-- Discussion containing changed information from previous version are highlighted. --
136
FXUS63 KUNR 221655
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1055 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

&&

.UPDATE...
-- Changed Discussion --
Issued at 1053 AM MDT Wed May 22 2024

Added low PoPs for a slight chance of showers this aftn/eve over
northeastern Wyoming and far western SD as depicted by some of the
CAMs in response to the weak disturbance moving through this
afternoon.
-- End Changed Discussion --

&&

.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 18Z TAFS Through 18Z Thursday)
Issued At 1039 AM MDT Wed May 22 2024

Isolated -shra/-tsra possible over northeastern WY, the Black
Hills, and far western SD this afternoon/evening, with local MVFR
conditions. Otherwise, VFR conditions are expected through the
TAF period. Westerly winds will begin to increase late Thursday
morning as a cold front moves through.

&&

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

$$

UPDATE...10
DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Pojorlie