Storm Lake, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

683
FXUS63 KFSD 200602
AFDFSD

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

.KEY MESSAGES...

- Patchy fog continues to develop early this morning. Dense fog
  is be possible with visibilities less than 1 mile especially
  along the Highway 20 corridor and into northwest Iowa.

- Isolated to widely scattered thunderstorms possible late
  Thursday afternoon and evening. 2-inch hail and 70 mph winds
  will be the primary risk.

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

- Slightly cooler weather, with mostly dry conditions can be
  expected for the upcoming weekend, with rain chances returning
  by Monday morning.

&&

.UPDATE...
Issued at 901 PM CDT Wed Aug 19 2026

Patchy fog looks to develop after about 2-3 am tonight mainly
along the I-29 corridor near and south of I-90 down into
northwest Iowa and the Highway 20 corridor given light winds and
clearing skies expected tonight in those aforementioned
locations. That said, there is lower confidence in the northern
extent of any fog up to I-90 since winds may be able to tick up
just enough closer to daybreak to prevent it from developing.
The better coverage and the better potential for fog with
visibilities a mile or less will be over the Highway 20 corridor
of northwest Iowa. Any fog should lift by 9 am, with perhaps
one or two low lying areas having fog linger through 9:30/10 am.

A few showers may creep into areas along and north of Highway
14 Thursday morning, though dry low level air should prevent
most of this from reaching the ground outside of some sprinkles.
The main concern comes in the mid-to-late afternoon as a front
moves into the area and helps provide some lift for a very
conditional thunderstorm threat. This is very conditional
because the better upper level support will be north and east of
the area and so uncertainty remains high if storms can develop
at all. But if a storm is able to develop and sustain itself, it
will be capable of severe weather with the potential for
supercellular storm modes given effective shear values around
40 kts.

Southwest Minnesota will be closer to the better upper forcing
and so the best chances of storm development will be there
(25-35% chance). The highest instability will be to the south
and west, but given the storm mode, large hail upwards of golf
ball size will be the main concern. Secondary threat is wind
gusts up to 65 mph due to fairly steep low level lapse rates.
This wind threat will diminish after sunset as storms lose touch
with the surface, though the hail threat would continue.
Farther south and west into the Missouri River Valley of
southeast South Dakota, storm chances diminish even further due
to the weaker upper level support (15-25% chance). However,
there will still be a threat for a very isolated supercell to
develop. This is even more of a highly conditional threat than
over southwest Minnesota, but these storms if they were to
develop would have a higher ceiling and could produce hail to 2
inches or perhaps slightly larger as the main threat. Wind gusts
to around 70 mph will be the secondary concern. No matter what
ends up happening, most areas will likely stay dry Thursday
afternoon and evening, but still keep that eye to the sky if you
have any outdoor plans! Main time frame for severe weather
chances will be 4 pm to midnight.

&&

.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...None.
MN...None.
IA...None.
NE...None.

&&

$$

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