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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
420 PM MST Thursday Nov 20 2014

Short term...this evening through Friday night
issued at 235 PM MST Thursday Nov 20 2014

Shortwave ridge will progress east through the region ahead of a
compact northern stream shortwave trough. This upper trough will
force a weak surface trough through the region Friday with weak cold air advection
ensuing in it/S wake. Dry weather will prevail in the period with
nearly all lsa remaining well north of the forecast area. Mild conditions can
be expected...with downslope westerly flow.

Areas of low stratus and fog have dissipated across most of the area
with dry westerly flow and insolation working in tandem. Mostly
clear skies will prevail into early evening...however...increasing
cirrus and middle cloud can be expected...which will limit T falls to an
extent. Pressure falls with warm air advection flow will support mild conditions on
the favored downslope areas of the Black Hills.

Middle cloud and cirrus will gradually exit the region Friday with ll
westerly flow and warm ll thermal fields supporting a fairly mild day
most places with highs in the 40s. Surface trough will shift through the
region late Friday...supporting slightly cooler ll thermal fields.
Sustained westerly flow Friday night should help limit temperatures falls
away from valleys...especially in the typical downslope areas of the Black
Hills.

&&

Long term...Saturday through Thursday
issued at 235 PM MST Thursday Nov 20 2014

Dry and mild conditions will persist into early Saturday before a
Canadian low pressure system sweeps cold and moist air southward
into the region. Chances for rain/snow begin late Saturday
afternoon...mainly across the northern and western County Warning Area. Rain will
quickly change to snow Saturday evening as colder air moves in.
Best precipitation chances remain across northeast Wyoming and the Black
Hills area Saturday night into Sunday. As the system shifts
east...winds on Sunday will be strong across northwest South
Dakota as a tight pressure gradient sets up across the Dakotas.
Winds will remain breezy on Monday as strong pressure rises move
through. On Wednesday...the next upper trough will begin swinging
through the northern plains states...bringing in another shot of
cold air and more widespread chances for snow.

&&

Aviation...(for the 00z tafs through 00z Friday evening)
issued at 418 PM MST Thursday Nov 20 2014

Local IFR ceilings/visibility due to St/fzfg possible in creeks/river
valleys tonight across the South Dakota plains. Otherwise...VFR conditions
expected through Friday.

&&

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

$$

Short term...jc
long term...15
aviation...helgeson

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