Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 211703
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1103 AM MDT SAT MAY 21 2016

.Update...
Issued at 1029 AM MDT Sat May 21 2016

16z surface analysis had low over central WY with frontal boundary
stretching north into MT and south into northwest UT. Well-defined
southerly flow over the CWA with pressure falls intensifying over
MT/ND which will result in very windy conditions over northwest SD
through this evening. Will hoist wind advisory to account.

12z guidance shows staunch SB/MLCIN on the SD plains through early
this evening due to lowered temperatures, cloud cover, and stratus.
Cap weaker over the Black Hills and northeast WY where 1-2KJ/kg
MUCAPE develops as weak shortwave rotates into the area this
evening, tapping into convergence along aforementioned frontal
boundary. 0-6km bulk shear increases to 15-20m/s and thus would
expect any convection to include potential supercells, which is
painted by current slight risk.

Looks like a decent setup for low clouds and fog tonight on the
eastern slopes of the Black Hills, so have added late tonight and
early Sunday.

&&

.SHORT TERM...(Today Through Sunday Night)
Issued at 212 AM MDT Sat May 21 2016

Current surface map places trof of low pressure across central
Montana into eastern Wyoming. Isolated tsra/shra continue from
northwest South Dakota into southwest South Dakota in area of weak
850-700mb theta-e advection. Shortwave ejecting out of western
CONUS low lifts into Montana late today with new low closing off
over western Montana. Surface low deepens along current trof...and
moves into northeast Montana this evening. Tight pressure gradient
across the forecast area with windy conditions on the plains of
western South Dakota. Stratus across western/central Nebraska will
soon spread into western and south central South Dakota. Forecast
soundings suggest stratus will linger through much of the day east
of the Black Hills. As a result, have trended downward with high
temperatures. Narrow axis of of instability, with 1000-1500 J/kg
MLCAPE, sits across northeast Wyoming this afternoon/early
evening. A few severe thunderstorms possible this afternoon/early
evening. Thunderstorm activity will spread eastward overnight with
850-700mb theta-e advection. For Sunday, surface trof moves into
central South Dakota. Threat for severe thunderstorms over eastern
portions of forecast area.

.LONG TERM...(Monday Through Friday)
Issued at 212 AM MDT Sat May 21 2016

Moist southwest flow will prevail for the extended
period through next weekend. This will bring a daily chance for
thunderstorms.

On Monday, upper level low pressure will continue to move slowly
across southern Saskatchewan. Some weak instability and lingering
moisture may allow for a few showers/storms around the Black Hills
area; however, most areas should remain dry. The Canadian upper low
remains nearly stationary during the beginning of the week, keeping
an upper level trough of low pressure draped across the Pacific
Northwest. By Tuesday/Wednesday, this trough will slowly swing
across the Rockies into the Northern High Plains. Energy associated
with this trough will bring chances for thunderstorms. The models
begin to diverge toward the end of the week into the weekend. The
GFS solution slowly washes out the trough over the plains as another
upper low dives into the Pacific Northwest. Then as the western low
deepens, a weak ridge develops over the forecast area on Saturday.
However the ECMWF solution spins a weak upper low over the Central
Rockies into the forecast area Friday. Regardless, there will be a
chance for thunderstorms with both solutions, especially with the
moist southwest flow.

Temperatures for the extended period should remain near seasonal
normals, but will trend cooler in areas with more rainfall.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1102 AM MDT Sat May 21 2016

Some MVFR ceilings may linger across portions of the
western SD plains this afternoon, with more widespread MVFR/IFR
conditions this evening and overnight as low clouds/fog/showers move
back in. Gusty southerly winds are also expected this afternoon,
especially across the western SD plains, with speeds of 25-40 mph
and gusts near 50 mph. Thunderstorms are expected to develop over
northeastern WY this afternoon and evening, with MVFR conditions and
gusty winds possible with any showers and storms.

&&

.UNR Watches/Warnings/Advisories...
SD...Wind Advisory until 9 PM MDT this evening for SDZ001-002-012>014-
     073.

WY...None.
&&

$$

Update...Helgeson
SHORT TERM...7
LONG TERM...MLS
AVIATION...Pojorlie



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