999
FXUS63 KUNR 271639
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1039 AM MDT Thu Jun 27 2024

.KEY MESSAGES...

-Strong upper trough will support showers and thunderstorm
 chances today and this evening, with some storms possibly severe.

-Cooler and drier for Friday and Saturday, then warming up for
 Sunday and early next week with chances for storms at times.

&&

.DISCUSSION...(Today Through Wednesday)
Issued at 136 AM MDT Thu Jun 27 2024

Semi-active flow pattern will continue through most of the period
as dominant northern stream fast flow continues. Deeper version
of these troughs will advect through the region today, supporting
increased chances for storms as well as warm conds. Pos theta-e
adv ongoing across the region tonight, with scattered storms
toward central SD per the LLJ. Activity will linger in central SD
through much of the morning, with stronger storms capable of
producing hail. Main show will be later this afternoon when the
system associated jet streak advects into the region and supports
increasing LSA. Sfc dryline will be in place in the western FA
with storms likely initiation INVOF this feature in addition to
forcing from the higher terrain of NE WY and possibly the BH if
the cap weakens enough. However, there looks to be a disconnect of
the deepest moisture and convection at first, with the deeper
moisture being shunted east toward central SD. Hence, storms that
initiate on the prefrontal trough will likely be high based and
may stay elevated above the capping inversion, esp over SW SD.
However, if storms can root in the BL further east, ample shear
(greater than 40 knots) and ample CAPE (greater than 2500 J/KG)
would be more than sufficient for rotating updrafts. Again, this
is very conditional. The best chance for severe storms will likely
be across northern NE WY and esp far NW SD where dynamic forcing
will allow storms to root into moisture rich BL, supporting a
liner convective system (with potential embedded supercells)
capable of large hail and strong winds, with more
isolated/scattered activity to the south. Storms will quickly exit
the region this evening with perhaps a few lingering cells after
midnight in central SD. Much drier and cooler conds expected
Friday. A secondary impulse and cool front will move through the
region in the afternoon, supporting chances for storms mainly in
the afternoon from the BH SE. Although CAPE will be limited at
less than 1000 J/kg, copious amounts of bulk shear (greater than
50 knots) would support isolated supercells in far SW SD. Things
cool off even more Sat, with highs in the 70s most places and dry
weather. Moisture begins to return Sunday with warmer temps. SE
upslope flow with increasing ll moisture may support a good setup
for storms on the BH, some possibly severe given moderate bulk
shear in place. This will all hinge on the degree of moisture
return. Semi-unsettled flow with near daily chances for storms can
then be expected next week with temps in the 80s and 90s.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1034 AM MDT Thu Jun 27 2024

Strong/severe thunderstorms will develop over northeastern
WY/northwestern SD after 20-22z today and spread east, some of
which will contain hail and gusty erratic winds as well as IFR
conditions. Otherwise, VFR conditions are expected.

A period of gusty, northwest winds will develop after 04-05z
tonight, especially over the plains just to the east of the Black
Hills where gusts of up to 35kt are possible. Winds will diminish
by 12z.

&&

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

$$

DISCUSSION/KEY MESSAGES...JC
AVIATION...Wong