381
FXUS63 KUNR 312043
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
243 PM MDT Thu Jul 31 2025

.KEY MESSAGES...

- Unsettled conditions at times into early next week

- Isolated strong to severe storms possible at times, including
  this evening across northeast WY and much of the area on Friday

- Areas of smoke from distant wildfires may impact the region at
  times

- Temperatures returning closer to seasonal normal next week

&&

.DISCUSSION...(This Evening Through Thursday)
Issued at 203 PM MDT Thu Jul 31 2025

Surface front this afternoon as parked on the edge of the Rockies,
with broad southeast flow resulting across the Plains. Aloft,
there is broad ridge across the Rockies well into Canada, but we
continue to see weak impulses move through the ridge. One of those
was coming out of CO and moving into southern WY. This wave is the
primary driver for the severe risk, as noted by the Slight Risk
in the Day1 SPC severe weather outlook extending into northeast WY.
These storms are delayed a bit from the previous forecast in
getting as far north as northeast WY, but 2000+ j/kg of CAPE with
around 30 kts of deep shear will be sufficient to support some
organized convection and a severe storm or two, with large hail
and damaging wind gusts being the primary hazard. Overnight, this
upper wave will drift into northwest SD, with a broken line of
showers and storms expected to develop east of the Hills from
western toward central SD.

Friday looks to be a rather busy day on radar, with 3 areas of
convection to watch. First, the shortwave moving into WY now will
become convectively enhanced, becoming more of an MCV (per the NAM
and several of the CAMS). By Friday afternoon, our continued
moist low level airmass will see 2000-3000 j/kg of CAPE develop,
with lift from the MCV combining with the instability to force a
fairly widespread area thunderstorms over central SD. This will
either happen in eastern sections of the UNR forecast area or far
western sections of the ABR area. The second area of forcing for
Friday will be the result of northeast winds coming around the
Hills, with convergence in the northern Hills resulting in
convective initiation in the Spearfish area by the early
afternoon. This would likely become supercellular, with this
activity drifting southeast toward Rapid City into the evening.
The HRRR hints at this idea playing out, so will have to keep a
close eye on this potential given the Sturgis Rally getting going
on Friday as well. The final area of concern for Friday will be
more for Friday evening, as we`ll likely see convection coming off
the higher terrain of CO/WY during the afternoon, that will again
drift across the UNR forecast area Friday night. The SPC kept and
Marginal Risk in the convective outlook for Friday, but the two
biggest concerns we have for severe weather for Friday would be
for the potential for supercells near the Black Hills, with the
CAMS also showing an MCS developing over central South Dakota from
whatever the MCV decides to do.

We would love to see we`ll finally dry out this weekend, with
temperatures warming back up, but that doesn`t look to be the case
right now, with the ridge of many weak impulses remaining. This
is expected to keep the afternoon/evening diurnally driven
activity going, though this will come with highs more commonly making
runs into the 80s outside of the Hills. We still expect
temperatures to continue to warm, with highs getting back closer
to the 90 mark by the middle of the week. Though warmer next week,
the threat for diurnally driven showers will continue, though the
EPS continues to show the precipitation chances quieting down
next week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1114 AM MDT Thu Jul 31 2025

Areas MVFR/IFR conditions due to low clouds (fog over higher
elevations of the Black Hills) will become VFR this afternoon with
the most stubborn IFR ceilings in/near the Black Hills. A band of
shra across western SD will slowly dissipate this afternoon.
Thunderstorms will develop over northeastern WY by early evening,
spreading across western SD overnight with local IFR conditions.
Areas of smoke from western wildfires may lower visibilities the
next few days.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION/KEY MESSAGES...MPG
AVIATION...Helgeson