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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
252 am MDT Friday Aug 29 2014

Short term...today through Saturday night
issued at 251 am MDT Friday Aug 29 2014

Strong shortwave trough is crossing the Central Plains into the eastern
Dakotas to upper Midwest. Zonal flow sits across the western Continental U.S. With
a much weak shortwave cross central/eastern Montana. Surrounding radar sites
showing -shra/thunderstorms and rain crossing eastern Montana to northern Wyoming. A surface cold front
also associated with the wave/precipitation is sliding southeast and is visible on
the bis/byz radars.

Cold front/upper wave will quickly push through the County Warning Area today
bringing chances for -shra/tsra. Latest timing appears to have the
area of precipitation over the Black Hills area by late morning...the continuing
eastward into the western South Dakota plains through the afternoon. Strong instability
developing ahead of the front over western/central South Dakota plains could result
in some strong storms this afternoon...but shear will be quite weak
so storm behavior is expected to be more pulse like. Temperatures will
range from the middle 70s to middle 80s...with cooler temperatures over northwest South Dakota/NE
Wyoming where the frontal passage occurs earlier this morning. Winds immediately
behind the frontal boundary will become breezy...with gusts to
around 30kt...but will be limited to an hour or two after the front
passes.

Weak ridging builds behind the departing wave tonight clearing out
precipitation late tonight to Saturday morning. The next longwave trough will
cross the Pacific northwest to northern rockies Saturday...with active SW flow
developing. Chances for rain showers/thunderstorms and rain will again return Sat aftn/evng.
Good instability and strong ll shear over northwest South Dakota will result in
strong to severe thunderstorms and rain Sat aftn/evng. Large hail and damaging winds
are the main threats.

&&

Long term...Sunday through Thursday
issued at 251 am MDT Friday Aug 29 2014

Active/progressive northern stream flow will continue through the
period with a strong upper level trough on track to advect through
the region sun...with a weaker shortwave impulse likely Monday.
Trough reloading over the NE Pacific Tuesday-Wednesday favored in European model (ecmwf)/GFS/Gem
solutions Tuesday...which will support shortwave ridging over the
northern plains with dry and warmer conditions Tue-Wed. No sig
changes from the previous forecast given very good deterministic
model consistency and continuity through the period...which is in
line with gefs/naefs means.

Upper level trough will be in the process of shifting east across
the northern plains Sunday. Associated middle level cold pool will
support steep ll lapse rates over the forecast area with residual ll moisture
in place. Diurnal heating coupled with bh/NE Wyoming terrain influences
should support scattered to isolated rain showers/ts especially south half of the forecast area.
Remained close to previous pop numbers with Middle Range chance
covering precipitation concerns. Otherwise...a weaker westerly flow impulse
still indicated in forecast models Monday...possibly supporting a
few shra/ts...although dry profiles will limit quantitative precipitation forecast and
coverage...especially away from the bh. Dry conds expected Tuesday-Wednesday with
mean deep layer ridging. The next NE Pacific upper level trough will
eject east into the northern plains Wednesday night into
Thursday...interacting with a Lee side trough. Increasing lsa Wednesday night
through Thursday will support increasing chances for shra/ts...far northwest
Sat night with probability of precipitation trending south through Thursday as the upper trough
shifts east with the reflected surface wave. Did bump probability of precipitation up to low
chance northwest 2/3 Thursday night.

Cool/below normal temperatures /5-10 degrees/ expected Sunday through
Monday...especially northwest areas. Warming conditions can be expected Tuesday-Thursday
most areas. However...northwest areas may see the start of a cooling trend
Thursday as the next cold front shifts southeast into the region.

&&

Aviation...for the 06z tafs through 06z Friday night
issued at 251 am MDT Friday Aug 29 2014

MVFR conditions in patchy fog may develop over parts of western
South Dakota late tonight/early Friday morning. A band of isolated
to scattered showers and thunderstorms will move into northeastern
Wyoming late tonight and will spread eastward into western South
Dakota on Friday morning. Thunderstorms will increase in coverage
over western and central South Dakota on Friday afternoon.

&&

Unr watches/warnings/advisories...
South Dakota...none.
Wyoming...none.
&&

$$

Short term...13
long term...jc
aviation...jc

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