Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 291023
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
523 AM CDT SUN MAY 29 2016

.SHORT TERM...(Today and Tonight)
Issued at 428 AM CDT Sun May 29 2016

A much nicer and quieter day ahead as the low pressure that brought
the rain and thunderstorms to the area Saturday has exited to the
east. This will allow drier air to settle in and with mostly clear
to partly cloudy skies temperatures will warm nicely. Highs should
be somewhere between about 77 and 82 in most locations. By late
afternoon and early evening southerly return flow across Nebraska
will push towards the Missouri River Valley. Soundings in this area
suggest a weakening cap but confidence in this happening is only
marginal so will just have a 20 percent chance to cover this.
Otherwise a weak wave will allow the low level jet to increase a bit
and converge over mainly central South Dakota. Suspect that a few
showers and thunderstorms will develop but coverage should not be
too widespread. With CAPE values likely less than 1000 j/kg and
shear values about 20 knots severe weather is unlikely.

.LONG TERM...(Monday through Saturday)
Issued at 428 AM CDT Sun May 29 2016

Changes will be underway on Monday, but expect these to be mainly
during later day. Fairly strong likelihood that any overnight
convection would begin to decrease heading through Monday morning as
leading elevated theta-e advection does the same with low level jet
refocusing back toward central South Dakota. At the same time, the
low level thermodynamic boundary should continue to push northward
toward or perhaps just north of I-90 by late afternoon, as long as
convection does not impede progress. Instability in models is very
unrealistic by Monday afternoon, and expect that MLCAPE will be much
closer to 900-1400 J/kg rather than the 2000-3000 J/kg in models. Do
agree with some capping to any surface based activity indicated in
profiles, but window should still be open for at least some spotty
elevated activity even into mid afternoon.  Looking at the other
component for severe threat, shear, values are fairly modest across
the area. Perhaps 30-35 knots in play for areas near/west of the
James Valley during the late day as westerlies aloft begin to
increase with approach of upper trough into northern high plains.
Shear does decrease a bit toward the south and east, even as low
level jet manages to veer during Monday night, but does remain
nicely distributed through the 0-6km depth with directional veering.

Conceptually, expect to see a few scattered storms develop near the
warm frontal boundary around I-90 in the very late afternoon and
evening, as cap weakens with diurnal support and help from lifting
forcing pushing into eastern South Dakota. Any of these storms will
eventually give way to more organized storms approaching from the
west during the evening and early overnight. Shear is marginal for
supercells toward the west, likely supportive of hybrid structures.
Shear vectors would suggest not holding onto discrete mode for too
long.  Further east, shear weaker though perhaps locally enhanced
near diffuse lower level frontal boundary. In addition to the
hail/wind threat mainly near/west of I-29, there may also be some
threat of flooding if storms form near the warm front with fairly
low propagation speeds and perhaps a tendency for redevelopment back
toward main wave.

Leading convection should push aggressively through CWA with pre
frontal wind shift. NAM is a terribly slow outlier compared to all
other solutions.  Suggestions of a trailing light precipitation area
with forcing from mid and upper cyclone working through post frontal
zone in southeast South Dakota during the morning.  However, more
significant and interesting will be where the synoptic boundary ends
up by afternoon and early evening. There is chance that the cyclonic
jet streak would hold up progression near the southeastern CWA
around KSLB and allow a small severe threat to linger for a few
hours, but this would be against most models at this time. Could
actually end up a fairly pleasant day further west through the James
Valley and westward as dry slot pushes into the area.

Overnight Tuesday night, will see a rapid end to any precipitation
during the evening with dry slot expanding eastward. Upper low over
the Dakotas will continue to shift eastward, with wrap around lower
cloudiness returning overnight and early Wednesday. System is
lifting somewhat more northeast, so should only be a low end chance
for any isolated showers to swing through on Wednesday, and mainly
only from areas along highway 14 and through southwest Minnesota.
Cooler trends worked into the forecast yesterday continue, and may
only see some mid 60s highs in some locations, and that counts on
some fairly decent mechanical mixing of brisk northwest winds along
with some partial clearing during the afternoon.

Thursday into early Friday should be dry and cooler with northwest
flow in control and surface ridge pushing across the area. Likely to
find some lows dip into the mid to upper 40s, but highs will start
to recover some on Friday to mid to upper 70s as southerly flow
returns. ECMWF has a curious wave on Thursday night which would at a
minimum increase mid clouds, and even shows some hint to isolated
convection. Worthy to watch, but at this time too much a fringe
solution to clutter the forecast.

Better and more consistent signal for precipitation on Friday night
and early Saturday as more defined jet structure digs southward into
the northern plains with frontal passage. Have included thunder, and
directional shear potential could make for a period to monitor the
severe threat.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 520 AM CDT Sun May 29 2016

Patchy areas of IFR fog will be possible through about 14z this
morning...but should quickly dissipate under a strong late May
sun. Otherwise VFR conditions are expected. Spotty thunderstorms
will be possible near the Missouri River Valley this evening but
coverage should remain fairly low.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...08
LONG TERM...Chapman
AVIATION...08



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