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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
633 am CDT Tuesday Sep 2 2014

Short term...(today and tonight)
issued at 403 am CDT Tuesday Sep 2 2014

Wave will exit to our east this morning...giving way to a mostly
sunny day across the region. Temperatures aloft continue to
warm...but as flow turns more southerly...do not think mixing will
be as good. Either way...most guidance seems too low given model 850
and 925 mb temperatures...thus continued on the warm side of
guidance. Looking for upper 70s to low 80s across the area...which
is around or just above average for this time of year.

Low level jet picks up tonight...increasing Theta-E advection and
isentropic lift over the region. Instability will increase as a
result...although expect the lower levels to remain capped as
warmer temperatures aloft advect in as well. Thus looking like any
convective activity will probably end up based around 800 mb or
so. Model consensus suggests best chance of seeing some elevated
shower or thunderstorm activity will be along and northeast of a
Huron to Sioux Falls to Storm Lake line. Based on low level jet
location and orientation this makes sense as well...so focused
probability of precipitation in this area. Think activity will probably just remain
isolated to scattered between 3 am and 7 am. NAM seems overdone
with instability originating from 800 mb...suggesting over 2000
j/kg. GFS and European model (ecmwf) solutions seem more reasonable...with 1000
j/kg or less of elevated cape. Storms will be based above the best
shear as well...so other than maybe some small hail...would not
expect anything too strong through 12z. With southerly flow
increasing overnight...warmed lows a few degrees into the low to
middle 60s.

Long term...(wednesday through monday)
issued at 403 am CDT Tuesday Sep 2 2014

Likely that isolated to scattered nocturnal elevated convection
will be ongoing at the start of Wednesday over the northeast half
of the County Warning Area. NAM solution rejected for several reasons...not the
least being some very unrealistic thermodynamic profiles...which
are allowing a much further south development into the approaching
cap. Will watch middle level focus for this convection pull off to
the northeast during the day. Other issue with Wednesday will be
threat for stratus development early morning to persist longer
into the day in the east to greatly impact temperatures. East of
I 29 likely to see very limited sunshine into early afternoon...
perhaps even a bit more pessimistic than going forecast. Areas to
the west will be poised to hold on to or break out into sunshine
much earlier...with temperatures varying from lower 80s east to lower 90s
west by afternoon. Very strong cap indicated by Wednesday
afternoon...so hard to picture being able to continue any threat
of convection past a small chance midday for evacuating high based
storms on nose of the 7-9c/km middle level lapse rates.

Strong Pacific northwest wave will start to knock down the flat ridging
across the northern plains by later Wednesday night...with
cyclogenesis across the northern plains...dragging a fairly strong
cold front toward the I 29 corridor by early Thursday morning.
Boundary strongly capped and convergence lacking...with deeper
dynamical lift forcing lagging behind with middle/high level
moisture. Prospect for any meaningful precipitation chances on
later Thursday and Thursday night are really not there...with just
a minimal threat for a higher based shower or even less likely a
thunderstorm along with divergence q axis slipping southeastward behind
boundary. Low level frontal boundary is very well removed to the
south of the area by later afternoon...with perhaps the leading
hint of lower based development by very late afternoon or early
evening along the far southeast corner of the County Warning Area.

The period from Friday through Sunday will transition back to
northwesterly flow...with a large cool area of high pressure
settling through the northern plains. This will drive boundary
southward...and other than a remote chance for a High Plains
development to brush the far southwest County Warning Area with an ill-timed
weak wave around Friday or Saturday...will be dry and quite
cool...with highs in the upper 60s to lower 70s...and lows in
the middle 40s to lower 50s.

Transition back toward zonal to weakly southwest flow toward the
end of the period...with low level southerly flow increasing along
with moisture content of air. With northern stream wave inducing
increase lower to middle level Theta-E advection by Sunday night and
Monday...continued to indicate a lower chance for thunderstorms.
Temperatures start to moderate with more clouds and increased moisture...
but still remain below normal through Monday.

&&

Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 628 am CDT Tuesday Sep 2 2014

VFR conditions expected today. Later tonight...could see isolated
to scattered thunderstorms...mainly along and northeast of a Huron
to Sioux Falls to Storm Lake line. However coverage is expected to
be too low to include in tafs at this time. Also will have to
watch the low level jet tonight. Could be near low level wind shear at khon...but
currently looks like it may stay just short. This increasing
southerly flow will also advect moisture north...and could have
some low stratus or fog by morning. Although at this time looks
like probability of this through 12z is low enough to keep out of
the tafs...but future shifts will have to monitor model trends.

&&

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

$$

Short term...chenard
long term...Chapman
aviation...chenard

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