Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 222021

Area Forecast Discussion
National Weather Service Sioux Falls SD
321 PM CDT Tue May 22 2018

.SHORT TERM...(This evening through Wednesday)
Issued at 321 PM CDT Tue May 22 2018

The primary concern over the next 24 h is the potential for heavy
rainfall overnight and the lower probability of severe weather this
evening and overnight.

At 18Z this afternoon, the surface warm front extended across
northern Nebraska from north of ODX to south of OFK. This front is
slowly expected to lift north into the evening. Both the 17Z HRRR
and 17Z RAP have recently begun to show convection developing this
afternoon.  This seems to be an model initiation issue as current
satellite does not show any indication of thunderstorm development
through 21Z. The only area of concern in the late afternoon and
early evening is the area of stratocumulus along and south of the
Missouri River.  At this point, there is no sign of storm
development but if temperatures can warm into the low 80s with dew
points in the upper 60s, that could allow an isolated storm to break
the cap. With 0-3 km helicity over 250 m2/s2 and CAPE over 2000
J/kg, any storms that form near the Mo River prior to sunset could
become severe.  However, the probability of any surface-based
development is low and would remain isolated.

The greater concern is toward sunset.  A weak upper level wave will
be moving north into northern Nebraska at that time. With low level
frontogenesis, MUCAPE of 1000-2000 J/kg and CIN < 25 J/kg, expect
storms to develop in the vicinity of I-90 within an hour or two of
sunset. Since these storms will be a little elevated, shear is only
30 kts meaning that storm mode is most likely to be multicellular.
Initial updrafts which have access to the best instability may be
able to produce quarter size hail but that threat should only last a
couple of hours - perhaps as late as 06Z.  A second wave will then
move into eastern South Dakota after 06Z. This approaching wave will
cause the low level jet to increase across Nebraska which will lead
to both enhanced moisture transport over the surface boundary and
increased convergence at the nose of the jet near I-90. Agree with
hi-res model solutions which show a significant increase in storm
coverage near or north of I90 after midnight. As storms grow upscale
a cold pool is expected develop and is likely to impede the
northward movement of the front. Corfidi vectors also show that the
MCS will have a slow movement to the east southeast. This will allow
for some areas to get 1 to 2 inches of rain overnight - with locally
higher amounts possible. Based upon the expected location of the low
level front and jet, the most likely area for heavy rain looks to be
from north of Sioux Falls to the Iowa and Minnesota border. However,
this axis could shift north if storms develop later in the night
allowing the front to move closer to Highway 14. Because Flash Flood
Guidance is around 2" in 3 h and the location of heavy rain is
somewhat uncertain, have not issued a Flash Flood Watch for tonight.
However, with brief periods of 1-2"/h rainfall rates possible,
ponding of water in urban and poor drainage areas could occur.

Storms are expected to exit to the east by late Wednesday morning.
With skies becoming most clear, rising mid level height and warming
temperatures above 700 mb, additional storms are not expected to
redevelop Wednesday afternoon.  With at least partly sunny skies and
southerly flow, temperatures will be in the 80s with 90 degrees
possible west of the James River. Dew points will be in the mid 60s
so it will also feel humid.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 321 PM CDT Tue May 22 2018

On Wednesday night, there is a strong shortwave that moves from
western Wyoming into eastern Montana.  There are a series of short
waves further to the south and east, but the main wave is located
across the far west. Winds increase out of the south, but the nose
of the low level jet appears to be focused to the north across North
Dakota. Portions of central South Dakota may be brushed by short
waves lifting to the northeast.  Therefore, have some slight chance -
 low end chance pops west of the James River Valley.

More significant chances of storms are expected on Thursday as a
short wave train moves across the area from southwest to northeast.
Have increasing speed shear with height, but increase is meager with
very little directional shear.  This once again results in 0-6km
bulk shear of 20-25 knots.  With that said, surface based CAPE
values of 2500-3000 J/kg may allow for isolated severe mainly for
hail and wind.

On Friday into next week, upper ridge develops across the Western
Plains leading to relatively dry conditions.  With the Gulf wide
open and southerly flow continuing, should see prolonged warm and
humid conditions. Have nudged temperatures upwards through the
weekend into next week, but not as much as the warm GFS would
suggest. Looks to be a quick transition to summer like conditions.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1228 PM CDT Tue May 22 2018

In the near term, MVFR cumulus could develop at Sioux City as a
lot of it has recently formed in their vicinity partially spawned
by earlier rainfall. For now, hedged the low deck at KSUX with a
scattered deck in case this happens. But the MVFR clouds will mix
into the VFR category by mid afternoon at any rate. Otherwise the
main story is tonight with a short wave lifting northward and a
warm front. Sioux Falls at KFSD will be the most impacted by this
event. Widespread TSRA is a pretty good bet right into the middle
of the night, with convection continuing just to the north and
northeast of KFSD for the remainder of the night possibly
impacting KFSD with TSRA possibilities for much of the night with
a VCTS. In the heaviest rainfall, visibilities could fall briefly
to IFR but only included MVFR at this time as that will be more
widespread. TSRA may also be in the KHON vicinity tonight with
KSUX likely too far south. Therefore left KSUX dry as the chances
for them receiving a stray thunderstorm is currently too skittish
to include in the TAF site. Lower end MVFR ceilings will likely
develop later tonight and Wednesday morning at KFSD and KHON with
a lot of rain cooled air around.




SHORT TERM...Schumacher
AVIATION...MJ is the U.S. government's official web portal to all federal, state and local government web resources and services.