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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
737 PM CDT Wednesday Aug 20 2014

Short term...(this evening through thursday)
issued at 331 PM CDT Wednesday Aug 20 2014

Plenty of complex interactions are occurring with the current
situation. First of all...a middle level short wave focused near 700mb
and an elevated warm front...has continually sparked elevated
convection in our southern zones...greatly stabilizing the
atmosphere through our southern and eastern zones. Synoptically...
the main short wave lifts to the northeast...well west and north of
our forecast area. One warm front sets up shop for much of the night
generally from the short wave in North Dakota...southeastward
through northeast South Dakota and into the northern parts of southwest Minnesota. To
the south...a second and primary warm front exists to the south of
the convection in Nebraska...where southern Nebraska is currently in
the 90s...and only middle 70s in Sioux City. Current thoughts are that
when the southern warm front lifts into our MO River Valley zones
this evening...that the portion of it roughly from Yankton...to
Sioux City and Storm Lake may be capped off due to the ongoing
stability. Therefore am initially focusing on the western portion of
the warm front near our south central South Dakota zones for a potential for
severe weather this evening...coupled with surface low pressure a
bit further west. It appears more and more that the storms may have
a difficult time getting tied to the surface or boundary layer...
lowering our chances for tornadic activity. But if storms were to
become based close to the surface...it would likely happen toward
south central South Dakota where instability to our west meets our humid air
around Chamberlain. Believe this is the location where an isolated
tornado could form this evening. Large hail is also certainly a
threat with enhanced updrafts...especially in our western zones this
evening where greater instability exists...and along the northern
wind shift in our northern zones and southwest Minnesota later tonight. In
extreme southeast South Dakota south of Sioux Falls...as well as most of
northwest Iowa and extreme northeast Nebraska...severe chances may be real
tough to realize due to the current stability...and their far
proximity from the main short wave.

Later...some stratus may develop due to some guidance from the
latest 12km NAM in our northern zones and into southwest Minnesota...but so
far left fog out of the grids. But just a heads up that fog could
develop in the very moist air generally along and north of I 90.
Skies will generally clear Thursday morning. By Thursday afternoon...
would not be surprised to see an isolated thunderstorm from Sioux
City northeastward in front of a slowly advancing surface wind
shift. And it remains very humid in our eastern zones so it will not
take much of a focus to fire off a few storms. In addition...the
Sioux City area will press heat index criteria for a couple of hours
Thursday afternoon...with a lot of 95 to 100 degree heat index
readings throughout northwest Iowa and southeast South Dakota.

Long term...(thursday night through wednesday)
issued at 331 PM CDT Wednesday Aug 20 2014

Anticipate most of the area remaining dry on Thursday night...though
with a lingering boundary located roughly through the Sioux City to
Spencer corridor cannot rule out an isolated thunderstorm early in
the night over our lower northwestern Iowa zones. It will be a mild
and relatively muggy night with lows in the middle and upper 60s.

On Friday will see increasing chances of thunderstorms...becoming
likely by Friday night...as low pressure deepens over the west
Central Plains and lifts northward...and synoptic lift increases
across the region as a shortwave ejects out of a western U.S. Upper
level trough. Does appear to be plenty enough instability and shear
around that severe storms will again be a possibility in the later
afternoon and evening. Apparent temperatures will be rather
unpleasant for Friday afternoon...with highs in the middle and upper
80s and dewpoints mainly lower and middle 70s...creating a heat index
of lower to middle 90s south of Interstate 90. Thunderstorm chances
then continue Saturday into at least early on Sunday as the
aforementioned low pressure and associated warm front lift up into
our area...though there are some model differences on how this plays
out exactly.

With the passage of the above mentioned shortwave...cooler air looks
to filter into the region...with below normal temperatures for
Monday through Wednesday. Monday may end up being mostly
dry...though model consensus would imply better precipitation chances
for Tuesday and Wednesday.

&&

Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 737 PM CDT Wednesday Aug 20 2014

Convective development at present on hold other than some spotty
high based showers over parts of SW Minnesota and northwest Iowa. Expect that
increase in low level jet later this evening should initiate
convection across southeastern South Dakota...and sufficient effective shear
to expect a few severe storms as well. MVFR to upper end IFR ceilings
and visibility potential remains for mainly kfsd and khon following
activity with low level flow weakening and moisture in place...but
should mix out by middle morning...with VFR conditions through 00z Friday.

&&

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

$$

Short term...mj
long term...jm
aviation...Chapman

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