452
FXUS63 KUNR 010449
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1049 PM MDT Sat Oct 31 2026

.KEY MESSAGES...

- Areas of fog through Sunday morning with dense fog near the
  MT/WY/SD/ND areas.

- Quiet weather with mild temperatures Sunday.

- Storm system to bring rain, strong northwest winds and light
  snow over the higher Black Hills on Monday.

&&

.UPDATE...
Issued at 925 PM MDT Sat Oct 31 2026

Latest nighttime microphysics RGB had stratus/fog slipping
southwest into far northeastern WY and slowly sinking
south/southeast over northwestern SD as develops. Latest T/Td
spreads and circumstantial evidence on highway webcams suggest
dense fog will likely continue to develop overnight into Sunday
morning over the northwest reaches of the CWA. Have hoisted a
Dense Fog Advisory to account.

Latest guidance still shows upper trough/low ejecting into the
central Plains Monday bringing robust synoptic lift leading to
plenty of precipitation for the CWA. Maybe even some convection
Sunday night. Latest ensemble guidance portrays highest QPF
totals east of the Black Hills with ECMWF EFI 70-90th percentiles
and some shift of tails action. Looks like a pretty good wind
event as surface low deepens with upper low. NBM continues to be
the strongest with other guidance somewhat weaker (EFI 50-70th
percentile wind gusts). Forecast Bufkit soundings similar. Will
continue watching event unfold with at least a high end Wind
Advisory event expected Monday/Monday night.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 1230 PM MDT Sat Oct 31 2026

Analysis (19z or 1 pm MDT): The longwave flow over the CONUS
remains in a trough (west) and ridge (east) pattern with much of
the northern Plains sitting between these two features allowing
for the SE winds to continue across the region and allowing for
our moisture source to be both the monsoon moisture pooled across
the Desert SW and portions of the Gulf. This has aided into the
formation of the foggy conditions we have seen Friday and again
this morning. This trend will continue and we should see patchy
fog again on Sunday morning. Latest VIS Satellite image shows most
of the fog has dissipated across much of western SD and with skies
becoming scattered to clear. With the modest WAA (Warm Air
Advection) into the region temperatures have moderated quickly
over the past several hours. Current forecast high temperatures
today look on track with much of the region seeing temperatures in
the low to mid 60s with pockets of lower 70`s possible.

Sunday: After what could be some patchy areas of fog to start the
day, we can expect another quiet and mild day on Monday with
continued breezy southeasterly winds. Temperatures will again warm
into the 60s across much of the area. The indicator of the shift
in the weather will be the increase in mid to high clouds as we
progress through the day on Sunday. We are tracking a large area
of low pressure that will be slowly moving through the
Intermountain West on Sunday and this will be the system that will
bring unsettled weather to the area on Monday and Tuesday.

Models show a strengthening Colorado Low system moving into
southern SD Monday morning. Some discrepancies remain on the
exact location of low pressure system by overall most models do
show a closed low somewhere in central SD Monday morning. This
system will be bring rain and wind to western SD. Focusing first
on the wind. This low pressure system will continue to deepen as
it moves into SD and become sandwiched between two ridges with one
remaining over the east coast and on developing across the west
coast a belt of strong NW winds develops on the backside of the
low and comes directly over much of western SD. Some of models
that were showing the highest winds have since backed off a bit
and have come into better agreement with most of the ensemble
members with current forecast agreement falling in the 45 to 55
mph range for wind gusts on Monday. If this transpires then Wind
Advisories will be hoisted. With the feature not developed in this
forecast package and location of the low still in question, no
wind highlights will be issued this afternoon.

This system will have decent amount of moisture so we are
confidence for high end precipitation chances across much of the
area on Monday and this should fall in the form of rain for much
of the plains and Black Hill Foothills. The best chances for some
light snow will be in the highest elevations of the Black Hills.
There will be a sharp gradient in precipitation totals which again
will depend on the exact track of the low itself, add in the
expected downslope wind component and this could eat into some of
the totals in these areas. Nonetheless much of area should see
some accumulating rains with the highest totals at this time
being for locations along and east of the river.

In the wake of the main system on Monday, a fast moving system
dives out of ND and pushes through the area quickly on Tuesday.
This will bring some cooler temperatures to the region and keep
those breezy NW winds in place but these won`t be as strong as
what we will with the system on Monday. Beyond Tuesday, the rest
of the week looks to be quiet with nice fall temperatures expected
before we potentially look at another system coming in just in
time for next weekend.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Sunday Night)
Issued At 1044 PM MDT Sat Oct 31 2026

IFR/local LIFR conditions (stratus/fog) near the SD/ND/MT border
and MVFR/IFR conditions (stratus) over south-central SD will
expand into much of northeastern WY and western SD tonight.
IFR/transient LIFR conditions will impact the KRAP and KGCC
terminals overnight into tomorrow morning. While conditions may
begin to slowly and temporarily improve to VFR later tomorrow
morning, rain is expected to move into the region after 18-19z
tomorrow with MVFR CIGs overspreading the area through the
afternoon and evening hours.


&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until noon MDT Sunday for SDZ001-002-012-078.
WY...Dense Fog Advisory until noon MDT Sunday for WYZ054-056-057-060.
&&

$$

UPDATE...Helgeson
DISCUSSION...Woodward
AVIATION...Wong