiting eastern CWA early Friday
morning, leaving upper ridge to slowly build into the region for the
holiday weekend. This will place main focus on continuation of above
normal temperatures. 850mb temperatures projected to be at/above the
95th percentile of climatology in the NAEFS, which could make this
holiday weekend feel more like the 4th of July than Memorial Day,
with highs in the upper 80s-lower 90s and lows in the lower-mid 60s.
Models show spotty precipitation chances within the upper ridge, but
timing/location ill-defined with little consensus, so will go with a
largely dry forecast through the weekend, with just some low chances
creeping in late Sunday night/Monday across our northern areas.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1217 PM CDT Wed May 23 2018

VFR is expected to be the dominant category at the TAF sites
through the 18Z TAF set. That said there are some challenges. In
the near term this afternoon, small, popcorn type showers and
isolated storms will develop, especially affecting the KHON and
possibly KFSD TAF sites. This will be closely monitored for
amendments when or if these small showers occur. Tonight, VCTS is
warranted at the KHON and KFSD TAF sites as activity develops
over the western plains and possibly nudges eastward later tonight
for the KHON and KFSD TAF sites. Not enough confidence to include
any TSRA potential in a tempo group so opted to go the VCTS route
as a heads up. Finally, as the low level jet increases tonight,
there will be a threat for low level wind shear.

&&

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

$$

SHORT TERM...JH
LONG TERM...JH
AVIATION...MJ


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