706
FXUS63 KFSD 181752
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1252 PM CDT Fri Sep 18 2026

.KEY MESSAGES...

- Isolated severe storms remain possible from roughly 4 pm to 9
  pm in portions of northwest Iowa. Damaging winds and perhaps
  an isolated tornado will be the main hazards if storms can
  develop.

- Fog is likely tonight into Saturday morning. For now
  widespread dense fog is not expected, but the better chances
  should be in and around southwest MN.

- Below normal temperatures will persist through next week.
  Additional rain chances will continue into early next week,
  but thunder chances are low and severe weather is not
  expected.

&&

.DISCUSSION...
Issued at 1251 PM CDT Fri Sep 18 2026

Shower and thunderstorm chances will likely come in tow waves
tonight, but focused on mostly northwest IA and northeast NE. The
first will be late afternoon and early evening, centered around the
potential for something close to surface based if enough heating can
occur. The warm nose is currently working towards Sioux City with
Wayne NE at 85 degrees and a 73 dew point. Latest soundings suggest
surface heating into the upper 80s will be needed to break the cap.
The best chance for this should be south of an Elk Point to Okoboji
line. If the cap can break CAPE values around 2500 J/kg are expected
with marginal shear and a freezing level around 14000 feet AGL. This
would support wind gusts to 65 mph and potentially an isolated
tornado if a storm can become rooted in the boundary layer. The
latest HRRR runs suggest that surface based development may tough to
come by with slightly elevated storms developing closer to Jackson
and Okoboji about 23-0z.

Later this evening and early overnight a wave moves northeast from
NE and may bring another threat for showers and storms. Currently
these should not be severe with marginal shear and instability
expected.

Weak north to northeast flow develops at the surface tonight and
continues through Saturday. Model soundings showing a saturated low
level air mass in place tonight into Saturday morning, shallowest
near the Missouri River and deepest near highway 14. Given the
weaker, cool flow suspect that fog will become fairly widespread
with some locally dense. Given the cooler northerly flow, this fog
will be slow to lift on Saturday morning and will likely become a
stratus layer through the day Saturday, leading to a cool and cloudy
day.

A slow moving low pressure will move onto the Plains Sunday into
Monday bringing the next threat for rain and isolated thunderstorms.
For now severe weather looks very unlikely. Heavy rain also looks a
bit unlikely with CAPE values pretty low. Could see some extended
periods of moderate rainfall, but unlikely to make a push towards
any flash flooding with river flooding also pretty low.

Broad upper level riding will build into the area behind the slow
moving low pressure, with temperatures expected to warm but remain a
bit below normal. The models are very disagreeable on some weak
upper level energy mid week next week so confidence on rainfall is
low.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1205 PM CDT Fri Sep 18 2026

Widespread IFR conditions will persist through the period,
possibly some LIFR in fog and stratus later tonight into
Saturday morning. Parts of northwest IA will see VFR conditions
for a few hours this afternoon. Otherwise scattered thunderstorms
will be possible in parts of southwest MN and northwest IA from
about 23z through 4z this evening, with spotty showers and
drizzle possible after that into Saturday morning.

&&

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

&&

$$

DISCUSSION...08
AVIATION...08