-- Discussion containing changed information from previous version are highlighted. --
375
FXUS63 KUNR 222258
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
458 PM MDT Wed May 22 2024

.KEY MESSAGES...

- Compact upper low brings storms, wind, bit of snow later
  Thursday into Friday
- Relatively active pattern expected through Memorial Day weekend
- Upper level ridge brings milder and drier weather mid-week next
  week

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 144 PM MDT Wed May 22 2024

20Z analysis shows upper lows over western Ontario/nrn MN and also
the Pacific Northwest, with a weaker upper low over srn
Saskatchewan. Between those upper lows is a weakly defined ridge
over ND/SD/NEB. A shortwave approaching the forecast area from the
west is combining with diurnal heating to produce a few light
showers over northeastern Wyoming and far western South Dakota.

For tonight the PacNW upper low will move into northwestern
Wyoming, leading to the development of a surface low over eastern
Wyoming. A low level jet will develop over the plains of western
South Dakota, expected to produce windy southerly surface winds
over south central South Dakota overnight.

Thursday into Friday, the low pressure system will strengthen as
is moves northeastward across the forecast area with the surface
low moving into northwestern MN by Friday morning. On Thursday, a
narrow tongue of 1-1.5KJ/kg SBCAPE will develop as moisture become
concentrated to the east of a dry-line over central SD with
25-35kt 0-6km bulk shear.

A few TSRA should develop over western SD, and possibly parts of
NE WY on Thursday afternoon. The best chance of strong to severe
thunderstorms will be over south-central South Dakota where the
instability will be greatest to the east of the dry-line. Any
thunderstorms that do develop are expected to move off to the east
of the forecast area by early Thursday evening.

On the west side of the upper-low, deformation zone forcing will
be robust, with moisture wrapping back into far northeastern
WY/northwestern SD where there is a 50-75% chance of >0.25" QPF.

Forecast soundings indicate that temperatures aloft will be cold
enough for some accumulating snow over the highest Black Hills
(10-30% chance 1-2"). As colder air surges into forecast area on
Thursday night, 3-5mb/3hr pressure rises would support headline
winds with 40-70% chance 40kt+ wind gusts on the western SD
plains, particularly just east of the Black Hills. Have trended
forecast winds upward toward NBM50/90th percentile to account.
Drier air will end any precipitation Friday afternoon.

Saturday, a weak disturbance supports low PoPs with chances for
showers and T-storms. Another upper low drops in for Sunday with
increased chances for thunderstorms, lingering into Memorial Day.
An upper ridge looks to bring milder/drier weather into the middle
of next week.

&&

.AVIATION...
-- Changed Discussion --
(For the 00Z TAFS Through 00Z Thursday Evening)
Issued At 455 PM MDT Wed May 22 2024

Isolated -shra expected over northeastern WY and far western SD
this evening. Westerly winds will begin to increase late Thursday
morning as a cold front moves through. MVFR conds will move into
NE WY Thur afternoon as rain develops.
-- End Changed Discussion --

&&

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

$$

DISCUSSION/KEY MESSAGES...10
AVIATION...JC