387
FXUS63 KUNR 230818
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
218 AM MDT Thu May 23 2024

.KEY MESSAGES...

- Compact upper low brings active thunderstorms, wind, and a bit
  of snow later this afternoon through early Friday
- Relatively active pattern expected through Memorial Day weekend
- Upper level ridge brings milder and drier weather mid-week next
  week

&&

.DISCUSSION...(Today Through Wednesday)
Issued at 213 AM MDT Thu May 23 2024

08z surface analysis had low over central WY, which is in
response to upper low per water vapour spinning over far southwest
MT. 35-45kt low level jet (per KUDX VAD/RAP forecast) east of
Black Hills producing modest theta-e advection over ND/eastern SD
and ACCAS/sprinkles. Effect of upper low main forecast concern in
the short term.

Today through Friday, upper low rolls through the CWA into
tonight and then lifts northeast as it opens up into the eastern
Dakotas Friday. WY surface low strengthens/moves northeast as it
pushes dryline/trough into the CWA today and then wraps a cold
front through the CWA late this afternoon/tonight. A narrow tongue
of buoyancy surges into far northern through eastern reaches of
the CWA. HREF forecasts 1-2KJ/kg SBCAPE over south-central SD by
21z with 500J/kg over far northwest SD. 25-35kt 0-6km bulk shear
increases to 30-40kt as jet streak ahead of upper low elongates
hodographs. Should see a narrow line of active convection along
dryline/trough late this afternoon with isolated severe
thunderstorms (damaging wind/large hail), especially over south-
central SD. If discrete supercells develop over south-central SD,
very large hail is possible. Strong/severe storms will quickly
move out of CWA this evening. Well-defined deformation zone
precipitation shield expected over northeast WY/northwest SD
(30-50% chance >0.25" QPF) and little precipitation over southwest
SD per dry slot. Wet bulb zero heights low enough for 1-2" of
snow (25-75th percentile) over the highest Black Hills. 3-7mb/3hr
pressure rises expected behind cold front tonight, peaking late
this evening. Good setup for strong/gusty northwest winds for the
squeeze areas of the Black Hills, so have hoisted high wind
watch/wind advisories to account. Probabilistic guidance seems low
for 50kt gusts from KRAP-KRCA region given bufkit momentum
transfer forecast, but EFI SoT painting strong wind potential
over much of western SD. Day shift can hone the wind speeds with
likely advisory expansion eastward for late tonight. Drier air
ends precipitation Friday afternoon. Temperatures will be near
guidance.

Saturday, a weak disturbance brings a chance of precipitation,
especially over the south. Another upper low/trough moves through
Sunday with likely PoPs warranted given synoptic forcing/moisture
availability. Lingering precipitation early Memorial Day gives way
upper ridge for the middle of next week with drier/milder weather
foreseen.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 934 PM MDT Wed May 22 2024

Westerly winds will begin to increase late Thursday morning as a
cold front moves through. MVFR conds will move into NE WY later
Thur afternoon as rain develops, lower cigs spreading east through
evening. Thunderstorms will be possible toward central SD in the
afternoon.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Wind Advisory from 8 PM this evening to 2 PM MDT Friday for
     SDZ012-013-026.
     High Wind Watch from this evening through Friday afternoon for
     SDZ031-072-073.
WY...None.
&&

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...JC