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497
FXUS63 KUNR 210337
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
937 PM MDT Thu Jun 20 2024

.KEY MESSAGES...

- Chances for strong to severe thunderstorms through Friday
  despite cool/cloudy/showery conditions.

- Significant precipitation will be possible over south central
  SD through Friday.

- Becoming hot and mostly dry this weekend into next week.

&&

.DISCUSSION...(This Evening Through Thursday)
Issued at 145 PM MDT Thu Jun 20 2024

At the upper levels, a longwave trough is positioned along the
western CONUS with high pressure over the east coast, creating a
southwest flow pattern across the Northern Plains. Water vapor
satellite indicates a few weak waves embedded in the flow being
pushed into the area, with ample moisture being advected into the
region from the Gulf of Mexico and the Pacific. At the surface,
there is an area of low pressure to the southwest, with high
pressure off to the northeast. This is resulting in weak easterly
upslope flow across the SD Plains, locking the low clouds into
place over most of the area.

For the rest of this afternoon and tonight, storms are ongoing
across portions of the area. MUCAPE of 1000-2000 J/kg is in place
and 0-6 km shear is around 40 kts, strengthening low level jet
this evening will all combine to produce strong to severe storms.
Another threat will be heavy rain across south central SD. PWs are
still predicted to be 150-200% of normal, and with 1-2 inches of
additional precipitation this evening after 1-2" this morning. The
6-hr FFG is now 1.5-2.0 inches, so could see some isolated
flooding issues, so have issued a flood watch for Tripp County
through Friday night. Additionally, expecting fog to expand across
much of the CWA.

Cool, moist airmass continues over the region Friday, and
temperatures will be only slightly warmer than today. Buoyancy
will increase across the warmer areas of northeastern WY and
southern SD again, and as a more potent wave approaches Friday
afternoon. Shear will be sufficient for supercells; however,
uncertain how far these storms will be able to progress across
much of our CWA with a staunch cap in place - perhaps again they
will stay mainly across southern SD. PWs will be high again, and
so heavy rain is expected...especially over south central SD. By
12Z Saturday, ensembles` total QPF 50th percentile have 2-2.5" for
south central SD. This exceeds FFG in Tripp County, thus plan to
continue the Flood Watch through 11Z Saturday.

The weather will begin to feel more like summer this weekend, as
a ridge begins to build over the Rockies. Highs will warm back
into the 80s Saturday, near 90 Sunday, and well into the 90s
Monday. Hot weather will continue through the week, and conditions
will be mostly dry, although cannot rule out an isolated
thunderstorm each day.

&&

.AVIATION...
-- Changed Discussion --
(For the 06Z TAFS Through 06Z Friday Night)
Issued At 935 PM MDT Thu Jun 20 2024

Widespread IFR conditions (low ceilings/lower VSBYS due to
fog/drizzle) are expected over western SD into far northeastern
WY overnight. MVFR/local IFR conditions will push into parts of
northeastern WY (including KGCC terminal) late tonight into Friday
morning. VSBYS should improve Friday morning, but ceiling
improvement will slow. Scattered/numerous showers/thunderstorms
will move through at times.
-- End Changed Discussion --

&&

.HYDROLOGY...
Issued at 145 PM MDT Thu Jun 20 2024

Showers and thunderstorms are expected through Friday night. Heavy
rainfall combined with wet antecedent conditions may result in
localized flash flooding across south central South Dakota,
especially Tripp County. PWs are forecast to increase to over 2
standard deviations above normal. Models indicate rainfall rates
of 1 to 1.5 inches/hour. Expected rainfall combined with the 1-3
inches of rain that has already fallen this morning will make
flash flooding possible through Friday night in Tripp County.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Flood Watch through Saturday morning for SDZ049.
WY...None.
&&

$$

DISCUSSION/KEY MESSAGES...MLS
AVIATION...Helgeson
HYDROLOGY...MLS