237
FXUS63 KUNR 131837
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1237 PM MDT Sun Sep 13 2026

.KEY MESSAGES...

- Widespread rainfall still looks likely overnight.

- Becoming windy on Monday, especially across the northwest.

- Fire weather concerns on Monday are the wildcard on Monday
  across the northwest.

&&

.DISCUSSION...(This Evening Through Sunday)
Issued at 1232 PM MDT Sun Sep 13 2026

Latest water vapor imagery shows 500hpa low spinning across
southern WA state with ridging setting up from eastern MT into
eastern CO. Upper flow across the Dakotas has turned more
northwesterly with broad cyclonic flow setting up across the Great
Lakes into New England. Broad high pressure still remains in
control across the southern tier of states. At the surface, low
pressure is setting up across southern WY. Southeast surface winds
ahead of this low are advecting in a lower cloud deck out of NE
into much of the forecast area. Temperatures at 18Z are in the low
to mid 60s.

The forecast through the next 24 hours presents some interesting
challenges. First in a series of shortwaves ejects into the FA
this evening, with strengthening 700hpa theta-e forcing and
moisture advection, as seen with increasing 850hpa dewpoints. This
should lead to increasing coverage of rainfall across the
forecast area beginning later this evening and continuing through
the night. Cant rule out some elevated convection. Hi-res models
are showing the potential for some of these storms to mix down the
strong 700hpa and 850hpa winds as well. The rain will be welcome
for sure. On Monday, the rain will push east of the FA during the
morning hours. The cold front sweeps through with gusty NW winds
found on the backside along with modest CAA. By 18Z Monday models
are showing from 4 to 5 mb/3hrly pressure rises on backside of
front. It appears that winds will become nearly linked during the
afternoon hours, so have opted to hoist a wind advisory for parts
of our northern FA during the day Monday. This also leads to a
tricky fire weather concern. Parts of our NW are at critical fire
danger levels. The unknown is how much rain they may receive
overnight, helping to put a small damper on those critical levels.
Additionally, there might only be a narrow window when RH values
and winds meet up for fire weather headlines. For now, am going to
forego any fire weather headlines and let later shifts evaluate.
Also, given CAA and anticipated cloud cover, lowered high
temperatures on Monday.

After Monday, the rest of the week still appears to be seasonable
temperature wise. Precipitation chances move back into the area
beginning late Tuesday night and generally continue through the
rest of the week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Monday)
Issued At 1119 AM MDT Sun Sep 13 2026

This forecast period presents a more involved scenario. For this
afternoon, conditions will remain gusty and VFR for the area. A
deck of lower clouds has moved in to south-central SD, which looks
to persist through the evening into tonight. MVFR/IFR cloud decks
will impact western SD tonight, lasting into Monday morning well
after sunrise. Also overnight, transient showers will impact
western SD and northeastern WY. Stronger storm cores may be
capable of MVFR/IFR conditions in the form of reduced visibilities
and lower ceilings if they are not already present. Alongside
possible TSRA, a front will move through, shifting winds to
northwesterly. This will impact KGCC around 11Z and KRAP around
16Z.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Wind Advisory from 11 AM to 9 PM MDT Monday for SDZ001-002-
     012>014-073-078.
WY...None.
&&

$$

DISCUSSION...Hintz
AVIATION...Schweigert