451
FXUS63 KUNR 292027 CCA
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
137 PM MDT Wed May 29 2024

.KEY MESSAGES...

- Thunderstorms this afternoon and evening; Severe storms are
  expected with damaging wind and large hail the primary hazards.

- Active pattern continues through the weekend with daily chances
  of showers and storms.

- Trending warmer by Saturday, with seasonable to above average
  temperatures expected next week.

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 134 PM MDT Wed May 29 2024

Upper level analysis shows upper ridge sliding eastward into the
Upper Midwest with upper trough axis centered through central
Idaho. Water vapor imagery shows shortwave pushing into eastern
Montana with ascent notable on IR/WV beginning to overspread
Montana and Wyoming. At the surface, diffuse low pressure/surface
trough extends from northeastern Wyoming southward into eastern
Colorado with a cold front extending from central Montana into
western Wyoming. A band of convection has formed east of the
Bighorns and has pushed into Campbell County.

Additional thunderstorms are expected to form across northeastern
Wyoming and western South Dakota over the next 1-3 hours, spreading
eastward through the afternoon and evening, some of which are
expected to be strong to severe. Ample instability is in place
(MUCAPE ~1000-2000 J/kg) with steep lapse rates (~8-9C/km) atop a
well-mixed boundary layer. Given the dry boundary layer, MLCAPE is
muted (~500-1250 J/kg) and MLCIN has been slow to erode across
western SD. Expect the remain MLCIN to weaken over the next 1-2 hours
with continued heating and increasing upper ascent. Additionally,
the dry/well-mixed BL has resulted in DCAPE ~1200 J/kg (per SPC
mesoanalysis), which will persist through the evening hours. Forecast
soundings show rather meager bulk shear (EBWD ~25-30 knots);
hodographs show ample curvature and shear within the lowest 3 km,
although modest winds aloft will limit the overall shear available.
Storm mode is expected to be multi-cellular with transient
supercellular characteristics. Given these ingredients coupled with
anticipated storm mode, expect mainly a damaging wind threat through
the evening. Regarding hail potential - modest shear will limit storm
organization, although sufficient instability exists for large hail.
Additionally, forecast soundings show relative lack of CAPE below the
freezing layer, which will result in increased residence time for
any parcel within the hail growth zone - this will help mitigate the
lack of shear throughout the cloud bearing layer. Ultimately, expect
the best hail threat will be limited to any stronger, longer lasting
updraft cores.

Deterministic and ensemble guidance both depict fairly active, zonal
flow through at least mid-week next week. This would result in near
daily chances of showers and thunderstorms the rest of this week
into next week.

Cooler temperatures will prevail tomorrow and Friday behind
the surface cold front before more zonal/southwesterly flow resumes
for next week and temperatures warm. Temperatures across the
higher elevations of the Black Hills are likely to fall into the
mid 30s tonight and Thursday night; some snow may mix in with rain
across the higher elevations of the Black Hills Thursday
night/Friday morning, although no accumulation is expected.
Ensembles and cluster analysis highlight broad ridging building
across the western half of the country by mid-week next week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1043 AM MDT Wed May 29 2024

Breezy southeasterly winds will continue across the western SD
plains this afternoon. Scattered thunderstorms are expected to
develop across northeastern WY early this afternoon, spreading
eastward into western SD later this afternoon and early evening
with the surface trough. At this time expecting storms to develop
just east of KGCC, but moving into KRAP around 00Z. Another round
of showers/storms will develop later this evening with the cold
front, which could affect both KGCC and KRAP. MVFR/IFR conditions
with gusty, erratic winds and hail may be encountered in and
around any storms. Additionally, some brief LLWS will be possible
across south central SD tonight as the trough moves through.
Behind the cold front, winds will become northwesterly and
somewhat breezy. Showers/storms will dissipate early Thursday
morning.

&&

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

$$

DISCUSSION/KEY MESSAGES...SE
AVIATION...Pojorlie