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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
204 PM CDT Sat Aug 30 2014

Short term...(today and tonight)
issued at 437 am CDT Sat Aug 30 2014

Isolated showers ahead of weak northern stream wave in west central
Minnesota have come to an end across SW Minnesota early this morning...and now
watching mass of stratus spilling southward across the northern
plains. Through at least midday...this stratus should be quite
persistent over much of the County Warning Area...eroding in the vicinity of the low
level ridge axis near the James Valley. Clouds may be somewhat more
persistent east...stymied by fairly weak flow toward i29 and
southwest Minnesota...but should see a better decreasing trend by
afternoon. This will impact potential highs today...which will
likely range from middle to upper 70s east...to middle 80s west.

Heading through the night...energy across the Pacific northwest will
begin to nudge out into the northern plains. Falling pressures to
the west will increase southerly wind flow...especially off surface
as a 40 to 50 knot low level jet develops by late evening...veering
toward the fsd County Warning Area by the final half of the night. Air at this level
remains fairly dry...with moisture source coming more aloft near or
above 700 hpa. All precipitation tonight is dependent on high based showers
and thunderstorms developing in warm air advection on nose of
stronger elevated mixed layer actually being significant enough to
break through the drier layer below. As initial pv lobe lifts out
across the area after 06z...could see at least a lower coverage of
precipitation...but certainly does not warrant more than a low
chance at this point with the degree of dry air at lower
levels...and with better convergence of the low level jet to the
north. Shaded the highest chance by later tonight across the Highway
14 corridor.

Long term...(sunday through friday)
issued at 437 am CDT Sat Aug 30 2014

Focus of this forecast is certainly on thunderstorm potential and
associated severe weather risk Sunday...during the middle of the
Holiday weekend. NAM seemed to be an outlier in pattern timing
and/or location, so have largely favored a GFS/European model (ecmwf) blend. With
this...anticipate any early morning elevated convection to lift
north of the area/diminish through middle morning...with primary
focus then shifting to southeast portions of the County Warning Area as one of
many waves slides through the middle level trough moving into the
northern plains. Location of low level boundaries may be impacted
by any morning convection...but on broad scale should see cold
front into western County Warning Area by midday...with warm front location a
little more uncertain. Preferred models point to strongest low
level warm advection along 850mb warm front extending into eastern
Nebraska at midday...then lifting northward into northwest Iowa
with the middle level wave by late afternoon/evening. This wave
accompanied by strong deep layer shear of 40+kt and MUCAPE values
in excess of 2500j/kg. Forecast soundings show a weakening but
still existent capping inversion above 850mb at ksux and other
points in our County Warning Area at 21z...but the approaching wave should help
overcome this...with the cap allowing for more discrete initial
cell development in the late afternoon to early evening. This
should be time of greatest severe potential... with large
hail...damaging winds and isolated tornados all possible.

Thereafter system should congeal into progressive cluster of storms
through the evening...which then shifts east of our forecast area by
early Monday morning. Due to expected progressiveness of system...do
not anticipate widespread flooding threat from training storms...but
isolated mainly urban flash flooding possible with deep moisture
providing potentially heavy rainfall rates.

Labor Day should be quiet and cooler with highs back into the 70s.
European model (ecmwf)/Gem both showing another weak wave tracking through the region
and bringing a chance of storms to the area Monday night.

Longer range Tuesday through Friday looks to start off as a fairly
quiet period...with cooler highs in the 70s Tuesday warming back
into the 80s Wednesday/Thursday. Fair consensus between the GFS and
European model (ecmwf) showing an increased precipitation potential by the end of the week.

&&

Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 158 PM CDT Sat Aug 30 2014

Stratus along and east of the James will be quite stubborn to
erode...but should fall apart with sunset this evening. With low
level moisture not able to adequately mix out...could see low
clouds and fog develop along and east of I-29 tonight. This may
impact both kfsd and ksux terminals. Could be some elevated
convection that develops after 09z...though expect covereage to
remain fairly low so have not worked into forecast at this time.

&&

Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&

$$

Short term...Chapman
long term...jh
aviation...

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