323
FXUS63 KFSD 301145
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
645 AM CDT Wed Sep 30 2026

.KEY MESSAGES...

- Dense fog remains most widespread east of Highway 81, with
  areas of dense fog farther west. A Dense Fog Advisory covers
  the entire forecast area through 10 AM. Visibility 1/4 mile
  or less will be prevalent through the morning commute.

- Additional showers will lift through the U.S. Highway-20
  corridor in northwestern IA this afternoon/evening. Rainfall
  amounts mainly less than 0.25 inch are expected, but totals
  nearing 0.50 inch may push into far southern portions of Ida
  and possibly Woodbury Counties.

- Drier conditions will temporarily return on Thursday and
  Friday with the potential for our next rain chances returning
  by Saturday.

&&

.UPDATE...
Issued at 220 AM CDT Wed Sep 30 2026

As anticipated by previous shifts, clearing and light winds has
allowed dense fog to develop through the southwest to central
portions of the forecast area early this morning. Have issued a
Dense Fog Advisory mainly east of Highway 281 for now, but
satellite hints at potential need for expansion farther west and
will continue to monitor.

As far as our rain chances over the next 24-30 hours, 30/00Z
models continue to focus band of heavier rain south of our CWA
as the remnant moisture from Hurricane Polo is drawn northward
ahead of a strengthening upper low swinging east-northeast out
of the Four Corners area tonight. We should see some increase in
mid-level clouds across our southeast (south of Yankton SD to
Spencer IA), but with the 850mb front projected to remain near
to south of a Sioux City to Storm Lake line, northward push of
more significant moisture will be limited by a dry low level
northerly flow north of the boundary. This could result in our
far southeast counties being near the northern edge of a very
sharp rainfall gradient, with additional rainfall potentially
ranging from only a tenth or two near Sioux City to Storm Lake,
to an inch or more only a county or two south. With relatively
good run-to-run consistency in this southern solution over the
past 24-30 hours of model runs, confidence has increased that
this system no longer poses a risk of flooding in northwest
Iowa, and the Flood Watch has been cancelled.

&&

.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 /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 645 AM CDT Wed Sep 30 2026

Widespread LIFR-VLIFR conditions, especially near and east of US
Highway 81, as we start this period. Visibility is expected to
improve to MVFR by 14Z-15Z with widespread VFR conditions across
most areas by 18Z. Exception will be in our far southeast, KSUX
to KSLB, where MVFR ceilings may linger as rain spreads north
out of Nebraska and central Iowa this afternoon and evening.

Winds remain light and variable through 01/06Z, with a light
northerly flow becoming more prevalent after 06Z.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 10 AM CDT this morning for SDZ038>040-
     050-052>071.
MN...Dense Fog Advisory until 10 AM CDT this morning for MNZ071-072-
     080-081-089-090-097-098.
IA...Dense Fog Advisory until 10 AM CDT this morning for IAZ001>003-
     012>014-020>022-031-032.
NE...Dense Fog Advisory until 10 AM CDT this morning for NEZ013-014.

&&

$$

UPDATE...JH
DISCUSSION...Rogers
AVIATION...JH