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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
1012 am MDT Wednesday Aug 27 2014

Update...
issued at 1012 am MDT Wednesday Aug 27 2014

Current surface analysis shows low pressure over far northeast
Colorado...with a nearly stationary frontal boundary from western Wyoming to
northeast Kansas. Upper level analysis shows low pressure over eastern
Utah...with trough covering The Rockies and much of the High Plains.
Skies are variably cloudy across the area...with areas of fog
lifting over eastern portions of the County Warning Area. Regional radars show
most of the shower and thunderstorm activity in two areas. The
main area is over southern and central Wyoming...moving north-
northwestward around the upper low. Another area of lighter
activity is located over central Nebraska into portions of southeast
South Dakota...moving north-northeast. Our area is dry at the moment...but
showers and storms will move in from the south and also develop
as the upper low moves across southern Wyoming/northern Colorado this
afternoon and evening...especially over the Black Hills and
southern portions of the County Warning Area. There should be a fairly sharp
cut-off to the coverage of showers... likely somewhere near or
just north of the I-90 corridor. The threat for severe storms
remains low with main area of instability staying south of the
area.

Have made a minor update to the forecast...mainly to lower probability of precipitation
across the area this morning and raise them over the Black Hills
this afternoon. Also made some adjustments to cloud cover for the
next few hours. Temperature forecast for today looks good at the moment...
but may have to tweak them a bit later on...depending on cloud
and shower coverage.

&&

Short term...today through Thursday night
issued at 251 am MDT Wednesday Aug 27 2014

Latest water vapor imagery shows an area of low pressure over
Nevada/Utah with a northern stream trough extending from central
Canada into the northern plains. The northern stream trough will
quickly move eastward as the cutoff low slowly propagates into the
central rockies. Southwesterly flow continues over the
central/northern plains.

For today the best forcing remains south of the County Warning Area. Best chances
of precipitation through Thursday will be across the far southern portions
of the forecast area as a series of waves cross the Colorado/Nebraska
area. Models are varying with timing of waves and how far north
the precipitation will extend within the County Warning Area. Either way quantitative precipitation forecast amounts will
be limited...except with the possibility of some stronger storms
crossing areas near the Nebraska border. High temperatures today
will once again be below average due to cloud cover. As the upper
low moves through Nebraska/Kansas...precipitation chances will diminish
across the area with dry conditions by Thursday afternoon/evening.

&&

Long term...Friday through Tuesday
issued at 251 am MDT Wednesday Aug 27 2014

Progressive northern stream flow will continue through the
period. Two main systems expected to affect the region in the
period...a weak northern stream trough Friday...and a more robust
longwave upper trough Sat night into Monday. Forecast models in
good agreement per wave amplitude and track. However...some
substantial differences begin to emerge in the Sunday night into
Monday period between the European model (ecmwf) and GFS...mainly the handling of
the northwest Continental U.S. Southern periphery ejecting impulse. The European model (ecmwf) has a
southern bias to the system with the bulk of lsa passing through
the forecast area as opposed to a more northern track in the GFS...which is
also faster. The European model (ecmwf) solution would support better chances for
rain in the forecast area however moisture limitations may preclude any
precipitation totals then.

Retained previous probability of precipitation for Friday afternoon...with a slight uptick and
expansion of slight mention. Forecast models continue to indicate a
surface trough stalling across the SW half Friday...with a passing upper
level impulse. Sufficient ll moisture ND resulting cape would be
conducive for convection in the afternoon...especially over the Black
Hills. Otherwise...a more impressive upper level trough will push
out of the NE Pacific and into the northern plains...supporting
increasing lsa Sat night through Sunday night. Better surface cyclone
development looks to remain well east of the region...supporting
more elevated/convective chances behind the surface trough rather than
widespread deformation rains. Hence...kept probability of precipitation in the slight to
low chance Cat for now...especially as deeper moisture will shift quickly
east out of the region. Kept most of the region dry Monday through
Tuesday...with upper level ridging commencing and dry profiles.

Seasonal temperatures expected in most of the period...with below normal
temperatures /5-10 degrees/ expected Monday and perhaps Tuesday behind
Sunday/S cold front...especially northwest areas.

&&

Aviation...for the 12z tafs through 12z Wednesday night
issued at 251 am MDT Wednesday Aug 27 2014

Scattered-isolated showers/thunderstorms expected across the southern half
through most of the period as a series of impulses cross the
region. Local MVFR/IFR conds experimental with heavier precipitation.
Additionally...IFR ceilings will spread north across SW and scentral
South Dakota this morning...with ceilings trending to MVFR late morning/early
afternoon. Ceilings may linger through the period there. Winds will
veer southeast around krap this afternoon...with the potential for MVFR
ceilings to spread into the rap terminal then. Have hinted at this in
the taf.



&&

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

$$

Update...26
short term...mls
long term...jc
aviation...jc

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