684
FXUS63 KFSD 211142
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
642 AM CDT Mon Sep 21 2026

.KEY MESSAGES...

- Rainfall and areas of drizzle continues early this morning.
  Although rain coverage will gradually diminish through today,
  areas of light drizzle may linger through tonight and perhaps
  into early Tuesday.

- Below normal temperatures will continue through mid-week,
  then gradually become more mild late in the week.

- Additional rain chances return Wednesday night into Friday,
  but severe weather risks remain low.

&&

.UPDATE...
Issued at 450 AM CDT Mon Sep 21 2026

Main area of rain has shifted north of I-90 through the early
morning hours as a mid-level dry slot wraps into the southern
half of the forecast area. Thick low cloud persist beneath this
dry slot, however, and weak lift atop the moist layer will aid
in continued drizzle/light rain chances today.

The upper low is slow to move east and while rainfall amounts
will taper off, models are trending a little slower in bringing
rain/drizzle to an end altogether. Have extended low (20-30%)
pops through tonight and later shifts may need to consider if
drizzle will linger into Tuesday in some areas as low clouds
persist.

The persistent cloud cover and occasional drizzle/rain chances
will keep temperatures on the cool side for these last couple of
days of astronomical summer with highs in the 50s today and near
60 on Tuesday. Autumn begins at 7:04 PM CDT Tuesday.

&&

.DISCUSSION...
Issued at 141 PM CDT Sun Sep 20 2026

Patchy drizzle remains possible late this afternoon into the evening
ahead of the incoming rainfall. The rain will move into the area
through the afternoon into tonight. Low pressure to the southwest
will swing saturation and lift aloft into the area this afternoon
with a dry slot working north towards the Missouri River. This seems
to suggest that the better potential for a more steady rainfall will
be closer to I-90 as the dendritic layer dries out south of I-90
during the evening, but remains saturated near and north of I-90.
Model soundings do hint at some weak instability near the Missouri
River which may help overcome the lack of saturation in the
dendritic layer during the evening and overnight hours, but the
rainfall would be more scattered in nature. If this weak instability
does not come to fruition, drizzle will be more likely towards the
Missouri River. Given how weak any expected instability will be,
thunderstorms are not expected.

As for amounts through tomorrow, model agreement is fairly high in a
swath of a half an inch to an inch and a half, focused near I-90 in
southeast SD, then east into northern parts of northwest IA. These
rainfall amounts are not expected to cause any flash flooding or
rapid rises along rivers.

Monday will see lingering showers across the area, with a more
focused area over northwest IA as the upper level low slowly moves
out of the area. Amounts will be lower through the day on Monday,
but parts of northwest IA could see a quarter inch of additional
rainfall. Otherwise Monday will be cool and cloudy with highs from
the upper 50s to mid 60s.

Monday night into Tuesday morning could see some fog with light
southeast flow and a fairly moist boundary layer.

Tuesday and Wednesday should both be dry and cool with highs in the
60s both days. Winds will increase on Wednesday with gusts around 30
mph possible near and west of the James RIver.

Wednesday night into Thursday will be the next better chance for
showers and thunderstorms. At this time severe weather and heavy
rain look unlikely.

Temperatures will rebound back to around normal Friday into next
weekend as westerly flow aloft develops. Periodic chances for
showers and thunderstorms are expected during this time.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 641 AM CDT Mon Sep 21 2026

More of the same through the next 24 hours with widespread IFR
to locally LIFR ceilings and occasional MVFR-IFR visibility in
fog and drizzle/light rain. Models to indicate a brief glimmer
of hope for parts of the Highway 14 corridor to break out of the
low stratus for a few hours this evening, but any improvement to
VFR conditions looks to be short-lived as MVFR or possibly lower
ceilings move back in prior to midnight.

&&

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

&&

$$

UPDATE...JH
DISCUSSION...08
AVIATION...JH