Scientific Forecaster Discussion
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1027 am MST Sat Feb 28 2015
Short term...today through Sunday night
issued at 222 am MST Sat Feb 28 2015
Cold northwest flow will prevail in the period. A weak
impulse and associated surface trough will advect through the region
today...with another stronger system Sunday. Both systems will
basically reinforce thermal fields in place. Although moisture
continues to be limited with the lingering modified Arctic airmass
in place. Today/S impulse/S cold pocket aloft will time with peak
heating...helping to support a weak convective response with steep
ll lapse rates. Most if not all of the lsa associated with Sunday/S
wave will pass north of the forecast area...with dry conds expected in the
forecast area...although a few high based snow showers/virga cannot be ruled
out...especially across the northwest half. Below normal temperatures will continue
through the period.
Today...very weak impulse and associated surface trough will continue to
advect through the region...supporting veering ll winds and
steepening ll lapse rates. Steepest ll lapse rates are expected over
the high terrain of Wyoming and the southern/central Black Hills where
terrain influences will prevail. In addition...local Southern Hills
eddy will aid in enhanced convergence supporting slightly better uvm
there. Given lack of appreciable moisture and weak upper level
support...not expecting more than a few light snow showers in the SW
third. Best chance for any light accums will be in the Lee of the
southern Black Hills...with a few tenths possible there. Weak upglide
will spread southeast this evening into SW South Dakota...possibly supporting a few
flurries or light snow...with little to no accums expected. Cold
temperatures expected...close to Friday/S numbers most places. Partly to
mostly cloudy skies will clear overnight tonight...with lows in the
single digits most areas.
Sunday...another northwest flow impulse expected into the region. This
system will be stronger than Saturday/S wave...although most if not
all of the lsa/uvm will pass north of the forecast area. However...forecast
models /and outputted bufr data/ suggest a few high based snow
showers could be possible across the northwest half given fairly steep ll
lapse rates. Given dry conds...mostly virga would be anticipated.
This wave will mostly affect the momentum fields...with a decent
uptick in northwest winds expected Sunday afternoon. Steep ll lapse rates
per insolation and mixing will force faster flow down supporting
breezy conds...especially on the South Dakota plains. Thermal fields will basically
see near neutral temperature advection...with temperatures similar to Sat/S
numbers...slightly warmer in NE Wyoming. Associated surface high will shift
southeast of the region Sun night...with gusty southerly return flow
developing over NE Wyoming and portions of the South Dakota plains.
Long term...Monday through Friday
issued at 222 am MST Sat Feb 28 2015
Weak upper ridge crosses the area for Monday...with milder air
advecting into the region from the west ahead of a longwave trough.
The warmth will be short lived...as an Arctic front will push
through the region late Monday night into Tuesday morning.
Temperatures will decline late Monday night and continue to decline
on Tuesday as strong cold air advection takes place...and Tuesday
night lows will drop below zero for all of the County Warning Area. There will be
some moisture behind the Arctic front...but snow amounts will
generally be light...around 1 to 3 inches for western parts of the
County Warning Area.
The GFS and European model (ecmwf) are in agreement of a strong ridge building into
the western US by midweek and persisting beyond the end of the
forecast period. As a result...expect generally dry conditions past
Wednesday with a warming trend for the County Warning Area. By Friday...afternoon
highs could be well into the 50s...about 10 degrees above
climatology for this time of year.
Aviation...(for the 18z tafs through 18z sunday)
issued at 1024 am MST Sat Feb 28 2015
MVFR ceilings will remain across northeast Wyoming and far western
South Dakota through this evening...with local IFR ceilings around the
Black Hills. Isolated snow showers are also possible across
northeast Wyoming into the Black Hills...with MVFR/local IFR visibility.