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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
821 PM MDT sun Aug 2 2015

Update...
issued at 815 PM MDT sun Aug 2 2015

00z kunr sounding showed substantial cap...even for MUCAPE. 02z
surface analysis had frontal boundary from south central Montana into
southeast Wyoming into west central Iowa. Overnight...weak east/southeast
low level jet will override boundary resulting in modest Theta-E
advection in a narrow band across far southern South Dakota. May be enough
to erase mucin as boundary layer moisture increases. Have
expanded smalls probability of precipitation a little bit overnight for these areas.

Rest of forecast in good shape.

&&

Short term...this evening through Monday night
issued at 300 PM MDT sun Aug 2 2015

Current surface analysis shows frontal boundary now from central
Montana through northeast Wyoming and across far northern Nebraska. Upper level
analysis continues to show ridge over the western Continental U.S....with
northwesterly flow over the northern plains. Monsoonal moisture is
in place over much of the western states and into the Southern
Plains. Skies are sunny across much of the area with temperatures in the
middle 80s to lower 90s across the plains...upper 70s to middle 80s in
the Black Hills. A few clouds remain across portions of northeast
Wyoming and southwest South Dakota...with isolated showers and thunderstorms to
the south of the County Warning Area near the frontal boundary. A couple of
buildups over the southern Black Hills have produced mainly
light showers/sprinkles so far. Winds are from the north to
northeast in most areas.

The hot and mainly dry weather pattern of the last week or so will
Cone to an end early this week as the ridge to the west breaks down
enough to allow a series of upper level disturbances to cross the
region. Chances for showers and thunderstorms will begin to increase
late Monday into Monday night...and especially on Tuesday.

For tonight...skies will remain clear in many areas. A low level
southerly jet will develop across western Nebraska and nose into
southwest South Dakota overnight. A shower or storm is possible over portions
of southwest into south central South Dakota overnight. Will add a slight chance
for showers and storms from Shannon County eastward to Mellette and
Todd Colorado. Lows tonight will be in the middle 50s to middle 60s with light
east to northeast winds in most areas.

Will see a very gradual increase in clouds on Monday from the
southwest as shortwave energy within the ridge moves northeast
across the Great Basin into the central rockies. The clouds and
weakening ridge will help to bring a slight cool down in
temperatures...with highs in the 80s on the plains and mostly 70s in the
Black Hills. Chances for showers and storms should remain limited
to portions of northeast Wyoming and southwest South Dakota in the
afternoon...with chances increasing a bit and spreading north and
east across much of the County Warning Area Monday night. The threat for severe
storms looks minimal on Monday at this point...but a strong storm
or two is possible across southern portions of the County Warning Area late in the
day and evening.

Long term...Tuesday through Sunday
issued at 506 PM MDT sun Aug 2 2015

Conditions become more active on Tuesday with daily thunderstorm
chances for the entire long term period. Latest models still
diverge on their solutions with the GFS showing on open trough over
the Pacific northwest closing off a low at 500mb over eastern
Wyoming. According to the GFS...this upper low becomes stacked at
the lower levels and tracks across southern South Dakota before
opening up over eastern South Dakota. The European model (ecmwf) keeps the system
as an open trough with a strong short wave passing through all of
South Dakota through Wednesday morning. NAM solution is similar to
the European model (ecmwf)...but keeps the strongest energy over northwestern South
Dakota instead of further south like the other models.
Regardless...have kept higher probability of precipitation in the forecast for Tuesday
into Wednesday. Quantitative precipitation forecast amounts will be tricky especially with the
model differences...but think some storms will have the potential
for an inch of rain in a short period of time. However...at this
time Don/T have confidence on where those areas may be.
Temperatures with this system will definitely be noticeable cooler
with highs struggling to reach the upper 70s. Even with the cooler
temperatures the area will be primed for thunderstorms as low
level easterly flow increases...boundary layer moisture and middle
level moisture surges northward from the Desert Southwest.
Cape/shear suggest some severe storms possible Tuesday...but main
threat will be locally heavy rain. This system on Tuesday will
linger into Wednesday morning before the next shortwave tracks
across northern Wyoming and enters the forecast area Wednesday
afternoon/evening. This wave is much weaker...but with moisture
already around...expected some isolated to scattered
showers/storms.

Models then show another shortwave for Thursday with the European model (ecmwf)
showing this wave stronger and further south than the GFS. At this
time will go with chance probability of precipitation...but lean more toward GFS solution.
On Friday the area appears to remain inbetween systems...thus should
see some slightly warmer temperatures again and maybe a chance for
afternoon convection with daytime heating. However...the extended
models try to develop an upper low over the Great Basin...
transitioning the area from zonal flow into southwest flow. This
should bring monsoonal moisture into the area. With energy ejecting
from the low...will see thunderstorm chances through the weekend.

&&

Aviation...for the 00z tafs through 00z Monday evening
issued at 506 PM MDT sun Aug 2 2015

VFR conditions expected through Monday afternoon. Elevated -tsra
may develop overnight across portions of far southern South Dakota.

&&

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

$$

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

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