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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
224 PM MDT Thursday Aug 21 2014

Short term...this evening through Friday night
issued at 222 PM MDT Thursday Aug 21 2014

Deep western noam trough will continue to support
active southwest flow over the region through the period...with
two staunch impulses affecting the region...after this evening/S
advancing wave. A wavering frontal boundary will provide the focus
for ll cyclogenesis and associated ll lift...supporting good
chances for rain and thunderstorms across the area...especially Friday
night. As of 20z a surface trough was entering the western forecast area with a
weak impulse ejecting NE out of Wyoming. This wave will continue to
support increasing chances for showers and thunderstorms over the
northwest half into this evening. A few cells have developed over the
Black Hills given weaker inhibition...with more activity expected
off the big horns and high terrain of Wyoming. Hires runs of the
nmm/arw/hrrr indicate that isolated/scattered thunderstorms may coalescence
into a bigger cluster of ts and progress NE across the northwest zones
this evening as the waves pushes NE. Best forcing for ascent will
remain northwest of the forecast area. Previous probability of precipitation work well with likelies across
the far northwest...with more isolated chances to the southeast along the
associated surface trough push where stronger capping and weaker lift
will be present.

Friday...a few lingering rain showers may be present over the eastern zones
Friday morning as the trough pushes southeast. This feature will stall in
response to the next impulse...the ejecting SW Continental U.S. Upper trough.
Water vapor imagery clearly shows ample deep layer moisture being
ingested into the system per a flow connection to karina. Hence
ample deep layer moisture will be available when this system crosses
the plains and a ll Gomex connection is established. Have precipitation
chances increasing through the day Friday. However...most rain showers/ts
may not arrive into the forecast area until Friday night given lingering
subsidence behind today/S wave. Forecast model consensus/and
run-run consistency per the track/evolution/and rain totals continue
to waver. However...there is a signal for heavy rain with this wave
given the deep moisture available. Very high precipitable water values in excess of
one inch in many locations...with some areas approaching two will
likely support an area of heavy rain with this system...especially where
800 mb-700 mb positive Theta-E advection is strongest. Where remains the question.
Latest guidance suggest the southeast forecast area. However...given the staunch upper
jet associated with the Pacific northwest upper trough...this area may shift northwest
with future model runs. At this point...feel a pop raise was
warranted with a locally heavy rain mention in the grids Friday
night. As for temperatures...E/NE flow with expected increased cloud cover
will support a cooler day northwest areas. Did adjust temperatures down there.

Sat...lingering rain showers/ts possible with a dry period expected SW half
in the after. Main northwest Continental U.S. Upper trough will eject NE Sat/Sat
night...supporting renewed surface pressure falls and reflected ll southeast
flow. Positive Theta-E advection regime should support rain showers/ts across portions
of the forecast area...especially the northwest zones Sat night. Have retained at least a middle
pop value all places...with likelies northwest...especially with ample/sufficient
deep layer moisture present per 700 mb Theta-E forecasts. Did lower temperatures
Sat given clouds and moist easterly ll flow...especially northwest zones.

Long term...Saturday through Thursday
issued at 222 PM MDT Thursday Aug 21 2014

Upper low lifts northeast into Montana Sunday...with dry slot
punching into western/central South Dakota. Best support for
precipitation will be north of the forecast area. Post frontal
stratus will linger through much of the day Sunday...especially
across northeast Wyoming/northwestern South Dakota. Upper low
continues to lift northeast into Canada Monday...leaving slow
moving upper trough across the northern plains. Below average
temperatures will continue through middle week...along with a daily
chance of showers/thunderstorms.

&&

Aviation...(for the 18z tafs through 18z friday)
issued at 222 PM MDT Thursday Aug 21 2014

Scattered showers and thunderstorms expected over northeastern
Wyoming and western South Dakota late this afternoon through
tonight...with local MVFR ceilings/IFR visibility. Area of stratus will develop
late tonight over northeast Wyoming/northwest South Dakota with
MVFR/local IFR ceilings.

&&

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

$$

Short term...jc
long term...7
aviation...7

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