904
FXUS63 KUNR 010915
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
315 AM MDT Mon Sep 1 2025

.KEY MESSAGES...

- Dense fog early this morning, especially on the western SD
  plains
- Cold front moves through Tuesday night with a chance of showers
  and thunderstorms
- Another cold front Thursday brings gusty northwest winds

&&

.UPDATE...
Issued at 312 AM MDT Mon Sep 1 2025

09z highway cameras and some surface observations showing dense
fog starting to expand. Given light winds, antecedent wet
conditions, and lack of high clouds, expect trends to continue.
Have hoisted a Dense Fog Advisory for a good chunk of western SD.
Will adjust through the early morning hours as needed.

&&

.DISCUSSION...(Today Through Sunday)
Issued at 202 AM MDT Mon Sep 1 2025

07z surface analysis had west/east-oriented ridge across the CWA
with trough exiting south-central SD to the south, taking the last
vestiges of precipitation with it. Stratus/fog in place over
northeast half of the CWA with a few spots getting thick. Will
continue to monitor the need for Dense Fog headlines. Rest of CWA
clear. Water vapour loop had upper ridge from the southwest CONUS
poking into the northern plains as narrow trough exits the CWA.

Today through Tuesday afternoon, upper/thermal ridge builds into
the CWA. Morning fog will mix out. Drier air, subsidence, and
increasing cap per thermal ridge will preclude mentionable pops.
Temperatures will be near guidance.

Tuesday night/Wednesday, cold front zips through the northern
Plains courtesy of Hudson Bay upper low shifting into the Great
Lakes. Moisture pooling along the front will promote weak buoyancy
in regime characterized by increasing 0-6km bulk shear per
increasing mid-level flow. A couple of stronger TSRA may develop
Tuesday evening over northwestern SD. Cooler temperatures are
foreseen for Wednesday.

Looking ahead, another cold front slips through Thursday/Friday.
There are some signs of gusty winds per deterministic guidance with
NBM (lesser extent LREF) showing a 30-50% chance of wind gusts
>40mph. Nothing to write home about yet for the first week of
meteorological Fall. Rest of the period looks seasonal/mainly dry as
CWA snuggled in between upper trough over eastern north America and
ridge over the western CONUS.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Monday Night)
Issued At 1021 PM MDT Sun Aug 31 2025

Areas of fog are possible aft 01/08z across the plains of western
SD, with isolated MVFR/IFR cigs/vsbys possible. The fog is
expected to lift by 01/14z with VFR conditions remaining in place
through the rest of the valid forecast period.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 9 AM MDT this morning for SDZ001-002-
     012>014-025-030>032-042>044-072-073-077-078.
WY...Dense Fog Advisory until 9 AM MDT this morning for WYZ060.
&&

$$

UPDATE...Helgeson
DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Hintz