Scientific Forecaster Discussion
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1106 PM MDT Friday Aug 29 2014
Short term...this evening through Saturday night
issued at 300 PM MDT Friday Aug 29 2014
Current surface analysis shows low pressure over western Minnesota...
with cool front now south and east of the County Warning Area. Upper level
analysis shows slow moving low over southwest Iowa...with shortwave
energy behind it moving across the western into central Dakotas.
Kudx radar shows isolated to scattered showers from north central
into southwest South Dakota...with the strongest cell over the southeast
slopes of the Black Hills. A few lightning strikes have been noted
with the cell over Fall River County. Northwest winds remain
breezy across portions of the South Dakota plains...but are gradually
decreasing. Temperatures are in the lower 70s to middle 80s.
A unsettled weather pattern can be expected through the Holiday
weekend...with numerous disturbances in near zonal flow bringing
occasional chances for showers and thunderstorms. The best chances
in the near term look to be late Saturday and during the day on
Sunday. Temperatures will be a bit above average Saturday and
slightly below average for Sunday and Monday.
For tonight...any lingering showers over south central South Dakota will end
early in the evening...with mostly clear skies into the overnight.
However...some low clouds could develop on the eastern edges of the
County Warning Area after midnight...with patchy fog also likely late
tonight...especially in the valleys. Lows will mostly be in the
50s with light and variable winds overnight.
Clouds will generally increase from west to east on Saturday...
especially in the afternoon...as the next disturbance approaches.
The best forcing will pass to the north of the area across eastern
Montana and ND late in the day and evening. South-southeast winds will
develop ahead of the next cold front...bringing warmer and moister
air into the area during the day. Highs will be in the upper 70s and
80s across most of the area...with dewpoints creeping back into the
upper 50s and lower 60s across the South Dakota plains by late in the day.
This will produce moderate instability over the area...especially
over northwestern South Dakota...with cape values of 1000 to over 1500 j/kg by
00z. Timing of the disturbance will be important with regard to
coverage and scope of potential for severe weather. As it stands
now...the best threat for a severe storm or two will be across
northwestern South Dakota...where shear will be most favorable. The threat
will diminish further to the south where the forcing is weaker and
cin could be sufficient enough to keep storm chances to a minimum.
The best chance for storms will hold off until late afternoon or
possibly early evening over western portions of the County Warning Area...with the
best chance elsewhere during the evening. Activity with this wave
should move out of the area from west to east by the overnight. The
next system will approach northeast Wyoming late Saturday night and move
across the forecast area on Sunday.
Long term...Sunday through Friday
issued at 300 PM MDT Friday Aug 29 2014
Cyclonic flow associated with upper trough will move through the
County Warning Area Sunday/Monday. Forcing with embedded shortwaves as well as
steep middle-level lapse rates will support scattered
-shra/-tsra...especially Sunday/Sunday evening. Temperatures will
be fairly chilly Sunday given cloud cover and low level cold air
advection throughout the day.
An upper level ridge tries to poke into the northern plains for the
middle of next week as an upper trough digs into western
Canada/northwest US. Amount of amplification becoming uncertain with
possible Flat Ridge and embedded shortwaves moving across the
US/Canadian border. This could bring a few wind shifts through the
area...but in general...a warmer/drier trend is expected as thermal
ridge builds overhead.
Aviation...for the 06z tafs through 06z Saturday night
issued at 1105 PM MDT Friday Aug 29 2014
MVFR ceilings are possible between 09z and 15z Saturday morning. Scattered
showers and thunderstorms are possible over northeastern Wyoming
and far western South Dakota on Saturday afternoon and evening.
Otherwise...VFR conditions will prevail.