651
FXUS63 KFSD 300004
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
704 PM CDT Tue Sep 29 2026

.KEY MESSAGES...

- Light showers will continue this afternoon mainly south of
  U.S. Highway 18 with up to an additional quarter of an inch of
  rain. There is a 20 to 30 percent chance for additional
  showers and isolated non-severe thunderstorms into this
  evening across far southeast South Dakota, far northeast
  Nebraska, and northwest Iowa.

- Gradually clearing skies overnight may lead to the rapid
  development of dense fog. Be prepared to allow for extra time
  during your Wednesday morning commute if fog develops.

- A moisture rich storm system will move across the Central
  Plains on Wednesday. A tight rainfall gradient will be in
  place, with the highest rainfall amounts south of the forecast
  area. Areas along the Highway 20 corridor may still see one
  half to one whole inch of additional rainfall. A Flood Watch
  is still in effect for parts of northwest Iowa if the higher
  rainfall amounts shift north.

- Expect drier and cooler conditions Thursday and Friday with
  the next chance for light rain north of Interstate 90 on
  Saturday.

&&

.DISCUSSION...
Issued at 238 PM CDT Tue Sep 29 2026

Regional radar as of 29.1930Z shows the majority of warm
advective rain showers have moved into areas south of U.S. Hwy
18 across southeast SD and northwest IA. These showers will
continue to move eastward into the late afternoon/early evening
with most areas drying out by 00Z. That said, some CAMs show
scattered convective activity re-developing along a trailing
surface boundary/front late this afternoon and evening mainly
along and south of a Yankton to Sioux Falls to Windom line. Area
soundings show some weak instability, so will keep chance
POPs/isolated thunder into the evening across far southeast SD
into northwest IA. Any lingering activity will diminish
overnight, although clouds will be slow to move out. Some
clearing across southeast SD will allow lows to drop into the
low to mid 50s, whereas temperatures will bottom out closer to
60 degrees for areas under cloud cover across northwest IA.

In addition, will need to watch for fog overnight. Given weak
near surface flow and ample boundary layer moisture from rain
today, could see rapid fog development if the broader cloud
shield dissipates or moves far enough east.

For Wednesday, a strong Four Corners closed low will begin to eject
east-northeastward, and with it will be remnant moisture from
Hurricane Polo. This is a very wet system for late September with
NAEFS PWATS above the 99th percentile. However, our southernmost
forecast area will be on the northern periphery of the highest
moisture as the surface low moves across the Central Plains and the
stationary front sets up across NE/IA. 00/06Z model runs last night
and now the 12Z suite this morning have trended south with the
highest QPF. There is likely to be a sharp gradient in rainfall,
ranging from amounts less than an inch to higher than 2-3 inches.
Areas north of U.S. Hwy 18 will likely remain dry all day. Current
trends suggest our forecast area will be on the lower end, but
given some lingering concern about a late model shift north,
will keep current Flood Watch in place and adjust as needed on
later shifts. Otherwise, Wednesday will be warm and mostly
cloudy with high temperatures in the low to mid 70s.

Some lingering showers are possible Thursday across northwest IA,
but the majority of the area will be dry with much needed dry
conditions continuing into Friday as surface high pressure moves
across the region. A cold frontal passage Wednesday night will also
usher in cooler air, keeping high temperatures Thurs/Fri mainly in
the 60s. As long as skies remain mostly clear, overnight lows will
dip into the mid to upper 40s.

Another weak wave moves through northwest flow aloft on Saturday,
bringing a 20-40 percent chance of showers to areas mainly north of
I-90. Synoptic 500 hPa flow begins to diverge between the major
models Sunday into early next week, but the consensus now is for dry
conditions with seasonable early October temperatures.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 703 PM CDT Tue Sep 29 2026

VFR Conditions for much of the CWA to remain in place
until around 5z and will then deteriorate all the way down to LIFR
for many areas east of highway 81 by approximately 9/10z. With
calming winds into the evening and clearing out of the clouds in
much of the CWA, fog potential stays high for the late overnight
hours into the morning. Higher confidence for areas of more dense
fog remain around KFSD and between highways 81 and 75 stretching up
to the northernmost edges of the CWA and down to KSUX. Confidence
for fog around KHON and west of highway 81 remains low and will most
likely at least settle into half of Beadle county and southward.
Winds to remain light and variable for the CWA through the period.
VFR conditions are expected to return to most of the area by 19z,
excluding much of northwest IA.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...Flood Watch through Thursday evening for IAZ014-021-022-031-
     032.
NE...Flood Watch through Thursday evening for NEZ014.

&&

$$

DISCUSSION...Rogers
AVIATION...05/Rafalski