664
FXUS63 KUNR 171123
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
523 AM MDT Thu Jul 17 2025

.KEY MESSAGES...

- Isolated strong/severe thunderstorms may occur through early
  next week as a series of disturbances traverse the northern
  Plains per zonal flow
- Warming trend expected into early next week with the warmest
  days Sunday through Tuesday

&&

.DISCUSSION...(Today Through Wednesday)
Issued at 200 AM MDT Thu Jul 17 2025

07z surface analysis had high pressure over the eastern Dakotas
with return flow over the western two-thirds of the CWA per
1-3mb/3hr pressure falls across MT/WY. This upslope flow helping
to maintain stratus deck over all but the northeast through east
edges of the CWA. Some fog around, especially over the Black Hills
where stratus intersects the higher terrain. Water vapour loop
had zonal flow across the northern tier states with next upstream
wiggle over southwest Canada. Concern this forecast revolves
around PoPs.

Today/tonight, southeast return flow continues which will slow
the erosion of stratus, but once the sun angle gets high enough,
deck should break up quickly. This delay in heating and recent QPF
supports keeping today`s MaxT a few degrees below guidance over
the northeast three-quarters of the CWA. Southwest Canada
disturbance will slide east-southeast assisting a weak low over
WY along lee trough. Sufficient buoyancy (750-1500J/kg mean
MUCAPE) and shear (40-50kt 0-6km bulk shear) develop by evening
creating isolated strong/severe thunderstorms if cap is overcome.
Most likely location would be near the MT/SD/ND borders where SPC
HREF suggests a couple of updraft/helicity tracks may occur. PoPs
focused on those locations tonight with SPC marginal risk (1 out
of 5) in place. 00z CSU MLP probabilities a bit higher.

Friday, WY surface low slides into northern NE with trough into
south-central SD focusing development of 2-3KJ/kg MUCAPE/30-40kt
0-6km bulk shear. CAP will be an issue again, but suspect trough
will be the focus for convective initiation with little defined
upper support. CAMs suggest this could be from KIEN-KICR late in
the day with large hail/damaging winds per SPC marginal risk (1
out of 5) in place. Temperatures Friday will be near guidance.

Saturday through early next week, not much change expected as
zonal flow continues, disturbances move through, frontal
boundaries oscillate, and thermal ridge expands into the northern
Plains. Temperatures will pop into the 90s Sunday through Tuesday
on the plains. Pretty typical second half of July pattern which
will support isolated strong/severe thunderstorms at times (SPC
Day 3 has marginal risk).

&&

.AVIATION...(For the 12Z TAFS Through 12Z Friday Morning)
Issued At 521 AM MDT Thu Jul 17 2025

Widespread MVFR/IFR (local LIFR over Black Hills) due to low
clouds, patchy drizzle, and fog will break up this morning leading
to gradual improvement to VFR conditions most places by 21z.
Isolated TSRA are possible tonight with the best chance of
strong/isolated severe thunderstorms (local IFR conditions) near
the MT/SD/ND border area.

&&

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

$$

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