Scientific Forecaster Discussion
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
531 am MDT Thursday Aug 28 2014
Short term...today through Friday night
issued at 302 am MDT Thursday Aug 28 2014
Current surface analysis shows low pressure over Saskatchewan...with
a frontal boundary extending through eastern Montana and central
Wyoming. Upper level analysis shows main low pressure area over northern
Colorado/southern Wyoming with a trough covering The Rockies and High
Plains. Latest radar imagery shows showers continuing to rotate and
lift north across the South Dakota plains. A little embedded convection is also
occurring in south central South Dakota.
As the upper low moves slowly east into the Central High plains...
shower/thunderstorm activity will quickly decrease from west to
east...with showers/storms continuing across much of south central
South Dakota. With a lot of low level moisture around...expect
areas of fog to linger into the morning hours across most of the
western South Dakota plains. By this afternoon...diurnal heating and lingering
instability could produce a terrain induced shower/storm over the
Black Hills. Thus left isolated probability of precipitation in the forecast. Temperatures today
will be warmer with decreasing clouds...especially across the northern
and western areas.
For tonight...clouds will continue to gradually decrease from
northwest to southeast...with mostly clear skies overnight. This
combined with light winds and surface high pressure to the
west...decided to add some fog for the South Dakota plains. Temperatures will be
On Friday...shortwave trough moves rapidly across Montana/ND pushing a weak
cool front into western South Dakota. Forecast soundings support 250-500j/kg
MLCAPE...with sufficient low level moisture/cape/shear for storms.
Most of the 500-700mb moisture remains north of the County Warning Area...however
could see scattered storms with an isolated strong storm across
northwest half of County Warning Area. Temperatures on Friday will be a little
cooler than today in the northwest...but most areas will see warmer
Long term...Saturday through Wednesday
issued at 302 am MDT Thursday Aug 28 2014
Progressive northern stream flow will continue through the period
with a long wave upper level trough on track to advect through the
region Sat-sun...with a weaker shortwave impulse possible Monday.
Trough reloading over the NE Pacific Tuesday-Wednesday favored in European model (ecmwf)/GFS
solutions...which would support mean ridging over the northern
plains with dry and warmer conditions. Main forecast trend is a
quicker arrival of the expected long wave trough Sat...with precipitation
chances increasing Sat afternoon western third. Otherwise...the
remainder of the forecast is on track at this time.
Upper level trough will push out of the NE Pacific and into the northern
plains...supporting increasing lsa Sat through Sunday. Lee side surface
trough will will interact with the approaching upper wave supporting
a focus for rain showers/ts activity over the western forecast area by Sat afternoon.
Ll southeast flow ahead of this system with moist upper level SW flow will
support increasing precipitable water over the region...with plenty of moisture
being available with this system. Some model indication for
sufficient deep layer shear and cape over the far northwest zones...especially northwest
South Dakota by Sat evening...supporting a possible severe threat there.
Something to watch. Have adjusted probability of precipitation up Sat afternoon and Sat
evening...with the primary southern periphery impulse expected to
shift NE across the northwest forecast area. Trailing surface trough with sufficient lsa
should support a good chance of rain showers/ts northwest third...with more
scattered/isolated activity across the remainder of the forecast area. Upper level cold
pool will support steep ll lapse rates over the forecast area Sunday with
residual ll moisture in place. Diurnal heating coupled with bh/NE Wyoming
terrain influences should support scattered to isolated rain showers/ts SW half of
the forecast area. Bumped probability of precipitation up over the bh. Otherwise...one more weaker
westerly flow impulse still indicated in forecast models
Monday...possibly supporting a few shra/ts...although upper level
support looks marginal at this time...especially with dry profiles. Dry conds
expected Tuesday-Wednesday with mean deep layer ridging.
Seasonal temperatures expected in most of the period...with below normal
temperatures /5-10 degrees/ expected Sunday through Monday...especially northwest areas.
The warmest day in period looks to be Sat...especially on the South Dakota plains.
Although there is some indication for warm conditions next Wednesday given
SW warm air advection flow ahead of the next potential ejecting NE Pacific upper
Aviation...(for the 12z tafs through 12z Friday morning)
issued at 517 am MDT Thursday Aug 28 2014
Upper level impulse and associated rains will wane and shift east
through afternoon...affecting mainly the far eastern forecast area...especially
scentral South Dakota. MVFR/IFR/local LIFR conds can be expected there. Any
IFR/LIFR conds will likely trend to MVFR through afternoon. VFR
conds expected elsewhere. MVFR/IFR conds expected to redevelop on
the South Dakota plains overnight /esp central SD/...with local LIFR/vlifr conds
possible in fog. Not expecting restrictions at krap or kgcc
tonight. Conditions will slowly improve late Friday morning.