378
FXUS63 KUNR 221040
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
440 AM 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...(Today Through Tuesday)
Issued at 223 AM MDT Wed May 22 2024

08z surface analysis had deep low over the MN arrowhead with weak
high over WY. Water vapour had shortwave over KVTN with leftover
deformation zone showers finishing up from KIEN-KICR. Drier air
filtering into the CWA per westerly low level flow. Next weather
maker over southwest BC per water vapour loop.

Today, weak shortwave ridge drifts through bringing drier/milder
air to the CWA. Residual moisture may allow isolated shra/TS over
the Black Hills during peak insolation. Temperatures will be near
guidance.

Tonight, BC upper low moves into northwest WY spinning up surface
low over eastern WY. Low level jet develops over the western SD
plains with modest theta-e advection and low/unmentionable pops
on the plains. Low will be near guidance.

Thursday into Friday, compact upper low strengthens as it moves
across the CWA. Surface low elongates and deepens as it splits
along surging dry-line. Narrow tongue of 1-1.5KJ/kg SBCAPE pools
ahead of dry-line over central SD with 25-35kt 0-6km bulk shear.
TSRA should develop east of the Black Hills later afternoon and
exit the CWA by early evening. Isolated severe thunderstorms
possible, with CAMS targeting south-central SD. On back side of
upper-low deformation zone forcing robust with moisture wrapping
back into northeastern WY/northwestern SD where there is a 50-75%
chance of >0.25" QPF. Forecast soundings cold enough for some snow
over the highest Black Hills (10-30% chance 1-2"). As colder air
surges into CWA Thursday night, 3-5mb/3hr pressure rises possible
which would support headline winds(40-70% chance 40kt+ wind gusts
on western SD plains), particularly just east of the Black Hills
(50%+ chance 48kt+ wind gusts). EFI guidance supports. Have
trended forecast toward NBM50/90th percentile to account. Drier
air will end precipitation Friday afternoon. Temperatures will be
near guidance.

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

&&

.AVIATION...(For the 12Z TAFS Through 12Z Thursday Morning)
Issued At 438 AM MDT Wed May 22 2024

Isolated -shra/-tsra possible over the Black Hills this afternoon
with local MVFR conditions. Otherwise, VFR conditions are expected
through the TAF period.

&&

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

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...7