Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202328

Area Forecast Discussion
National Weather Service Sioux Falls SD
628 PM CDT Tue Jun 20 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 312 PM CDT Tue Jun 20 2017

Unsettled weather returns late tonight into Wednesday, bringing
chances for thunderstorms, some of which could become severe. A
difficult forecast is projected over next 12 to 24 hours due to
moderate confidence on location and time of occurrence of any
thunderstorm development.

The first round of thunderstorms is expected late tonight into
Wednesday morning.  As a warm front lifts northward from central
Nebraska late tonight, warmer and more low-level moisture will
increase across the region. A narrow instability area of 800-1500
J/Kg, on the nose of a strong low-level jet of 40+ knots located
between central South Dakota, and west of the James River could
bring isolated to scattered thunderstorms. With an uncapped
atmosphere above 800 mb, and 0-6 km bulk shear of 40-50 knots,
marginal thunderstorms could develop from west to east, albeit some
of them could become strong to severe as they progress eastward.

Uncertainty still remains in terms of timing, occurrence, and
intensity of any storms as the main shortwave and cold front slides
eastward in the afternoon. The main questions are: will these
morning storms overcome the cap as they move east ahead of the
front? Will the morning storms be strong enough to inhibit any
chance of thunderstorms re-developing later in the day? Models
continue to show a strong cap with 700 mb from 12-15C through late
afternoon. However, Hi-Res models (NAMNest, HRRR, NMM) prog
scattered to multicells storms developing west of the Missouri river
in central South Dakota in the nighttime, and continuing across the
forecast area through late afternoon. Therefore, isolated strong to
severe storms could develop with some of these storms producing
mainly strong gusty winds, and isolated large hail. If this trend
verifies, chances for thunderstorms in the late afternoon and
evening will diminish. Moreover, if cloud cover decreases through
the day, with rich-boundary layer moisture, moderate instability
with MUCAPE from 3000+ J/kg, and LLJ reinforcing early in the
evening, strong to severe storms could develop mainly east of the I-
29 early in the evening.

Wednesday looks to be the warmest day of the week, though
temperatures will be very conditional on thunderstorms and cloud
coverage. Highs will generally be in the 80s east of the I-29 to
lower 90s west.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 312 PM CDT Tue Jun 20 2017

Biggest questions for Wednesday evening and overnight will be the
impacts of morning convection, stout EML, and lack of strong
forcing.  Most convective allowing models in reasonable agreement
today really putting a damper on widespread severe storms
redeveloping behind morning shortwave activity in the evening hours.
 Some concerns that any morning convection could further stabilize
the environment over the CWA, in addition to shoving boundaries
further south.  Model soundings continue to show a very stout cap,
even as late as 7 or 8pm, and maintain 700 mb temperatures greater
than 12C.  Forcing along the boundary is really not that strong,
with the greatest forcing tied closer to a shortwave crossing North
Dakota into Minnesota Wednesday evening.  That said, will not
discount the convection risk, especially with trailing cold front
driving southward out of North Dakota late in the afternoon.

Perhaps the best risk for strong to severe convection develops late
Wednesday night into Thursday morning along or north of the I-90
corridor as a more potent shortwave enters the Plains, forcing
convection AOA the 700 mb layer.  The track of this convection will
be key to Thursday afternoon redevelopment, as there would be strong
potential for an outflow boundary to reinforce the synoptic front
just south of the CWA. While the instability and shear are not the
greatest combination, there would still be potential for a few
elevated hail storms Thursday morning. Should the track of this
upper wave slow down, it`s possible to have a bit more robust growth
by the early afternoon hours, especially as storms begin to track
towards NW Iowa.

Cool and dry air settles in for Friday, with highs cooling towards
the 70 degree mark. That trend continues into next weekend as very
broad synoptic troughing develops over the northern US. Recent
guidance has trended further south with the rain chances into
Saturday morning with only widely scattered afternoon instability
showers to the area. A closer look at temperatures suggests we may
be only a couple degrees away from the record coldest highs for the

Cool weather diminishes somewhat to start next week as highs slowly
climb back to the 80s with dry conditions.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 628 PM CDT Tue Jun 20 2017

Chance of scattered convection returns after 06Z tonight, and
continues through the TAF period. At this time, favored window
for storms in the forecast area is 09Z-17Z. However, confidence
in timing at any specific point remains low, and thus opted to
leave TSRA mention out of our TAFs for this issuance. However,
possible that a small window will be added if confidence increases
later this evening.




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