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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
346 PM CDT Friday Jul 11 2014

Short term...(this evening through saturday)
issued at 346 PM CDT Friday Jul 11 2014

Last warm afternoon for a while today with temperatures in the 80s along
and south of I-90 and 90s to the north where clouds have cleared
out. Weak surface pressure trough has moved through northern
counties and is currently oriented NE-SW through Sioux Falls causing
a brief shift in winds to some northerly component behind it.
However...new short wave moving in from the west has triggered weak
cyclogenesis to the SW...and pressure falls associated with this are
backing winds farther south. Hence...winds in and around Sioux
Falls will likely stay light and variable this afternoon between the
two features with more of an easterly component farther south...but
still light. As a result of these continued light winds...
aforementioned cloud cover will likely have a hard time mixing out
in areas where it has not already.

As we head into this evening...a relatively pronounced low level
jet will develop across Nebraska. Currently appears based on model
guidance that nose of low level jet will remain south of our County Warning Area which makes
precipitation forecast difficult initially. Think that areas north of I-90
will almost definitely stay dry through the overnight...south of
I-90 more difficult to say. Have kept chance probability of precipitation in for this evening
but increased into likely category after 06z as a result of enhanced
low-level Theta-E advection along and to the north of the nose of
low level jet and surface boundary draped across Nebraska. Some guidance is
suggesting that bulk of precipitation stays south of Missouri River so have
highest probability of precipitation in our Nebraska counties and around Sioux City. Probability of precipitation in
Sioux Falls do not increase to likely until around sunrise although
the area between the river and I-90 is the most likely to bust.
Should also be enough elevated cape left late into the evening to
support thunderstorms as well...and with 0-6 km bulk shear vectors
around 30-40 knots...could be just enough storm organization for some
small or marginally severe hail in any storms that do move through
tonight.

Although northern areas will stay dry overnight...everyone will see
a chance of showers to start the day Saturday associated with DCVA
ahead of short wave. Shower threat is then exacerbated by cold
front moving in from the northwest. Models place front in our
northern zones around 18z and then push it southward through the
afternoon...likely clearing through Sioux Falls somewhere around
21z. Behind the front we will see cooler and drier weather and a
switch to northerly winds...but still some cloud debris left over
through around sunset. Highs should be in the upper 70s and lower
80s with warmest temperatures west where showers will clear out quicker
allowing for more heating.

Long term...(saturday night through friday)
issued at 346 PM CDT Friday Jul 11 2014

Cold front will be approaching from the north on Saturday night as
upper level low starts to amplify sliding toward the Great Lakes.
GFS a bit faster with progression...but not unreasonable given the
position of the jet. Wherever the interaction between southward
advancing front and jet entrance occurs will define the low end
chance for afternoon shower/thunder...with just an isolated high
based showers along the frontal zone further north during the
morning. Overall...have shifted this a bit further south and
later into the diurnal cycle on Sunday...but should be a short
duration threat at any areas south of I 90. Possible that north
edge of better development could bump up through the Missouri
Valley later Sunday night as main pv lobe starts to wrap southward
through Minnesota and interacts with the location of lower to middle
level frontal boundary. Instability is pretty weak...but enough to
mention thunder.

Main upper lobe wraps through on Monday as low closes off across
the Great Lakes. Main pv lobe will swing through earlier in the
day... with slower solutions perhaps holding on into early
afternoon in the southeast. Models will have to be awfully wrong
on thermodynamic structure to end up with enough depth to mixed
layer to involve any serious ice processes/charge separation...so
kept mention of precipitation free of thunder. Did go with a
diurnally weighted isolated shower chance in the eastern County Warning Area
closer to better upper support and coldest air aloft. After trough
axis drops through...will likely see somewhat lesser threat by middle
to later afternoon. Winds are likely underdone on what should be a
formidable mixing day...and have raised most areas 5-7 knots over
initialization...giving good potential for a gusty 20 to 35 miles per hour.
Full mixing supports only highs in the middle 60s to near 70...and
this ignores the colder European model (ecmwf) which would be a full 2-4 degrees
cooler. Statistical guidance showing strong signal to go cooler...
and operational MOS also should reside on the warm side due to
climatological impacts.

For Tuesday through Friday...a fairly quiet period with no
discernable precipitation threat as northern plains struggle to
recover from the cooler early week airmass. Northerly to northwest
flow will persist into Thursday...at which time some ridging will
attempt to build back into the northern plains. Tuesday will start
very cool...with some readings not too far above records on
Tuesday morning. A gradually moderation to temperatures expected...
with normal perhaps in sight after Friday.

&&

Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1244 PM CDT Friday Jul 11 2014

Mostly calm weather this afternoon before more trouble overnight.
Subtle surface pressure trough moving through has induced wind shift
at Hon...but with surface pressure falls to the SW being driven by
new short wave moving in...will be in a squeeze play between these
forcings so have kept wind mostly light and variable at taf sites
this afternoon. Some cumulus may develop this afternoon to northwest...otherwise
present cloud deck remains ceiling concern. Into tonight aforementioned
short wave approaches and combined with low level Theta-E advection
will trigger shower development after 06z. Should be enough elevated
cape left for periods of thunder as well. Have kept rain chances in
at sux into morning but all sites should begin to clear out by midday
with approaching cold front.

&&

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

$$

Short term...Connelly
long term...Chapman
aviation...Connelly

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