Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 232327

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
527 PM MDT Tue Jul 23 2019

.DISCUSSION...(This Evening Through Tuesday)
Issued at 139 PM MDT Tue Jul 23 2019

The latest GOES-16 satellite images and RAP analyses indicated a
pronounced upper ridge over the Rockies with mostly sunny skies
across the CWA. Some cumulus were forming over the BLKHLS in the
1000 J/kg MLCAPE air mass, but the -100 J/kg MLCIN and lack of
synoptic forcing were keeping things in check. A few storms were
trying to form over the Big Horn Mountains, but these were not
making it out over the plains. Thus, think NERN WY should stay
free of storms today given marginal lapse rates and moisture. A
southerly low-level jet later tonight should focus any storms to
the northwest of the CWA.

The models are in overall good agreement through at least 84
hours, with the ridge moving east and breaking down on Wednesday,
followed by FROPA Wednesday night and Thursday morning, with only
minimal thunderstorm chances Thursday and Friday. A plume of steep
lapse rates and PWs of 125-150 percent of average will move across
the area later Wednesday, resulting in 2000-3000 J/kg of SBCAPE
for much of the CWA. Deep-layer shear will be favorable for some
supercells, and high cloud bases should favor storms with gusty
winds. Coverage may be somewhat limited, so the SPC marginal risk
looks good at this time.

FROPA should occur from 06-18z on Thursday, which should produce
some gusty northwest winds at least early in the day on Thursday.
A few storms may redevelop over the BLKHLS, and if the boundary
hangs up over SCNTRL SD it is possible that a few storms could
fire there as well. Otherwise, flow will be quasi-zonal through
Friday with limited moisture and instability, so thunderstorms
will be at a minimum.

West-northwest upper flow should persist Saturday through next
Tuesday, with a more notable wave and another FROPA later Saturday
and Saturday night. This should result in additional scattered
thunderstorms across the CWA, followed by a relative lull in
storms Sunday and Monday. Temperatures will continue to be near
average, but the humidity will be higher than average.


.AVIATION...(For the 00Z TAFS Through 00Z Wednesday Evening)
Issued At 526 PM MDT Tue Jul 23 2019

Mostly VFR conditions will prevail through Wednesday morning. TSRA
over the Black Hills with local IFR conditions will end early this
evening. Additional storms will develop Wednesday afternoon with
local MVFR conditions.




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