Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 232000

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

.SHORT TERM...(This evening through Thursday)
Issued at 259 PM CDT Wed May 23 2018

In the near term, broken line of thunderstorms funneling north
through the James River Valley.  Could see an isolated storm or two,
but with CAPE near 1500 J/KG and weak shear, generally expect these
storms to remain below severe limits.  Expect these storms to decay
in the early evening hours as heating dies away.

Challenging forecast heading into tonight with waves of convection
expected to propagate into the forecast area.  Storms across western
South Dakota into western Nebraska this evening are expected to move
into central South Dakota towards 06z.  If they hold together, they
are moving into a more favorable environment as low level jet is
most concentrated across central and eastern South Dakota.  CAMs
suggest as these clusters of storms approach Central South Dakota,
the may try to organize and expand into a complex.  Not a lot in the
way of additional forcing mechanisms, as low level jet is nearly
parallel to the low level boundary across Central South Dakota. One
concern is 0-3 km shear is fairly weak, so elevated storms may
produce outflow that races out ahead of the complex stabilizing the
near surface layer. Therefore, have the highest pops concentrated in
the west, with lower pops trailing into the I-29 corridor and areas
east after 12Z Thursday. Forecast soundings suggest a nearly
uncapped environment from near 850-800 mb, so there is the potential
of storms if we can get a trigger. It is unknown how the outflow
boundaries will interact with the low level jet and convective
clusters around the area.

If we can destabilize throughout the day on Thursday, interesting
set up with stationary front/ dry line type feature remaining across
central South Dakota as shortwave approaches from the southwest late
in the day. Models suggest CAPEs could exceed 3000 J/kg with very
limited shear.  0-6 km bulk shear is near 15-20 knots, so think
storms will struggle to organize. Biggest concern for afternoon
convective potential and high temperatures will be the lingering
cloud debris from morning convection.  With weak winds through the
layer, cloud could hang around longer than expected.

.LONG TERM...(Thursday night through Wednesday)
Issued at 259 PM CDT Wed May 23 2018

Friday through the holiday weekend looks warm and dry as an upper
level ridge develops across the plains.  Have kept forecast dry, and
with 850 hpa temps pushing 20C, expect highs in the upper 80s and
90s through much of the weekend.  At this point, Saturday appears to
be the warmest day with widespread 90s.Dew points in the lower 60s
will push heat index values towards advisory criteria, but appears
to be a few degrees on the cool side at this point.

Upper level trough works into the west coast and inches toward the
forecast area early next week. Spokes of energy break off the low
and lift northeast bringing sporadic chances of precipitation Monday
through Wednesday to the region.


.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.




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