Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 221654

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1054 AM MDT Tue May 22 2018

.DISCUSSION...(Today Through Monday)
Issued at 159 AM MDT Tue May 22 2018

07z surface analysis had lee trough from central MT into eastern
CO. Earlier convection died across far southwest SD and convection
just northeast of the Big Horns passing just north of northeast
WY. Water vapour loop had upper low spinning over southern NV, and
this is the main forecast maker in the short term.

Today through Wednesday, upper low will reach northeast MT by 12z
Thursday. Upper ridge over the CWA early this morning shifts east
during the day as upper low pushes northeast. Decent impulse of
energy moves into the CWA for this afternoon/tonight as a 90kt jet
streak moves through. Decent surface response with main low
developing over north central CO and inverted trough merging with
lee trough into central MT. Sustained southeasterly boundary layer
flow will collect at least 1-2KJ/kg MLCAPE by afternoon over the
CWA, particularly over western SD. Initially, 0-6km bulk shear
around 15m/s, but as jet streak arrives tonight and low level jet
develops, shear increases to 15-20m/s. Synoptic lift should overcome
weak MLCIN by early afternoon with CAMS showing rapid increase in
TSRA coverage by 21z. Activity will congeal into an MCS this evening
and shift north through Wednesday morning. Initial activity shows
marginal risk of severe hail/wind, but probably transforming into
more of a wind risk overnight. Initial storm movement will be
fairly slow, but will increase tonight as PWATs reach 150%. Later
shifts may need to assess heavy rain threat, but for right now
will put into outlooks. Discombobulated atmosphere Wednesday
morning will recover by afternoon with good chances for robust
solar insolation given clearing skies. Another day of
strong/isolated severe storms possible given instability/shear.
Temperatures will be near guidance.

Thursday through Saturday, upper/thermal ridge build into the CWA
with late spring/early summer temperatures. TSRA threat appears to
be limited.

Sunday/Monday, another upper low barrels out of the Rockies into the
plains with increasing PoPs warranted. Temperatures will continue to
be quite warm.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1052 AM MDT Tue May 22 2018

Widespread thunderstorms are expected across northeast Wyoming
and western South Dakota late this afternoon through the overnight
hours. IFR conditions are possible with the strongest storms,
along with strong winds and hail. Conditions will improve
Wednesday morning.




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