472
FXUS63 KUNR 102317
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
517 PM MDT Wed Jul 10 2024

.KEY MESSAGES...

- Becoming hotter with heat headlines possible through the weekend
- Limited precipitation chances, but marginal risk isolated
  severe thunderstorms later Thursday
- Elevated fire weather conditions

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 210 PM MDT Wed Jul 10 2024

20Z analysis shows upper/thermal ridge building toward the
forecast area from the west. At the surface, high pressure is
located over the central/eastern Dakotas with a surface trough to
the west of the forecast area over MT/WY. The resulting surface
pressure gradient will lead to increasing southerly flow with a
low level jet developing along the WY/SD border overnight.
Downslope flow flow may keep low temperatures noticeably warmer in
the northern foothills zones tonight. Also, with the LLJ
increasing mid-level moisture return, may see a couple of light
showers/virga over parts of NE WY/far wrn SD overnight.

On Thursday, most of the region will see temperatures well into
the 90s with a few locations in SW SD possibly reaching 100F. The
surface trough will cross northeastern Wyoming on Thursday,
reaching the Black Hills area around the time of peak heating on
Thursday afternoon. With southeasterly upslope winds to the east
of the trough increasing low level moisture, may see isolated
thunderstorms develop along the convergence zone over the Black
Hills, with convection initiating as early as 200 PM. With SBCAPE
around 1000J/kg to the east of the surface trough and 0-6km bulk
shear of 25-35kts any thunderstorms that do develop will have the
potential to become severe. Due to the warm temperatures and
expected high bases, the main severe threat will be damaging wind
gusts, although large hail will also be possible, especially over
the Black Hills.

A south/southwest low level jet will develop over western South
Dakota on Thursday night, bringing a slight chance of
nocturnal thunderstorms to parts of southwestern and south central
South Dakota, with relatively warm overnight temperatures in the
downslope areas along the eastern slopes of the Black Hills.

For Friday through the weekend, due to the influence of the
upper/thermal ridge, high temperatures will approach or exceed
100F degrees in many locations. Due to the heat, stray thunderstorms
will be possible each day, especially over the Black Hills. A heat
Advisory may be needed for parts of the forecast area.

From Sunday night into early next week, the upper ridge will
flatten, leading to only slightly cooler weather next week with
near daily chances for very isolated thunderstorms.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Thursday Evening)
Issued At 509 PM MDT Wed Jul 10 2024

This afternoon, isolated thunderstorms have formed off the
Bighorns and have slowly moved eastward, but should remain well
west- southwest of KGCC. A few isolated thunderstorms are possible
again after 17-18z Thursday across the Black Hills, although
confidence is low re: impact to KRAP and have kept thunder
mention out of the TAF. Otherwise, VFR conditions will persist
through the forecast period.

A strong low- level jet will develop this evening, primarily
across western SD, with winds above ~300-400 feet AGL increasing
to 15-25 knots out of the southeast. Thursday, a trough axis will
push through NE WY, shifting winds out of the NW at KGCC by
15-16z. Expect southerly winds at KRAP through the forecast
period, although some WSW component is possible during peak
heating Thursday.

&&

.FIRE WEATHER...
Issued At 210 PM MDT Wed Jul 10 2024

Elevated fire weather conditions are expected through the
weekend. Minimum afternoon relative humidities will dip in the
teens and 20s with the lowest readings over northeastern WY/far
western SD (10-15% likely). Wind speeds and gusts will generally
be less than 25 MPH, precluding fire weather headlines, but later
forecasts will need to reassess. Nighttime relative humidity
recovery will be poor over the higher terrain of northeastern
WY/Black Hills.

&&

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

$$

DISCUSSION/KEY MESSAGES...10
AVIATION...SE
FIRE WEATHER...10