Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
713 PM CDT Thursday Sep 3 2015

Short term...(this evening through friday)
issued at 411 PM CDT Thursday Sep 3 2015

Upper ridging providing the hot and muggy conditions across the
central states will nudge slightly to the east tonight and Friday as
the first of several shortwaves takes aim at the central and
northern High Plains. For the majority of the region
tonight...mostly clear skies will persist overnight along with
southeast winds around 10 miles per hour. This will allow for mild temperatures
with lows a few degrees either side of 70.

Weak shortwave tracking northeast out of the central rockies lifts
into central South Dakota late tonight through Friday morning. With the low
level jet increasing overnight and plenty of elevated
instability...scattered to isolated showers and thunderstorms are
expected to form in central and western portions of Nebraska this
evening and eventually lift into south central South Dakota after midnight.
Activity tracks north northeast and dissipates by midday Friday. A
cannot rule out a few stray high based storms lingering in the
northwest quadrant of the County Warning Area into the afternoon...but with the
shortwave pulling away to the north...the main support mechanism
will be lost. Otherwise...another hot and humid day is on tap with
highs in the middle 80s to lower 90s...tempered some by increased
middle and high clouds.

Long term...(friday night through thursday)
issued at 411 PM CDT Thursday Sep 3 2015

Challenge through the start of the longer range deal with attempting
to time convective threat across the area. Tail of wave drags
through the northeastern County Warning Area on Friday evening...with overall little
fanfare as largely orphaned by main system lifting northward into
Manitoba. Could yet see a couple of rogue showers/thunderstorms
during the evening over the far north/northeastern County Warning Area...but have
generally cut back probability of precipitation for much of the night as area remains in
tendency for weak ridging...despite a persistence in the low level
jet and presence of a marginally unstable airmass. Models have
splashed a considerable deal of convection especially the last half
of the night right over The Heart of the County Warning Area...but with a lack of
focus with any boundaries well north and west and exit of the wave
early in the period...hard to justify too much of a broad brush.
There is leading forcing of lift from next upstream wave which
approaches the Missouri Valley late night...and have maintained
first chance in the South/West during this time.

Saturday again will work toward being a bit less warm with a greater
degree of cloud cover with middle level subtropical moisture along with
a weakly unstable middle level lapse rate...but high levels of humidity
are locked in for yet another day or two. Will have next in the weak
wave train lifting northward through the region and should force at
least a scattered coverage for showers and thunderstorms over the
northwest half of the County Warning Area. Lift

Going into Saturday night...will start to see influence of the main
western trough across the plains as low level jet gets one last
boost. With lower level boundary leaking toward the northwestern
County Warning Area...some better convergence likely to enhance precipitation threat
as lift forcing swings across during the evening...and have
maintained likely probability of precipitation which will work east/southeast through the
night as low level jet veers eastward with leading wave. While
severe threat still looks fairly meager...a few spots north of I 90
on Saturday evening may end up with just enough shear to favor a
more organized Mode...but still have concerns that instability will
come up shy of model expectations given the degree of cloudiness and
threat for precipitation through the day.

Off surface flow takes on a more southwest tilt on Sunday...and this
should act to limit precipitation threat after early day lingering
areas diminish east...until lobe of divergence q swings eastward during the
afternoon and early evening across the frontal boundary. Warm temperatures
aloft will help to cap deeper vertical motions for much of the
day...but steep lapse rates may try to keep some altocumulus castellanus around much
of the day...especially south/east. Should get some better coverage
late day as front pushes into parts of SW Minnesota/northwest Iowa...which will be
fairly quick to exit during the early evening. Again...with veered
winds...shear is diminished quite a bit and is more unidirectional
outside any local impacts near the boundary. Probably remains a
fairly small severe threat...but little better thermodynamics would
open up a conditional wind/hail threat a bit more over the eastern
County Warning Area late day. Much lower dewpoints push in behind frontal boundary
which should reach near the James River by midday...and into
northwest Iowa by early evening.

Cooler and less humid Labor Day as wedge of low level drying settles
in. As deeper trough wanders across southern Manitoba and Ontario by
early Tuesday...will get jet entrance working across the area. 850
hpa boundary and some middle level frontogenetic forcing will spread
back across the area and once can overcome the dry layer...will
probably find a meld of showers and isolated thunderstorms across
the area...especially south. Will not take long for this to push
through on Tuesday...perhaps lingering Tuesday evening in the far
south and brushing the south central with tail end later Tuesday
night. Otherwise...have coordinated removal of the precipitation
chances from Wednesday through Thursday given northern stream
dominance and invasion of dry air. Models have some fairly
significant timing differences and longwave progression after
Monday. Seasonably cool to mild temperatures expected Wednesday and


Aviation...(for the 00z tafs through 00z Friday evening)
issued at 712 PM CDT Thursday Sep 3 2015

VFR through the period. A few gusts from the south greater the 25
miles per hour tomorrow after about 16z.


Fsd watches/warnings/advisories...
South Dakota...none.


Short term...
long term...Chapman

National Weather Service Glossary of Abbreviations