Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 152313

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
513 PM MDT Tue May 15 2018

.DISCUSSION...(This Evening Through Tuesday)
Issued at 229 PM MDT Tue May 15 2018

20z surface analysis had weak ridge over the CWA. Water vapour
showed upper ridge building from the northwest US into the
northern plains with a very weak shortwave over south central SD.
Upper ridge assisting mid-level warming today, so MLCIN continues
to increase, but 0.5-1KJ/kg MLCAPE exists east of the Black Hills.
Weak TSRA rooted over the higher terrain of the Black Hills and
under south central SD shortwave. Activity will quickly die with
decreasing solar insolation and upper ridge continuing to build
east. Temperatures will be near guidance.

Wednesday, upper ridge slips east as upper trough moves into the
western CONUS with an upper low over CA and another over western
MT. Weak spoke of energy extending from MT feature will push
through the CWA during the day. Forcing fairly weak. MLCAPE around
1KJ/kg with 50-100J/kg MLCIN. 0-6km shear increasing by afternoon
to around 15m/s. Latest guidance looks less favorable for
organized severe storms, but still expect a few strong storms
tomorrow afternoon/early evening, especially near the Black Hills.
Temperatures will be 10-15F above normal.

Wednesday night, cap quickly reestablishes itself, but 30-40kt
low level jet will bring ACCAS/isolated TS to south central SD
overnight. Lows will be near/slightly above guidance.

Thursday, upper low moves into UT with a decent shortwave moving
through the CWA during the afternoon/evening hours. Weak surface
low may develop along developing frontal boundary from eastern MT
into southeast WY with lee trough merging into feature. Moisture
return looks pretty good with long southerly fetch bringing 50s
Td/s to the area. NAM/GFS show a tongue of 2KJ/kg+ MLCAPE over
western SD with increasing shear. Most guidance shows convection
firing by late afternoon along surface boundaries and then moving
east. Timing of higher winds aloft to lengthen hodographs may be
the discriminator between a a couple of severe storms to an
organized severe event. Slight risk paints best areas nicely.
Locally heavy rain may also occur given PWs around 1".
Temperatures will be 10-20F above normal for highs. Activity will
move east in the evening, but additional precipitation will occur
with synoptic lift ahead of upper low.

Friday and Saturday, upper low shifts slowly northeast into the
northern plains with the GFS showing the circulation weakening as it
dies over the CWA. Unsettled weather will continue with GFS
suggesting a few strong storms possible across the south Friday
afternoon if daytime heating can tap into well-defined shear.

Sunday through Tuesday, initial system is absorbed into main flow
as yet another upper low approaches for the middle of next week.


.AVIATION...(For the 00Z TAFS Through 00Z Wednesday Evening)
Issued At 510 PM MDT Tue May 15 2018

Isolated showers and thunderstorms will occur over parts of
southwestern and south central South Dakota early this evening.
Otherwise, VFR conditions will prevail through 18Z on Wednesday.
Scattered showers and thunderstorms are possible over parts of
northeastern Wyoming and the Black Hills area on Wednesday




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