556
FXUS63 KUNR 202030
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
230 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...(For the 18Z TAFS Through 18Z Friday)
Issued At 1113 AM MDT Thu Jun 20 2024

Widespread MVFR/local IFR ceilings across much of western South
Dakota into portions of NE Wyoming will continue through this
afternoon and evening. Local MVFR/IFR visibility in fog will
persist around the Black Hills. Scattered showers and
thunderstorms are expected today and tonight with local IFR
visibility near any storms. Low ceilings will persist through the
overnight hours, with some gradual improvement after 14-16z
Friday.

&&

.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...SE
HYDROLOGY...MLS