Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 211212

National Weather Service Sioux Falls SD
712 AM CDT Thu Sep 21 2023

.SHORT TERM...(Today through Friday)
Issued at 359 AM CDT Thu Sep 21 2023

Today: A slightly cooler and dreary day will be on tap for the day.
Taking a look across the area, scattered showers and thunderstorms
continue to develop west of the James River thanks to a couple
hundred Joules of instability and a strengthening LLJ ahead of a
weak mid-level wave. Expect these showers and thunderstorm to slowly
progress northeastwards over the next several hours before gradually
dissipating as the activity approaches the I-29 corridor. Looking
aloft, upper-level ridging continues to build just east of our area
shifting our flow aloft to the southwest. At the surface, the SPG
begins to tighten just west of I-29 bringing marginally breezy
southeasterly winds back to the the region.

Shifting gears here, a secondary shortwave will "ride the ridge"
into our area this afternoon triggering more scattered showers and
thunderstorms to develop in portions of southcentral and
southeastern SD. However, expect this developing activity to mostly
be confined to areas along and west of the James River as drier more
stable air is advection into the our area from the southeast.
Nonetheless, with the better corridor of instability expected to be
in the same area; some strong to severe storms will be possible this
afternoon and evening with the primary threats being large hail and
damaging winds. Otherwise, lingering stratus from this morning
showers and thunderstorms will keep skies mostly cloudy east of I-29
for the majority of the day. This along with southeasterly flow should
keep temperatures a bit cooler than the previous day as highs peak
in the mid 70s to low 80s across the area.

Tonight: Looking into the evening and overnight hours, our attention
shifts to the western CONUS as a closed upper-level low (ULL) begins
to push towards the Rockies as the main axis of the ridge slowly
moves to our east. With this shift, any lingering showers and
thunderstorms will finally be allow to progress eastwards overnight.
However, the severe threat will diminish shortly after sunset.
Otherwise, the increased precipitation chances and consistent cloud
cover will help our overnight temperatures take a slight dip into
the mid 50s to low 60s for lows.

Friday: Heading into the start of the weekend, the wet and dreary
conditions continue as the previously mentioned ULL moves into the
Rockies. An area of large-scale ascent aloft (mentioned in the
previous morning discussion) continues to surge northeastwards
towards central/eastern SD & NE ahead of a stronger wave that
rotates off our main low Saturday afternoon to evening. With
deterministic guidance continuing to show this wave getting tied to
the 850mb warm front and the nose of an upper-level jet, expect
scattered showers and thunderstorms to develop over central SD /
northcentral NE and slowly push northeastwards as we head into the
overnight hours.

Increasing instability and shear ahead of the surface boundary
could lead to some strong to severe storms with the primary
threats being large hail (with discrete cells) and damaging winds
(more linear storms). A notable secondary threat will be locally
heavy rainfall especially with developing convection. Nonetheless,
while areas along and east of I-29 will have pretty good chances
of seeing showers and thunderstorms; the main severe threat will
be focused along the Missouri River corridor. Otherwise, with
consistent cloud cover, southeasterly winds, and increasing
precipitation chances; our overall temperatures will continue to
decrease as highs peak in the mid to upper 70s for the day.

.LONG TERM...(Friday through Wednesday)
Issued at 359 AM CDT Thu Sep 21 2023

The latest models and ensemble output indicate that Saturday into
Saturday night will be the best threat for a more widespread area of
showers and thunderstorms. By 0z Sunday the GEFS shows the 500 mb
low pressure center near the Missouri River along the ND/SD border
with a similar location of the surface low. Instability also
predicted to surge into the area as this low tracks northeast and
slows across ND. A strong mid level wave/dry slot will drive
northeast into the area on Saturday afternoon and should be the
driving force for additional development. With the potential for
upwards of 2000 J/kg CAPE and moderate shear, severe weather will be
possible, especially near and east of I-29. However, the big caveat
is how much convection is ongoing Friday night into Saturday morning
which will really make a big difference on location and strength of
the instability. Also still some timing differences as a slightly
slower wave could result in more widespread issues. One difference
noted between the GEFS and EC Ensemble are precipitation amounts.
The EC is coming in with about twice as much as the GEFS.

Sunday into Monday models continue to diverge on how the pattern
will resolve itself.  Overall the low pressure should weaken and
fade away as a trough digs into the west coast and upper level
ridginess starts to build onto the Plains. The low level flow should
remain on the mild side with scattered chances for showers. Overall
model soundings not very impressive with the better chance Sunday
afternoon east of I-29.

Tuesday into Wednesday will see upper level ridging spread across
the area with warming in the low levels. Lower chances for
precipitation are expected with temperatures near to a bit above
normal, especially on Wednesday.


.AVIATION...(For the 12Z TAFS through 12Z Friday Morning)
Issued at 706 AM CDT Thu Sep 21 2023

VFR conditions with occasional MVFR conditions are expected for
this TAF period mostly due to the developing showers and
thunderstorms this morning and this evening. Taking a look across
the area, scattered showers and thunderstorms continue to develop
just east of I-29. Expect this activity to follow the Missouri
River eastwards over next several hours before gradually
dissipating as it approaches I-29.

A secondary round of showers and thunderstorms will likely develop
in central SD this afternoon in response to a secondary wave.
While this activity should be confined to our River Valleys and
areas west of the James, decided to keep prevailing showers and
thunderstorms in for KHON for most of the day. Added in some TSRA
to the KSUX TAF since this follows the better corridor of
instability. Otherwise, expect a dreary day with plenty of VFR
stratus and breezy southeasterly winds to continue for most of
the TAF period with gusts between 20-30 kts possible at times.




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