532
FXUS63 KUNR 171737
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1137 AM MDT Tue Sep 17 2024

.KEY MESSAGES...

- Dynamic negatively-tilted upper trough brings slight risk of
  severe weather later this afternoon/evening (mostly wind)
- Elevated fire weather conditions continue today, but now extend
  into Wednesday
- 50mph wind gusts for south-central SD late this afternoon/evening
- Low confidence long term forecast with cool weather potentially
  delayed until the weekend

&&

.DISCUSSION...(Today Through Monday)
Issued at 229 AM MDT Tue Sep 17 2024

08z surface analysis had low over southeast MT. Mostly clear
skies per nighttime RGB save for some mid-high clouds over
northwest SD. Water vapour loop had upper low vigorously spinning
over NV, which will be our main short term weather concern.

Today/tonight, upper low lifts northeast as it becomes negatively
tilted. Robust lift on nose of 100kt jet streak will deepen
surface low over southeast MT. Trailing front accelerates from WY
into western SD late this afternoon/this evening. Initially,
buoyancy will be limited with high-based convection over WY. CAMS
show developing squall line pushing into western SD with
0.5-1KJ/kg MUCAPE available this evening. Environment
characterized by moderate deep-layer shear, which will enhance
thunderstorm strength. Main threat will be damaging wind gusts,
although marginally severe hail could occur. Activity will
quickly exit the CWA by 06z. Gradient ahead of surface low will
promote gusty southerly winds ahead of the front, particularly
over south-central SD where chance of 45+ mph wind gusts 50%+,
supported by Bufkit momentum transfer profiles. Have hoisted Wind
Advisory for late afternoon/evening to account. Low confidence
for wind speeds behind the front later tonight, but there is
potential for near advisory westerly winds. Temperatures will be
near guidance.

Wednesday, stacked low spins over southeast MT with tight
pressure gradient over the CWA. Potential exists for headlineable
wind speeds per some deterministic guidance. However,
probabilistic guidance portrays low chances for 45+ mph wind
gusts. Will have to watch closely given subjective pattern
recognition. Backwash moisture lends credence to low pops along
the WY/MT/SD/ND border areas. Temperatures will be near guidance.

Thursday through next weekend, west/northwest flow aloft expected
with periodic shortwave troughs bringing PoPs. Confidence is low
with details as probabilistic guidance swinging around (not
surprising given Fall right around the corner) over the last few
days. For now, the major cooldown that was thought possible has
been pushed back to the weekend. The chance of >0.25" QPF during
any 24-hour period in the long term is less than 30%.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1137 AM MDT Tue Sep 17 2024

VFR conditions will continue through this afternoon with breezy
south-southwest winds. A fast moving band of showers and
thunderstorms will move from southwest to northeast across
northeastern Wyoming and western South Dakota Dakota late this
afternoon and evening. It will reach the southern part of
northeastern Wyoming and far southwestern South Dakota 22Z-00Z
and exit northwestern/south central South Dakota around 06-08Z.
IFR/LIFR conditions along with wind gusts of 40-50kts can be
expected with the stronger segments of thunderstorms. A few gusts
to around 60kts are also possible. VFR conditions will return
after the thunderstorms pass and continue through Wednesday
morning. Low level wind shear is possible across the western SD
plains late tonight. Then gusty southeast winds are expected after
sunrise through Wednesday morning.

&&

.FIRE WEATHER...
Issued At 229 AM MDT Tue Sep 17 2024

Elevated fire weather conditions will persist into Wednesday.
MinRH today will be 17-30%, but gusty southerly winds are expected
ahead of a front. Strong/severe thunderstorms are possible with
the front late this afternoon/this evening. Then, gusty southwest
winds are expected behind the front, persisting into Wednesday.
MinRH Wednesday will average 20-30%, but areas of 15-20% are likely
over southern SD. The cooldown for the end of the week looks to
show up for the weekend. Thus, Thursday and Friday will be warmer
and drier than previously thought.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Wind Advisory until midnight MDT /1 AM CDT/ tonight for SDZ044-
     046-047-049-077.
WY...None.
&&

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...13
FIRE WEATHER...Helgeson