Area Forecast Discussion
Issued by NWS Rapid City, SD

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525
FXUS63 KUNR 210836
AFDUNR

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

.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 12Z TAFS Through 12Z Sunday Morning) Issued
At 212 AM MDT Sat May 21 2016

The lingering moisture from overnight storms will help to
develop an area of low stratus and patchy fog across parts of
western South Dakota and the eastern foothills early this morning.
MVFR to IFR conditions are possible in the fog/stratus. By mid-
morning this will lift and isolated showers/thunderstorms will begin
to develop across the higher terrain of the Black Hills. This
afternoon, scattered to numerous showers and thunderstorms will
develop across northeastern Wyoming and the Black Hills. These
storms will move east tonight across western South Dakota. MVFR
conditions and gusty winds are possible with any showers or
thunderstorms.

&&

.UNR Watches/Warnings/Advisories...
SD...None.
WY...None.
&&

$$

SHORT TERM...7
LONG TERM...MLS
AVIATION...MLS



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