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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
302 am MDT Sat Apr 18 2015

Short through Sunday night
issued at 302 am MDT Sat Apr 18 2015

Current surface analysis shows low pressure over far western
Kansas...with a weak cold front to the north of the forecast area...
across northern Minnesota and ND. Upper level analysis shows low pressure
remaining centered across southern Colorado...ever so slowly moving
east-northeast. Skies are mainly cloudy across much of the
area...though there are still some breaks in the clouds across far
northwestern South Dakota and western portions of northeast Wyoming. Kudx radar
shows scattered to numerous showers across southwest and south
central South Dakota...moving to the northwest. A lot of the activity has
been drying up as it moves toward northwestern South Dakota...where airmass
remains rather dry. Regional radars show much more activity to the
south across Nebraska...with embedded thunderstorms as well. This
activity is moving north-northwest and will bring the bulk of our
rainfall during the day.

Short range models seem to be in pretty good agreement now with
timing and track of the system over the next 24 hours...with several
bands of showers expected to move up and through much of the County Warning Area
through the day and into this evening...before cold front over the
Pacific northwest moves through and pushes the system quickly out of
the region late tonight and Sunday morning.

Showers will continue to move north-northwest into the area today as
the upper low over southern Colorado moves slowly east-northeast into
eastern Colorado and stronger upper energy lifts into the County Warning Area. Showers
will be most abundant and heaviest over southern South Dakota this
morning...with the entire area seeing at least scattered activity
during the afternoon. Rainfall amounts of a tenth to a half inch can
be expected in many areas...but some locations from southwest into
south central South Dakota will receive up to an inch or possibly more where
the heavier showers go through. Campbell Colorado Wyoming and far northwest South Dakota
will be the areas likely to see the least amount of rainfall with
this system. Isolated thunderstorms are expected as well across the
area. Highs will mostly be in the 50s...with some lower 60s possible
over far northern and western portions of the County Warning well as
toward the Winner area.

Cold front and shortwave system aloft will push into northeast Wyoming
late in the day and then move east across the rest of the area
tonight. This system will help to accelerate the upper low out of Colorado
and push it quickly across the Central Plains tonight and Sunday
morning. Showers will taper off gradually from west to east
tonight...with most of the activity over with by late tonight. A
little snow could develop over the Black Hills before the precipitation ends
overnight. Gusty northwest winds can be expected behind the passage
of the front...but the strongest winds should be rather short-lived.

A cool and much drier airmass will be in place for Sunday as high
pressure approaches the northern High Plains. Before it does...a
tight pressure gradient will remain over the western Dakotas...
resulting in strong northwest winds for much of the day. A Wind
Advisory will likely be needed for at least the northwest and west
central South Dakota plains during the winds could gust to around 50
miles per hour. Highs will be in the low to middle 50s across much of the
plains...with 40s in the Black Hills. Winds will diminish toward
sunset and skies will become mostly clear overnight. Lows Sunday
night will be in the 20s and lower 30s.

Long term...Monday through Friday
issued at 302 am MDT Sat Apr 18 2015

Deep eastern noam upper trough will support blocked downstream
flow...with western noam ridging and potential Rex type blocking
over the western Continental U.S.. deep large scale upper low over the
eastern Pacific will support a southern periphery vortex into the SW
Continental U.S. By middle next week with antecedent downstream ridging. This
will support a psuedo baroclinic zone over the region with the
warmest codns expected over NE Wyoming. Dry conds will prevail until
toward the end of the which time western Atlantic
blocking looks to diminish...supporting an eastward shift of the
western noam ridge and the eventual ejection of the SW Continental U.S. Upper
low. Current determinstic and mean ensemble solutions track this
system through the Central Plains...with most lsa passing south of
the forecast area. probability of precipitation to low chance and slight chance Wednesday-
Sat. Some indication for a ll cold air advection surge on the backside of this
potential system...which could support some snow accums...especially over
the higher terrain.

Given dominant easterly flow...temperatures are expected to be near or
slightly below seasonal norms in the majority of the period...with a
cooler trend toward the end of the week...especially if appreciable precipitation


Aviation...for the 12z tafs through 12z Sunday morning
issued at 302 am MDT Sat Apr 18 2015

A slow moving upper low will slowly spread rain from south to
north across the area through today. Across western South Dakota...MVFR
ceilings/visibilities are expected across all but far northwestern South Dakota. Parts of northestern
Wyoming will also remain VFR. Some LIFR ceilings/visibilities expected over the
higher terrain of the Black Hills. Conditions will improve from
northwest to southeast beginning late Saturday afternoon behind a
cold front.


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


Short term...26
long term...jc

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