911
FXUS63 KFSD 311145
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
645 AM CDT Sat Oct 31 2026

.KEY MESSAGES...

- Showers and isolated thunderstorms continue into Saturday
  morning, especially east of the James River. Locally heavy
  rain and lightning are the main threats.

- Outside of showers, drizzle and fog are expected into mid
  Saturday morning. Fog is most likely west of the James River.

- Additional scattered showers and isolated storms Saturday
  night into early Sunday.

- More widespread rain returns later Sunday night into Monday
  night. Moderate to high (60 to 90%) chance for more than an
  inch of rain. Flooding is unlikely, but not zero. Additional
  rainfall is likely on Tuesday.

- A low confidence threat for low topped supercells Monday
  afternoon and evening in far southeast SD, northeast NE and
  northwest IA.

&&

.UPDATE...
Issued at 316 AM CDT Sat Oct 31 2026

Somewhat busy start to Halloween weather-wise. Showers and storms
continue trekking northeast, mainly near and north of I-90 at the
time of this discussion. Still enough elevated instability to see a
fair amount of lightning but severe weather is not expected.
Precipitation is moving relatively slowly and with locally moderate
to heavy rain, rainfall amounts since midnight are over an inch near
and north of Sioux Falls with more widespread 0.25" to 0.50"
observations. Flash flooding is not expected, but could see some
localized ponding of water for areas. Additional showers/storms are
moving north across northern NE and IA.

Further west toward the James River, showers are much more isolated.
Light winds have allowed fog to develop, with some dense fog across
south central and central SD. This should dissipate by mid morning.

No significant changes to the forecast outside of refining pops for
the next few hours. Expect a break in the rain through at least
early afternoon. However, a couple of short waves begin moving north
through the afternoon/evening, which brings the return of rain and
isolated storm chances. Elevated instability remains weak (a couple
hundred Joules) so lightning would be the main concern. Chances
increase from south to north tonight.

Still quite a bit of variance toward early next week with regards to
the surface low. Looking at 00z Tuesday (6 PM CST Monday), the
31.00z NAM is the furthest south (northeastern NE) and the GFS is
the furthest north (US Hwy 14) with the ECMWF and Canadian both
elsewhere in southeastern SD. Still have to keep an eye on the
potential for a few stronger storms. See the Discussion for more
details.

&&

.DISCUSSION...
Issued at 148 PM CDT Fri Oct 30 2026

Showers remain likely tonight into Saturday morning as low level
moisture deepens, roughly the lowest 5 km, and marginally dry,
cooler air above this layer allows for weak instability, generally
200-500 J/kg CAPE. This should generate mainly showers, but a few
lightning strikes will be possible. Rainfall amounts will mostly be
light, but some pockets of a tenth of two will be possible.

Saturday night into Sunday should be mostly dry, but moist neutral
soundings and lingering moisture/cloud cover may allow for some
patchy drizzle and showers. Overall low impact, but likely not an
entirely dry period.

A fairly well agreed upon trough of low pressure will move onto the
Plains Sunday night, deepening on Monday as a strong southern stream
jet strengthens. This should provide that area with widespread
rainfall and if everything comes together timingwise, the threat for
an isolated tornado over southeast parts of the area.

All models continue to indicate a widespread 1-2" of rainfall from
Sunday night into Tuesday morning. Very little disagreement for the
most part on timing and location of the rainfall. Given a fairly wet
pattern the past 6 weeks or so river flooding could start to become
a problem. But for now, the latest hydrologic modeling suggests at
least 2.5-4" of rain would be needed to push rivers to action stage
and minor flooding, so for now chances of river flooding are pretty
low (<20 percent). Also suggestions that 1.5" of rain in an hour
could produce some localized flash flooding, but for now the
rainfall rates should not be quite that high. All in all, it looks
like a band of warm advection rainfall will lift northward Sunday
night into Monday. Later Monday afternoon into Monday night a dry
slot could move in and if convection does not develop, a dry period
or period of drizzle will be possible. Then later Monday night into
Tuesday wrap around rainfall will move through.

While this is a very low threat probability, we will need to keep an
eye on the development of surface low pressure Monday afternoon as
the upper level wave moves through. Starting to see indications this
surface low pressure could end up near southeast SD/northwest IA
which could bring about a threat for some low topped
supercells/isolated tornadoes. The 6z ensemble data has the EC
placing this SFC low in southeast SD late Monday afternoon while the
GEFS and Canadian Ensemble are in eastern NE. Obviously a big
difference so trends will need to be monitored. The 12z GFS is now
placing the surface low in southeast SD late Monday afternoon as
well.

Tuesday will likely see some wrap around rainfall continue, but
should diminish throughout the day. Wednesday through Friday should
be mostly dry and cooler with highs from the upper 40s to mid 50s.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 638 AM CDT Sat Oct 31 2026

MVFR and lower stratus expected through much of the period.
Scattered showers and isolated thunderstorms continue through
mid to late morning, moving northeast over the area. Seeing some
visibility at or below 2 SM at times with locally heavier rain
and TS. Additionally, fog west of the James River is leading to
reduced visibility, including some visibility below 1 SM. Fog
lifts mid to late morning.

May see some VFR stratus at times, but overall MVFR or lower
conditions expected. Rain showers return this evening into
tonight, but confidence is low on timing/coverage, so kept
PROB30 groups at all sites.

South to southeast winds increase late this morning with gusts
around 20 knots into the evening, strongest west of I-29. Winds
taper down tonight.

&&

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

&&

$$

UPDATE...SG
DISCUSSION...08
AVIATION...SG