713
FXUS63 KFSD 200828
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
328 AM CDT Thu Aug 20 2026

.KEY MESSAGES...

- Patchy fog continues this morning. Dense fog has developed
  near US Highway 20 into far southern South Dakota, with
  visibility below half a mile at times.

- Isolated to widely scattered thunderstorms may develop this
  afternoon and evening. Two inch hail and 75 mph winds are the
  main hazards the strongest storms.

- Warmer temperatures continue on Friday, with another very
  conditional risk for thunderstorms in the afternoon and
  evening.

- Slightly cooler weather for the weekend. Saturday should
  remain dry with rain chances returning to portions of the area
  Sunday morning.

&&

.UPDATE...
Issued at 327 AM CDT Thu Aug 20 2026

Patchy fog, including dense fog, has developed across the southern
portions of our forecast area. More isolated dense fog has developed
elsewhere. Issued a Dense Fog Advisory for the US Hwy 20 corridor
into far southern SD through 9 AM CDT, although may need to extend
in time or expand in area before then.

Focus remains on the chances for thunderstorms later today into
tonight. May see some showers this morning, mainly for southwestern
MN, as WAA and a weak mid level vort lobe swing south ahead of the
front. Soundings continue to show a robust dry layer, so expect
sprinkles to very light showers at best. Cold front pushes through
the area today/tonight with 20.00z and later guidance quite varied
in the timing. Again, better forcing is to our north and east across
central/northern MN with the main wave but the proximity to the
forcing should keep more scattered activity in southwestern MN vs
more isolated convection elsewhere.

However, one change in guidance over the last 24 hours has been the
upward trend in instability, with a majority of guidance showing
2500 J/kg or more of MUCAPE just ahead of the front - focused south
of I-90 into NE. Have seen a small trend downward in the extent of
the strongest cap, but there still will be around or more than 100
J/kg of CIN. Shear values remain supportive of rotating updrafts,
and may be strong enough to support hail around 2" (or more) despite
mid level lapse rates around 6.5 deg C/km. Drier low levels support
strong wind gusts to 75 mph. Tornado threat is low, but will have to
watch if a stronger storm is able to be more surface based and along
the front this afternoon. Storms will be progressive enough to limit
flash flooding potential, although could see some brief heavy rain.
No changes to the timing of the severe weather, with the greatest
chances between 4 PM and midnight. Stay weather aware through the
day today and into tonight.

Storms exit the area by early overnight. May see patchy and locally
dense fog develop again for the Friday morning commute.

&&

.DISCUSSION...
Issued at 1256 PM CDT Wed Aug 19 2026

THIS AFTERNOON: Rain continues to move southeast this afternoon,
with gradual subsidence setting in behind the passing mid-lvl
vorticity. There remain a very low risks for a few elevated
storms to form along the MO river this afternoon, and then
further northwest into central South Dakota where a narrow
tongue of increased boundary layer moisture remains.

TONIGHT: Much of the evening and overnight hours is expected to
remain quiet and dry. While a few showers and thunderstorms may
form over central and eastern ND and perhaps extreme northern
South Dakota. Winds at the surface will turn light and variable,
and with recent rain and mostly clear skies some areas of fog
may develop by daybreak. The lowest visibilities may form along
the I-29 corridor with locally dense fog possible at times.

THURSDAY: Another embedded wave within the northwesterly flow
pattern is expected to drop through the Plains and into the Upper
Midwest on Thursday. This wave will push a surface front into the
Tri-State are in the afternoon with breezy southwesterly winds ahead
of the front in the morning.  Soundings ahead of the boundary by
afternoon show moderate instability upwards of 2500 J/KG in a
few locations. However they also show at least 100 J/KG CIN,
which could serve as a big limiting factor for convective
development in the afternoon. The greatest forcing along the
boundary remains focused from northern Minnesota into central
Minnesota, with much weaker surface convergence along the
boundary further southwest into northern Nebraska. This is
likely why, outside of any significant moisture/thermal
advection in the boundary layer, CAMs remain very isolated in
convective development by late afternoon and early evening. That
said, given effective shear of 35-40 knots, any updraft could
rotate and increase the hail size towards the 1.5-2" mark.

FRIDAY: We may see a quick warmup on Friday into the middle and
upper 80s as brief mid-lvl ridging moves into the Tri-state area. By
the afternoon however, model guidance is all over the place
suggesting perhaps another weak trough passes through the
Plains/Midwest late in the day. Should this wave materialize, it
could drive a front and some associated convection through the
eastern Dakotas Friday afternoon and evening. This remains a very
low-probability event, so will keep PoPs on the lower end of
guidance.

SATURDAY-SUNDAY: Much of the upcoming weekend will be dry as
Canadian high pressure moves through the region. Light northerly
winds and weak CAA aloft will promote a very pleasant afternoon with
northerly surface winds and lower humidity. Some modest return
flow and weak warm advection may develop over the western
Dakotas and Nebraska panhandle into Sunday. Medium range
guidance has slowed down this eastward progression over the past
2 days, now indicating that precipitation chances should remain
west of the CWA until Sunday night into Monday. High
temperatures Sunday will be very similar to Saturday, in the
upper 70s to lower 80s.

MONDAY-TUESDAY: Precipitation chances increase early next week as a
shortwave crests the top of the growing mid-lvl ridge over the
central CONUS.  Model spread in both timing and location of
precipitation will keep these PoPs only around the 20-30%
range. Temperatures do show a warming trend into next week, with
growing probabilities of highs reaching or exceeding the 85
mark at times into mid-week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 101 AM CDT Thu Aug 20 2026

Areas of fog, some locally dense, will be the main aviation
concern through the mid-morning hours today. Confidence has
increased on this potential up to the I-90 corridor, though the best
chance of dense fog is along the Highway 20 corridor into northwest
Iowa. Any fog should lift by 20.14-20.15Z.

Though there is a good chance at least one thunderstorm will
develop somewhere over the area late this afternoon and evening,
confidence remains low on where it develops and the overall
coverage of storms. For this reason, left out -TSRA from the
TAFs. There is still a 15-25% chance of storm at KFSD and KSUX;
however the slightly better chances (25-35% chance) remain over
southwest Minnesota.

A cool front will bring a shift in winds from the south-southwest to
the northwest as it moves through areas west of I-29 during the day
today. This front may stall in between the James River and I-29 by
the late afternoon and so a wind shift is unlikely to occur east of
I-29. Winds will become light and variable again anyway to
close out the period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 9 AM CDT this morning for SDZ070-071.
MN...None.
IA...Dense Fog Advisory until 9 AM CDT this morning for IAZ020-031.
NE...Dense Fog Advisory until 9 AM CDT this morning for NEZ013-014.

&&

$$

UPDATE...SG
DISCUSSION...Dux
AVIATION...Samet