556
FXUS63 KUNR 241131
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
531 AM MDT Tue Jun 24 2025

.KEY MESSAGES...

- Marginal to slight risk (1-2 out of 5) of severe thunderstorms
  this afternoon/evening
- Unsettled Wednesday with a marginal risk (1 out of 5) of severe
  thunderstorms
- Much warmer Thursday through Saturday afternoon with lesser
  chances for thunderstorms

&&

.DISCUSSION...(Today Through Monday)
Issued at 206 AM MDT Tue Jun 24 2025

07z surface analysis had low pressure over western CO with trough
into western ND. Water vapour loop had mean trough from central
Canada into the southwest CONUS. Main wave spinning over southern
NV with a couple of wiggles heading toward the CWA. Modest low-
level jet (30kts per KUDX VAD wind profile), 800-700mb theta-e
advection, and 100-500J/kg MUCAPE creating isolated/scattered
shra/TS near the WY/MT-SD/ND border early this morning. Expect
this convection to bubble through sunrise, then tail off.
Attention then turns to severe weather potential today.

Today/tonight, NV upper low inches eastward shooting lead
shortwave into WY this afternoon/evening. Surface trough becomes
warm front as weak low develops along it in central WY. Return
flow along warm front pools 1-2KJ/kg SBCAPE by 21z. Convergence
along warm front combined with shortwave should be sufficient to
break cap with 50kts 0-6km bulk shear. Supercells most likely
initial convective mode with large/very large hail possible. CAMs
(per updraft tracks) seem to favor left-moving supercells over
northeastern WY and this is plausible given forecast hodographs.
Per orientation of convection to surface low and warm front
position, wouldn`t be surprised to see enhanced 0-1km shear, so
isolated tornado threat still present, but CAMs sure don`t seem
to emphasize right movers over northeastern WY. Storms should grow
into clusters this evening and track east/northeast overnight.
Significant MLCIN is forecast to be present on the SD plains, so
how far surface-based convection makes it into SD is suspect.
Temperatures today will be seasonal/near guidance.

Wednesday, upper low lifts northeast as it weakens into an upper
trough. Sustained southeasterly flow may thrust "semi-humid"
conditions into far western SD (50-80% chance Td>60F) which will
support modest buoyancy and sufficient shear for isolated
strong/severe thunderstorms. However, forcing
mechanism/organization is uncertain. Temperatures will be
seasonal/near guidance.

Thursday through Saturday afternoon, west/southwest flow aloft
will promote thermal ridge building into the northern plains and a
return to hot weather, especially Friday when MaxT pushes into
the mid 80s to upper 90s. TSRA chances much less, but not zero.
Saturday night into early next week, slightly unsettled weather
develops per northwest flow aloft and a series of weak fronts.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Wednesday Morning)
Issued At 527 AM MDT Tue Jun 24 2025

Areas of MVFR CIGS over northeastern WY/southwestern SD will lift
to VFR this morning. Leftover shra/TS over far northwestern SD
will end this morning. Isolated/scattered strong/severe TSRA will
develop after 20z over northeastern WY, moving into western SD
tonight. Any storm will have local IFR conditions and the
potential for hail and strong/erratic winds. Behind the storms
later tonight, easterly upslope flow will promote IFR/MVFR CIGS
over much of the area.

&&

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

$$

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