676
FXUS63 KFSD 210159
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
859 PM CDT Sun Sep 20 2026

.KEY MESSAGES...

- Rainfall overspreads most of the area through tonight,
  gradually ending west to east on Monday.

- Totals rainfall amounts with this system will average about
  0.50" to 1.50" in most locations through tomorrow.

- Below normal temperatures will persist through next week, but
  gradually become more mild late in the week.

&&

.UPDATE...
Issued at 855 PM CDT Sun Sep 20 2026

No major changes to the forecast through tomorrow. Rain
continues across the area as a low pressure system pivots into the
area. Some elevated instability over northwest Iowa and adjacent
areas of southwest Minnesota and southeast South Dakota will lead to
pockets of moderate rainfall through the night. Flash flooding and
rapid river rises are still not expected, but be prepared for some
wet roads on your morning commute tomorrow. Once again
temperatures are not expected to drop all that much through the
night given the cloud cover, so expect morning lows Monday to be
in the 50s across the majority of the area.

An overall damp and dreary day is once again in store for
tomorrow as showers linger and areas of drizzle fill in the
gaps. The rainfall totals expected for this event through
tomorrow remain in good shape, see main discussion below for
more. Just like temperatures don`t drop too much overnight,
don`t expect too much of a rise into Monday afternoon. Highs
look to be mainly in the 50s, though a few lower 60s will be
possible near and west of the James River where at least a few
peaks of sunshine will be possible in the late afternoon.

&&

.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 /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 651 PM CDT Sun Sep 20 2026

A few breaks in the MVFR/IFR stratus are out there to start the
period, but expect cigs to lower again in these areas as the
evening progresses. We`ll continue to see this low stratus
continue into the day on Monday, with drizzle likely at times
especially near the Missouri River Valley. Overnight tonight we
will also see some showers with locally moderate rainfall
especially around and between the I-90 and Highway-18 corridors.
Storms remains unlikely, though a rogue rumble of thunder
cannot be ruled out (<10% chance). Showers will very slowly
drift southeastward through the day on Monday. Winds will be
mainly out of the east-northeast through the period, around 5-10
kts. Some of the heavier showers may be able to mix down a wind
gust to around 20 kts overnight tonight.

&&

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

&&

$$

UPDATE...Samet
DISCUSSION...08
AVIATION...Samet