819
FXUS63 KUNR 180515
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1115 PM 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 through Wednesday
- Low confidence long term forecast with cool weather potentially
  delayed until the weekend

&&

.UPDATE...
Issued at 725 PM MDT Tue Sep 17 2024

At this time, the threat for severe storms has diminished
considerably across northeast WY and the Black Hills area, as
temps have cooled and buoyancy has decreased behind an outflow
boundary from earlier convection that is now located just to the
east of the Black Hills. The main severe threat over the next few
hours now appears to be from southeast of the Black Hills into
portions of the west central SD plains, where warmer temps, along
with better buoyancy and shear, linger ahead of the outflow.
Gusts of 60 to over 70 mph are possible with the stronger storms,
along with some small hail. Further west into the Black Hills
area, far northeast WY, and northwest SD, cannot rule out some
strong gusts to around 50 mph with some of the storms moving north
across the area. Lingering severe potential should diminish
toward late evening as atmosphere stabilizes further and main
shortwave energy starts to move north and east of the area. Not
much change needed to the forecast at this time, but will make
needed adjustments as the evening progresses.

&&

.DISCUSSION...(This Evening Through Tuesday)
Issued at 228 PM MDT Tue Sep 17 2024

The latest analysis places the upper low over western Wyoming,
producing southwesterly flow aloft over the forecast area. A
surface low is centered over SE WY/WRN NEB/NE CO with breezy
southerly winds over western SD. As the surface low moves
northeastward into South Dakota early this evening the surface
pressure gradient will tighten, increasing the winds over south
central South Dakota. A wind advisory remains in effect there into
this evening.

The upper low will lift northeastward as it becomes negatively
tilted this evening. A jet streak approaching the region from the
southwest will deepen the surface low. A band of thunderstorms is
expected to develop over northeastern Wyoming late this afternoon
and accelerate eastward across western and central South Dakota
through this evening.

Initially, buoyancy will be limited with high-based convection
anticipated over WY. CAMS continue to show a developing squall
line pushing into western SD with 0.5-1KJ/kg MUCAPE available this
evening, along with moderate deep-layer shear, which will enhance
thunderstorm strength over western SD. The main severe threat
will be damaging wind gusts, although marginally severe hail could
occur. Activity will quickly exit to the east of the forecast
around 06Z.

Thee tightening surface pressure gradient to the east of the
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. A wind
advisory remains in effect through this evening over south central
SD.

On Wednesday, the stacked low will be located over southeastern
MT resulting in a with tight surface pressure gradient over the
forecast area. May eventually need a wind advisory for parts of
western SD on Wednesday, but will hold off for now to reduce
confusion with the expected strong to severe thunderstorm winds
tonight.

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 varying greatly. 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 06Z TAFS Through 06Z Wednesday Night)
Issued At 1115 PM MDT Tue Sep 17 2024

Showers and thunderstorms will continue to move north-northeast
across northwest to south central SD, exiting the area by around
09z. Gusty winds will continue to accompany stronger storms. VFR
conditions can be expected tonight and Wednesday, along with
breezy/windy south to southwest winds in many areas. Some low
level wind shear can be expected across at least the Black Hills
area late tonight into early Wednesday, including KRAP, with
stronger southwesterly winds just off the surface. Some wrap
around showers are likely on Wednesday from far northeast WY to
northwest SD as upper level system passes north of the region.

&&

.FIRE WEATHER...
Issued At 228 PM MDT Tue Sep 17 2024

Elevated fire weather conditions will continue through Wednesday.
Strong/severe thunderstorms are possible beginning over
northeastern Wyoming this afternoon, reaching the Black Hills
during the late afternoon/early evening and then spreading
eastward across the rest of western and south central South
Dakota during the evening. Gusty southerly winds are expected
ahead of the front, especially over south central South Dakota
where wind gusts up to 50 mph are possible during the late
afternoon/evening. The line of thunderstorms may be accompanied by
strong to severe wind gusts as they move through. There is a good
chance of wetting rain with any thunderstorms.

Elevated fire weather conditions are expected through Wednesday
due to gusty southwest winds. On Wednesday, the MinRH 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.
&&

$$

UPDATE...26
DISCUSSION/KEY MESSAGES...10
AVIATION...26
FIRE WEATHER...10