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Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1112 am MDT Thu Sep 21 2017

Discussion...(today through wednesday)
issued at 135 am MDT Thu Sep 21 2017

Anchored Pacific northwest upper level trough will continue to support a
stalled ll baroclinic zone over the region as yet another impulse
is in the process of advecting across the region. Pos Theta-E adv
will support a few light showers across the far northwest early this
morning. Otherwise, a weak cold front will settle back south into
the forecast area today supporting a wind shift and nothing more given very
dry ll profiles. It will be a warm day across the southeast half where
highs in the 80s and 90s are expected ahead of the front. The next
stronger upper level trough will eject into The Rockies Friday,
supporting renewed pos Theta-E advection over the forecast area. Lsa will
increase late Friday with diurnal heating forcing a few shra/ts
over eastern WY, and potentially along the stalled sfc front
across scentral South Dakota where renewed cyclogenesis will be ongoing. A
few of the Wyoming showers may advect into western South Dakota in the evening,
being of an elevated nature by then. NE ll flow on the back side
of the deepening low will keep northwest half temps tame with another
warm day expected over scentral South Dakota. Active SW flow will remain
over the region through the weekend with bouts of lsa with each
passing impulse as the ll baroclinic zone will remain in place but
be biased southeast with each impulse, supporting much cooler temps
across the region this weekend. Parts of the region could see a
decent rainfall where fgen and moisture convergence aligns best.
Cold air advection will also support a strong cooling trend this weekend under
persistent ll NE flow, with much cooler more Fall like weather
returning to the region. Many locations will struggle out of the
40s both Sat and Sunday. The cooler temps, wetbulb effects, and
nocturnal timing still looks to support some higher elevation snow
this weekend, esp Sat night where some light accums are still
possible over parts of the Black Hills. The cool, showery weather
will persist into next week as mean troughing lingers over the
central Continental U.S.. warmer weather looks to return by the middle to end
of next week as another round of western Continental U.S. Ridging is favored
in GFS/CMC/ECMWF solutions.


Aviation...(for the 18z tafs through 18z friday)
issued at 1112 am MDT Thu Sep 21 2017

VFR conditions can be expected this afternoon and tonight. A cold
front, across the Black Hills area at midday, will slowly move
east across western South Dakota this afternoon, stalling out over central
South Dakota tonight. Breezy south to southwest winds can be expected ahead
of the front this afternoon, with breezy northwesterly winds in
many areas behind the front.


Fire weather...
issued at 135 am MDT Thu Sep 21 2017

Rh's will fall into the upper teens to around 20 percent over SW
South Dakota this afternoon with breezy westerly winds. This will support
extreme grassland indices there. However, red flag conds are not
expected with rh's remaining above 15 percent combined with
marginal wind speeds.


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


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