391
FXUS63 KUNR 211908
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
108 PM MDT Fri Jun 21 2024

.KEY MESSAGES...

- Storms this afternoon and evening.

- The risk of flooding continues across south central SD, with more
rain on the way

- Becoming hot and mostly dry this weekend into next week

&&

.DISCUSSION...(This Evening Through Friday)
Issued at 103 PM MDT Fri Jun 21 2024

At the upper levels, there is high pressure across the southeastern
CONUS and a trough over the west coast, the positioning of these two
features are allowing moisture to advect into the northern
plains. Shortwave energy moving through southwest flow is
contributing to thunderstorms across the Dakotas. At the surface,
a boundary extends through eastern Wyoming into Colorado, with a
warm front extending from the Nebraska Panhandle into southeastern
SD. Southern and western portions of the area are beginning to
break-out of the low clouds/stratus, as temperatures are rising
into these areas. This is already leading to thunderstorm
development.

For the rest of today and tonight, expect scattered showers and
storms to develop as a more potent shortwave moves east northeast
into the area. As the energy combines with the persistent
easterly low-level winds and moisture, will see storm development.
MUCAPE of 1000-2000 J/kg combined with 35-50kt shear, should be
sufficient to promote supercell storm structures capable of hail
and damaging winds. The strong cap should limit the severe
potential across much the SD Plains.

Nonetheless, another round of rain will move across the CWA tonight.
PWs remain 150-200% of normal, and so heavy rain will again be
possible. If additional precipitation falls over south central SD,
mainly Tripp County, additional flooding is expected. HREF chance
of >1" of precip falling over south central over the next 24
hours is 50-70%. Thus, will keep the Flood Watch going there.
Considered expanding to Todd and Mellette, but latest CAMs are
backing off on storm coverage later this evening and overnight.
(Models are pushing the convective activity further east and
south). Therefore, will leave Todd and Mellette out of the Flood
Watch.


Summertime weather is expected 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. Conditions will be mostly dry, although
cannot rule out an isolated thunderstorm each day.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1114 AM MDT Fri Jun 21 2024

Widespread IFR/LIFR conditions will continue across the area
today, with gradual improvement in visibility and ceilings this
afternoon. Scattered showers and thunderstorms will redevelop
this afternoon and evening. IFR/LIFR ceilings will return to much
of western SD after 03-05z tonight, before slow improvement after
12z Saturday morning. VFR conditions expected area-wide after
15-18z Saturday.

&&

.HYDROLOGY...
Issued at 103 PM MDT Fri Jun 21 2024

Keeping flood watch going over Tripp County through tonight. PWs
remain 150-200% of normal, and so heavy rain will again be
possible. If additional precipitation falls over south central SD,
mainly Tripp County, additional flooding is expected. HREF chance
of >1" of precip falling over south central SD over the next 24
hours is 50-70%. Thus, will keep the Flood Watch going there.

Considered expanding to Todd and Mellette, but latest CAMs are
backing off on storm coverage later this evening and overnight as
models are now pushing the convective activity further east and
south). Therefore, will leave Todd and Mellette out of the Flood
Watch.


&&

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

$$

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