Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 231132
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
532 AM MDT Wed May 23 2018

.DISCUSSION...(Today Through Tuesday)
Issued at 214 AM MDT Wed May 23 2018

08z surface analysis had trough from central MT into southwest SD
through central WY. Weak low along it over southeast MT and
better low over northeast CO. Water vapour showed upper low
spinning over northern UT with southwest flow aloft over the CWA.
Main shortwave action over eastern WY/western NE on nose of 90kt
jet streak. 30-40kt low level jet supporting MCS over SD. Upper
low main concern this forecast.

Today/tonight, upper low will take a stroll into northeast MT with a
decent shortwave trough south into eastern WY. Southeast MT low
strengthens some supporting continued southeast boundary layer flow.
After some early morning ST/BR/leftover SHRA/TS, solar insolation
should be in full swing by late morning with implied subsidence
behind this morning`s shortwave. Atmosphere recovers with 1-2KJ/kg
MLCAPE. MLCIN will hold on a bit later than yesterday, but will
eventually crack across the far southwest later this afternoon per
CAMS/synoptic forcing. 0-6km bulk shear around 20m/s. Expect some
discrete convection late this afternoon over WY/far southwest SD
with the usual hail/wind threat and then it turns into more of a
wind threat as an MCS develops and 0-3km shear profiles bulge into
the evening hours. PWATs high, but storm movement decent, too.
Will have T+ wording in forecast late this afternoon/early this
evening. Temperatures will be near guidance.

Thursday into Saturday, upper ridge builds into the northern Plains
as another upper trough dives into the Rockies. Thermal ridge keeps
trending warmer and thus, temperatures, which will be at least
20F above normal on the SD Plains. Can`t rule out a stray TS
during the period.

Sunday into Tuesday, upper trough/low moves from the Rockies into
the northern Plains bringing a period of active weather with good
chances for TS.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Thursday Morning)
Issued At 526 AM MDT Wed May 23 2018

Rain showers across northwest SD will continue to move north and
clear the forecast area by 14Z. Widespread MVFR to IFR ceilings
are present across the area with expected improvement to VFR by
17Z. Scattered showers and thunderstorms will re-develop across
northeast WY and western SD late this afternoon and evening, with
severe storms likely. Damaging winds gusts are likely from these
storms.

&&

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

$$

DISCUSSION...Helgeson
AVIATION...15



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