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242
FXUS63 KUNR 211720
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1120 AM MDT Tue May 21 2024

.KEY MESSAGES...

- Relatively active pattern expected through Memorial Day weekend

&&

.DISCUSSION...(Today Through Monday)
Issued at 244 AM MDT Tue May 21 2024

08z surface analysis had low over central NE with front into
southwest MN and trough into the eastern Dakotas. Water vapour
loop had shortwave over eastern CO with left exit region of 110kt
jet ahead of it and right entrance region of 100kt jet over the
northern Plains. Features conspiring to produce broad band of
showers/isolated TS over southeast reaches of the CWA coincident
with best lower troposphere frontogenesis. Another upper trough
over MT/ID. Disposition of these features main short term forecast
concern.

Today/tonight, main shortwave ejects into the upper Midwest
deepening surface low as it races to the MN arrowhead and rapidly
deepens. Main synoptic lift pushes east of the CWA this morning.
Low level boundary layer flow becomes north/northwest today, and
then west tonight, which will entrain drier air into the CWA
marking a sharp cutoff in precipitation over northwest SD. Further
south, weak buoyancy develops as weak shortwave slides southeast
across southwest reaches of the CWA. HREF supports this scenario
by pushing main synoptic precipitation east of CWA this morning
and best chance of >0.10" QPF over southwest third of CWA this
afternoon/early evening per showers/isolated TS. Have tried to
paint these thoughts in PoPs with a reduction in the north and
diurnal maximum near the Black Hills/shortwave this
afternoon/early evening. Temperatures this afternoon will be near
guidance in the north and slightly below in the south per higher
afternoon pops. Westerly boundary layer flow should scour
moisture out tonight with clearing skies and the potential for
patchy frost over northeast WY (30-50% chance per HREF).

Wednesday, shortwave ridge provides a pleasant day for most folks
with perhaps a stray shower over the Black Hills. Next upper low
drops from the northern Rockies into the northern Plains for
Thursday. Robust theta-e advection on low level jet may bring
ACCAS late Wednesday night, but main effect arrives Thursday with
modest moisture return/sufficient shear for a marginal risk of
severe storms over south-central SD late Thursday.

Friday through Memorial Day weekend looks unsettled. After a cool
start Friday, seasonal temperatures expected with (mostly)
diurnally driven chances for showers/storms. Given eventual
changes in timing/location for each disturbance, did not stray
from guidance.

&&

.AVIATION...
-- Changed Discussion --
(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1119 AM MDT Tue May 21 2024

Scattered showers and isolated thunderstorms are expected through
today with MVFR/local IFR ceilings and visibility. Conditions will
trend toward VFR across the forecast area after 22-00z tonight.
-- End Changed Discussion --

&&

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

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...SE