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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
1030 PM MDT Friday Sep 4 2015

Update...
issued at 832 PM MDT Friday Sep 4 2015

Raised probability of precipitation even further (after an earlier update) across northestern Wyoming and
parts of far western South Dakota given cluster of storms that area. Much of
this activity should push north of the County warning forecast area by midnight...although
additional upper level energy moving in could spark a few more
storms overnight. Gusty winds the main threat with storms this
evening.

&&

Short term...this evening through Saturday night
issued at 142 PM MDT Friday Sep 4 2015

19z surface analysis had low over central South Dakota with frontal
boundary from northeast ND to the Black Hills to central Wyoming with a
trough into western Kansas. Early morning shortwave northeast of area
with 80kt jet streak/shortwave energy over western Colorado heading
northeast into southeast Wyoming. This activity will roll into the County Warning Area
late this afternoon/early evening and slowly move northeast
overnight...which is in good agreement with most forecast model
guidance. Temperatures will be warmer than guidance in the east
and near to the west.

Saturday...upper trough now over the northwest US will spill into
rockies and into lift into Montana. Frontal boundaries will remain in
place...and perhaps move a bit to the northwest/west during the day
with return flow ahead of pressure falls associated with trough. 1-
2kj/kg MLCAPE pools near the Montana/SD/WY/ND border area of the County Warning Area with
mlcin gone in the afternoon. 15-20m/S 0-6km bulk shear available.
Potential exists for isolated supercells...but implied subsidence
behind tonight/S shortwave and any debris clouds could interfere
with peak daytime heating with main synoptic forcing heading north
of the County Warning Area Saturday night...along with a good dry slot. Will
mention isolated t+ in the far northwest for now. Cold front will
enter far northwest reaches of the County Warning Area late Saturday night.
Temperatures will be near guidance in the west and above in the
east.

Long term...Sunday through Friday
issued at 142 PM MDT Friday Sep 4 2015

Medium range models still in very good agreement on upper low
passing to the north and west of the area on Sunday. Cooler air
will push into the area behind passing cold front...with brisk
west to northwest winds. High temperatures will range from around
70 in northwest South Dakota...to near 90 in far south central South Dakota.

On Monday...zonal flow will develop for the early part of next
week...transitioning to northwesterly flow for the latter part of
the week. This should result in seasonably warm and mostly dry
conditions for much of the week. Weak warm air advection combined
with and upper shortwave may produce a few showers and storms
across the South Dakota plains on Monday and Monday night.

&&

Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1026 PM MDT Friday Sep 4 2015

MVFR/IFR conditions in thunderstorms and rain possible across far northwestern South Dakota through 08z.
Elsewhere...isolated high based -tsra through the night. Mainly VFR
conditions during the day Saturday.

&&

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

$$

Update...Johnson
short term...helgeson
long term...15
aviation...Johnson

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