902
FXUS63 KUNR 262050
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
250 PM MDT Wed Jun 26 2024

.KEY MESSAGES...

- Increasing chances for showers and thunderstorms late this afternoon
  and tonight, with a marginal risk for severe storms across far
  southwest South Dakota late this afternoon and early evening.

- Enhanced risk (level 3 of 5) for severe storms across much of
  the forecast area Thursday afternoon and evening as a cold
  front and upper level system approach the region.

- Cooler and drier for Friday and Saturday, then warming up for
  Sunday and early next week.

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 250 PM MDT Wed Jun 26 2024

Current surface analysis shows nearly stationary frontal boundary
from northern OK to central CO and southwest WY. High pressure is
centered to the north of the region. Upper level analysis shows
ridging over the Rockies, with high pressure over NM. Upper trough
is located along the Pacific NW coast. Embedded energy within the
ridge is noted on water vapor imagery over WY, with showers and
thunderstorms developing over the higher terrain of southern WY
and northern CO. A few showers are also trying to move into
northeast WY from the west. Skies are partly to mostly sunny, with
the most clouds over northeast WY into southwest SD. Temps are
mostly in the mid 70s to mid 80s.

As energy moves through the ridge and over the area late today and
tonight, chances for showers and thunderstorms will increase across
much of the area, especially southwest into south central SD. MUCAPE
values to around 500 j/kg are in place across southern parts of
northeast WY to far southwest SD this afternoon with moderate shear.
There is potential for isolated strong to marginally severe storms
to clip these areas, especially across Fall River and Oglala Lakota
counties, though the better potential should stay south of the area
across southeast WY and western NE. Elsewhere, isolated to scatted
showers and some thunderstorms will overspread a good portion of the
forecast area through the evening hours. Best chances for beneficial
rain will generally be across far southwest into south central SD. A
southerly low level jet will strengthen later this evening and
overnight over western into central SD, helping to keep showers and
some storms going overnight into early Thursday, especially toward
central SD.

Some lingering showers are likely over far eastern areas Thursday
morning as the low level jet starts to move east of the forecast
area. Upper trough will move over the northern Rockies Thursday as
ridge shifts east of the region. Frontal boundary to the southwest
of the region will push northeast into the area as a warm front late
tonight and Thursday. A cold front will track across the Rockies on
Thursday, approaching the area late in the day, with low pressure
moving from western MT to southeastern MT. A pre-frontal
trough/dryline is likely to develop ahead of the cold front and move
across northeast WY in the afternoon. Breezy southerly winds to the
east of the trough will bring increasing moisture, with 60s to near
70 dewpoints on the SD plains and MLCAPE values of 1000 to over 2000
j/kg from the Black Hills area to central SD. Shear is progged to
increase some during the day, but is still fairly modest in most
areas, strongest across northwest SD into ND. Thunderstorms will
start to develop by mid afternoon across western and northern areas.
Supercell potential looks to be most favorable from far northeast WY
to northwest SD, but they could develop anywhere across most of the
area. The pre-frontal trough could be the focal point for more
organized storms as it crosses northeast WY into the Black Hills
area later in the afternoon. Cold front should gradually catch up to
the trough as it crosses western SD during the evening. Enhanced
risk for severe storms (15 to over 30 percent chance) covers much of
the forecast area for Thursday PM, with large hail and strong wind
gusts the primary threats. Damaging wind gusts may become the bigger
threat as time goes on late Thursday into Thursday evening. An
isolated tornado or two is a possibility as well with various
boundaries moving across the area and elevated low level SRH across
western SD. Main activity should exit east of the area mid to late
evening.

Cooler air will gradually push into the area later Thursday night
and Friday behind the front, with brisk northwesterly winds
developing. As upper low passes north of the area and trough swings
through the northern Plains on Friday, some showers and
thunderstorms are possible, mostly in the afternoon and evening. A
secondary cold front passes late Friday, bringing gusty winds and
even cooler air for Saturday. Saturday looks to be a dry day with
highs in the upper 60s and 70s. Warmer air quickly returns for the
second half of the weekend and early next week, with above average
temps in most areas. With the warmer temps, near daily chances for
showers and storms will likely return for the rest of the extended
period.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1125 AM MDT Wed Jun 26 2024

Isolated/scattered showers/thunderstorms are expected this
afternoon through tonight with brief local MVFR conditions
possible, mainly across northeastern WY and southern SD. Early
Thursday morning, some MVFR ceilings may develop across central
SD. Otherwise, VFR conditions are expected. LLWS may develop
across southern SD after midnight.

&&

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

$$

DISCUSSION/KEY MESSAGES...26
AVIATION...Pojorlie