Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 171619

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1019 AM MDT Fri Aug 17 2018

Issued at 1005 AM MDT Fri Aug 17 2018

Well-defined shortwave over WY will continue to move east this
afternoon and evening. With 1000-2000 j/kg CAPE, have increased
POPS for the afternoon. Light smoke will remain over forecast area
today with thicker smoke to the north.


.DISCUSSION...(Today Through Thursday)
Issued at 259 AM MDT Fri Aug 17 2018

Current upper level analysis shows ridging centered over the region
with a upper level trough moving onshore in Washington, Oregon, and
northern California. Across the northern High Plains, skies are
mostly clear beneath surface high pressure.

Upper level ridge will remain in place today with subsequent warm
and dry weather. Smoke from western wildfires will filter back into
the region today. As the ridge shifts east throughout the day, a
shortwave trough will clip the area this afternoon and evening.
This wave coupled with increasing moisture aloft will give a
chance for a thunderstorm or two during the afternoon and evening.
Chances of precipitation are best over the Black Hills and
southwestern South Dakota where moisture, instability, and lift
will be most favorable. While the best chance of severe weather
will remain south of the forecast area late today, a well-mixed,
dry sub-cloud layer will support marginally severe wind gusts with
any strong storms that do form during the afternoon and evening.

Unsettled weather returns this weekend as the trough
centered over the Pacific NW will move into the Northern Plains
Saturday and Sunday. Strong moisture advection will occur ahead of
lee trough Saturday afternoon along with fairly steep mid-level lapse
rates that will result in 1500 to 2500 J/kg of MLCAPE. Shear
will be marginal with bulk shear values generally 30 knots
or less across much western South Dakota, limiting a more organized
severe weather threat. Despite the marginal environment, the most
favorable region for severe weather Saturday will be south-
central SD where CAPE and shear will be maximized. Strong winds
and marginally severe hail are the most probable severe hazards
Saturday. The other main concern will be heavy rainfall.
Widespread rain along with precipitable water values well above
normal (> 1.5") and slow storm motion could result in some
localized flooding Saturday into early Sunday morning.

Showers and storms will linger through late Sunday as the system
departs. A cold frontal passage late Saturday will bring gusty winds
and much cooler air behind it. Highs on Sunday will only reach the
upper 60s to mid 70s...very fall like to end the weekend.

A quick moving wave will clip the region Monday night with
showers and storms possible late Monday. Otherwise, the rest of the
week will be drier and warmer with a few waves bringing periodic
chances of showers and storms through the


.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 1005 AM MDT Fri Aug 17 2018

VFR conditions expected through the TAF period. Some thicker smoke
over nwrn SD, but it is not restricting vsby.




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