Area Forecast Discussion
Issued by NWS Rapid City, SD
FXUS63 KUNR 170958
Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
358 AM MDT WED AUG 17 2016
.SHORT TERM...(Today Through Thursday Night)
Issued at 321 AM MDT Wed Aug 17 2016
Upper level analysis has ridge building across the Rockies and
Northern Plains with broad trof entering the Pacific Northwest.
Skies are mostly clear across the Northern Plains with the
exception of some mid level cumulus and isolated sprinkles across
central SD. At the surface, high pressure resides across the
Dakotas with weak lee trof across central Montana. Temperatures at
3 am are in the 60s, with light southerly winds.
Today...Upper level ridging will move across the Northern Plains
today, producing the warmest day of the week. High temperatures
will rise into the upper 80 to mid 90s across the plains.
Southeasterly winds will increase ahead of an approaching surface
low, and dew points will be in the 50s. Instability will
increase, with models` estimations of 21Z MLCAPE range from 250
to 2000 J/kg across central SD - but this area will remain capped
through mid afternoon. Further west, showers and thunderstorms
are expected to develop in the afternoon, mainly across the higher
terrain of the Black Hills, northeastern WY, and northwestern SD
where upper forcing will assist development. The most likely
location for any strong to severe thunderstorms will be across far
northwestern SD, where 0-6 km shear is expected to be around 30
Tonight...Upper ridge begins to break down as strong upper
shortwave slides across southern Canada. Showers and storms will
continue across northern and western portions of the forecast
area, with additional development across south central SD. Again,
shear remains on the weak side, so widespread severe weather is
not expected. Only isolated storms are expected to persist across
northern SD after midnight. Southerly boundary layer flow will
persist overnight, keeping temperatures in the 60s through early
Thursday...Upper level ridge breaks down as strong Canadian trof
begins to drop into the Northern Plains. Strong surface cold
front will drop south along with the upper forcing, producing
more widespread precipitation and a wide range of temperatures
across the area. Highs will range from the lower 70s across parts
of Northeast WY and the Black Hills, to near 100 degrees in far
south central SD. Strong forcing with the front combined with
strong cooling aloft will set up steep lapse rates/strong
instability across the area by the afternoon. Strong to severe
thunderstorms are expected across much of the area, with hail and
damaging wind gusts the main concern. By Thursday night,
widespread showers and thunderstorms are expected across much of
the area, with the severe threat quickly shifting to the south and
east. Low temperatures will fall into the 50s to lower 60s.
.LONG TERM...(Friday Through Tuesday)
Issued at 321 AM MDT Wed Aug 17 2016
Vigorous shortwave rotating through upper trof swings through
Montana/Wyoming Friday. Combination of strong Q-G forcing and
steep lapse rates will result in widespread shower/thunderstorm
activity. Clouds and precipitation will keep highs in the 60s,
15-20F below average. Trof axis shifts into eastern
Dakotas/Minnesota on Saturday. Models have been consistent in
dropping secondary shortwave through the western Dakotas Saturday
afternoon, bringing isolated/scattered showers and thunderstorms.
The upper trough moves into the Great Lakes region Sunday as an
upper ridge builds across the Rockies into the northern plains.
This will lead to dry weather and warmer temperatures for the
first half of next week.
.AVIATION...(For the 12Z TAFS Through 12Z Thursday Morning)
Issued At 321 AM MDT Wed Aug 17 2016
Isolated showers/thunderstorms possible across south central
South Dakota early this morning, then across portions of northeast
Wyoming into western South Dakota this afternoon and evening.
Local MVFR conditions with the precipitation, otherwise VFR
conditions through tonight.