487
FXUS63 KFSD 181811
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
111 PM CDT Tue Aug 18 2026

.KEY MESSAGES...

- Mostly dry this afternoon, but isolated thunderstorms may
  develop over SW MN and NW IA this evening. Marginally severe
  hail and 60 mph could be possible, but risks remain fairly
  low.

- Scattered showers and non-severe thunderstorms slide through
  the MO river valley early Wednesday. The rest of the day looks
  dry with highs in the 70s/80s.

- Late Thursday afternoon and evening thunderstorms possible
  along an approaching front. Best chances in Northern and
  Central MN, but isolated risks also further southwest. Large
  hail and localized strong winds main risks.

- Mostly dry conditions heading into the weekend with highs
  either side of 80 degrees. Morning rain on Sunday is possible.

&&

.DISCUSSION...
Issued at 106 PM CDT Tue Aug 18 2026

THIS AFTERNOON: A weak frontal boundary continues to sink southward
early this afternoon, with noticeable strato-cu field
developing along the Highway 20 corridor. The remainder of the
afternoon is likely to remain dry in most locations, outside of
very isolated development along this aforementioned boundary.

THIS EVENING/TONIGHT: Some guidance shows isolated development this
evening along a weak convergence zone in SW Minnesota and northern
Iowa. This convection may also be sparked by a weak area of
vorticity dropping through the Dakotas from the northwest. Any storm
that develops will be in an area of MLCAPE AOA 1000-1500 J/KG, with
sufficient shear to promote perhaps a rotating storm.
Anticipating the severe weather risks to remain low.

Further west, additional convection is expected to form over the
Black Hill region this evening as a stronger wave enters the area.
The track of this wave, along with how the LLJ propagates overnight,
should push some of this activity into south central SD after 1am.
MUCAPE remains quite low, with marginal mid-lvl lapse rates, so
widespread severe weather is not anticipated.  Though, a few gusty
wind gusts could remain possible through 3am. There remains
reasonable agreement that this convection could track along the MO
river valley into eastern Nebraska by daybreak.  This is an area
where higher PoPs will be focused.

WEDNESDAY: The departure of this shortwave should lead to a period
of subsidence and quiet weather well into Wednesday. Cloud cover may
be more substancial over the southern half of the CWA thanks to mid-
upr clouds from convection. By the afternoon southwesterly surface
winds will return, allowing high temperatures to rise into the upper
70s to lower 80s. Convection risks in the afternoon and evening may
be quite narrowed to a zone ahead of an approaching surface front in
north central SD.  Further southeast, there doesn`t seem to be any
strong focusing mechanism, with weak convective inhibition likely
keeping the area free of any isolated larger storm growth. While
surface and low-lvl focus remains absent, we`ll need to watch the
presence of any weak vorticity in the NW flow. Further northwest,
any convection that develops along the frontal boundary may try to
track southeast, though lessening instability should limit the
overall severe risks.

THURSDAY: Any precipitation risk continues to look minimal heading
into Thursday morning as a stronger EML begins to build into the
region.  Southwesterly surface winds overnight may keep overnight
low temperatures from falling significantly. The aforementioned
surface boundary from Wednesday will sink southeast through the
day on Thursday providing a focus mechanism for storms by late
afternoon and evening. MLCAPE approaching 2000-3000 J/KG may develop
along this boundary as dew points try to quickly surge northward.
The lingering questions for convective development are focused
on the track of mid-lvl vorticity in the late afternoon, along with
convective inhibition ahead of the boundary. The track of shortwave
energy could favor more development in central and northern Minnesota,
with only isolated storms further southwest along the boundary. While
instability is greater, shear is a bit more marginal suggesting
less organization to anything that develops.


FRIDAY-MONDAY: Mid-lvl heights begin to rise over the Rockies as the
ridge amplifies. The lingering, but sharper, NW flow pattern
over the Dakotas may bring one more wave into the region Friday.
However the southward passage of Thursdays front should push
the best boundary layer moisture south of the area. The ridge
axis begins to push eastward as we move into the upcoming
weekend. A pleasant Saturday is likely in the region with highs
either side of 80. Increasing low-lvl warm advection into Sunday
could bring elevated showers and thunderstorms eastward into
Sunday morning, along with more clouds and slightly warmer highs
into the lower to middle 80s. At this time, rain coverage may
be limited with only a 20-30% PoP.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1212 PM CDT Tue Aug 18 2026

Scattered CU will continue to develop this afternoon along a
couple weak frontal boundaries. Some greater concentration of
MVFR CU will be near Highway 20 into this evening. Winds will
remain light from the west to northwest, though considerable
variability will be expected this evening. A rogue thunderstorm
could be possible along the Ridge of SW Minnesota this evening.

Elsewhere, VFR conditions persist as scattered convection forms
in the West River areas overnight. While uncertainty persist in
the track of convection overnight, the greatest potential will
be along an west of the James River valley into daybreak.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

$$

DISCUSSION...Dux
AVIATION...Dux