Scientific Forecaster Discussion
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1116 am MDT Friday Aug 29 2014
issued at 952 am MDT Friday Aug 29 2014
Current surface analysis shows low pressure over northeast
South Dakota...with cool front stretching southwestward across South Dakota into
central Wyoming. Upper level analysis shows slow moving low over
southeast Nebraska...with shortwave energy behind it moving across the
northern High Plains. Kudx radar shows isolated to scattered
showers from far northwest South Dakota through the Black Hills and across
far eastern Wyoming. A couple of lightning strikes have been noted with
the activity so far. Winds have been gusty across parts of the South Dakota
plains behind the front...especially in the Rapid City area. Winds
will gradually diminish into the afternoon. Temperatures are in the middle
60s to middle 70s.
Will keep chances for showers and a few storms across the area
through the afternoon. The quicker movement of the system will
likely push most...if not all...precipitation out of the area by the
evening. Have made a minor update to raise winds across the plains
for late this morning. Also tweaked clouds and probability of precipitation chances for
the rest of the day.
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
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 18z tafs through 18z saturday)
issued at 1113 am MDT Friday Aug 29 2014
Isolated -shra will move across the western South Dakota plains this
afternoon with scattered -shra/-tsra possible later this
afternoon...mainly east of a line from kd07 to kien. Local MVFR
ceilings/visibilities possible with any -tsra. Local MVFR ceilings may sneak into
northeast Wyoming this afternoon...but should lift into VFR ceilings as
heating continues. Otherwise...VFR conditions will prevail
through Saturday morning.