339
FXUS63 KUNR 152334
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
534 PM MDT Sat Jun 15 2024

.KEY MESSAGES...

- Strong to severe storms are expected to develop by mid-afternoon,
crossing western SD through early evening.

- Gusty northwest winds are expected behind a cold front tonight,
with advisory criteria possible for a short time.

- Nocturnal storms will be possible Sunday night, some of which
could be severe.

- Another round of strong to severe storms will be possible
Monday, mainly across central SD.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 1226 PM MDT Sat Jun 15 2024

Upper level analysis shows broad ridging over much of
the CONUS, with a trough pushing into the Pac NW. Weak shortwaves
are moving through the trough, and the one responsible for this
afternoon`s thunderstorms is moving through WY. Its surface
trough is pushing into the western Dakotas, evident by a line of
showers/storms. Southerly winds ahead of the trough are advecting
warm, moist air into the CWA: temps are warming into the 80s, and
dew points across western/central SD are in the 50s and 60s.

By mid-afternoon, MLCAPE is expected to reach 1500-2500 J/kg across
the western SD plains. Upper shortwave will begin to slide through
our CWA, with the best upper divergence across NW SD. Supercells
will be supported by 0-6 km shear increasing from ~30 kts early
afternoon to ~45 kts early this evening. Large hail and damaging
winds will be possible, and a tornado or two cannot be ruled out,
but high LCLs will be a limiting factor. Storms will dissipate
and move out of our CWA by mid to late evening.

The cold front will slide through the western Dakotas overnight,
behind which pressure rises of 3-4 mb/3 hr would support strong
northwesterly winds. With sustained winds of 20-30 mph and gusts of
40-50 mph possible, advisory criteria may be met; however, forecast
soundings indicate this would occur for only a couple of hours.
Therefore, will forgo advisory issuance.

Breezy winds Sunday morning will diminish through the day.
Temperatures will be noticeably cooler behind the cold front, with
highs in the 70s to lower 80s, warmest across southern SD.

Upper flow becomes southwesterly late Monday, as another low
pressure system moves into the Pac NW. Weak shortwave energy
emanating from this trough will cross the northern plains Sunday
night. With increasing 850 mb theta-E advection across southern SD
and strong shear, late night thunderstorms (some severe) are
expected to develop along a boundary between low pressure to the
south and high pressure to the north. PWATs of 140-160% of normal
will support heavy rain as well.

Cool high pressure will sink into SD Monday. With breezy easterly
winds, highs will be only in the 60s most places. The exception will
be south central SD: with highs in the 70s and dew points in the 60s
again, decent MLCAPE will develop by mid-afternoon. As the Pac NW
trough digs into the Rockies Monday afternoon, shortwave energy will
once again move over the CWA. As a surface trough slides through the
northern plains, showers/storms will develop. Showers are expected
over the cooler parts of the CWA, but strong to severe storms will
be possible over central SD, where shear and buoyancy will be
supportive.

Below average temperatures will persist through Wednesday. Mostly
dry weather is expected Tues/Wed, although isolated storms cannot be
ruled out. Chances for storms increase towards the end of the week,
when temps warm back near average (highs in the mid-70s to near 80).

&&

.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 531 PM MDT Sat Jun 15 2024

Isolated/scattered thunderstorms will push across western SD
through this evening, ending by midnight. Gusty, erratic winds
and brief MVFR conditions possible with stronger storms. A fast
moving cold front will cross the area after 03z, bringing
localized northwest wind gusts around 40kts, especially just
east/northeast of the Black Hills. Winds will gradually diminish
by Sunday morning. Widespread VFR conditions expected Sunday.

&&

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

$$

DISCUSSION/KEY MESSAGES...Pojorlie
AVIATION...Johnson