Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
1158 am CDT Sat Sep 20 2014

Short term...(today and tonight)
issued at 324 am CDT Sat Sep 20 2014

Cold front has stalled across the southeastern forecast area this
morning...but is expected to sag across the remainder of northwest
Iowa through around middle morning before exiting the region.
Convection which fired along this boundary early this morning has
finally exited the forecast area. Further west...a band of high
based light rain showers with a few claps of thunder has formed
along a weak area of 700mb frontogenesis and is drifting southeast
across south central South Dakota. Models not handling this
precipitation well...but think that it will hold together through
around middle morning and will be mainly sprinkles or a few hundredths
at best.

Cold air advection will be slow to arrive today...holding off until
later this afternoon and evening in the wake of a secondary cold
front quickly dropping through the region. Highs today will remain
mild in the middle 70s to lower 80s. This front will be associated with
a minor shortwave tracking south southeast across the upper
Mississippi Valley. This fast moving wave may clip the northeastern
forecast area near the Buffalo Ridge. Weak instability in this area
will support some isolated shower or thunderstorm activity middle
afternoon through early evening...but severe weather is not
anticipated...and precipitation amounts will be on the light side.
Breezy northwest winds develop later this afternoon into the
evening...mainly across western and northern counties. Winds
decrease some overnight under clear skies...leading to much cooler
lows in the upper 40s to lower 50s.

Long term...(sunday through friday)
issued at 324 am CDT Sat Sep 20 2014

High pressure moves overhead on Sunday resulting in a pleasant day.
Should be mostly sunny with just some diurnally driven cumulus. Will be a
bit cooler as well with highs in the upper 60s to low 70s. Sunday
night will be the coolest night of the forecast period with good
radiational cooling allowing US to drop into the 40s. High pressure
moves off to the east on Monday...with large scale upper level
ridging building into the plains. However we will have to deal with
an upper low that will begin to meander into the plains. Think the
daylight hours Monday will generally br dry though...with mostly
sunny skies and highs in the low to middle 70s.

Forcing and moisture will increase Monday night into Tuesday as the
upper low slowly pushes east. This system will be fighting to
overcome a pretty dry antecedent rainfall progression
east will probably be slow. Thus on Monday night the majority of any
shower activity should stay along and west of the James River. The
threat of showers will push east across much of the County Warning Area Tuesday into
Tuesday night. Thinking this period ends up pretty wet with at least
off and on rain showers. As the trough axis slowly moves east and
moisture slowly decreases...shower coverage should decrease
Wednesday into Wednesday night...but still will probably be a few
around. Not seeing much in the way of instability with this kept the thunder mention limited. Future shifts may be
able to remove thunder altogether. With the cloud cover and showers
highs will be cooler Tuesday and Wednesday...mainly upper 60s to
lower 70s.

By Thursday into Friday we begin to see the upper low dissipate.
Just how quickly this happens is uncertain with the GFS and Gem
lingering the low nearby through the week. Either way would expect
rain chances to decrease...but cloud cover could be an issue if the
low remains nearby. Main uncertainty is how warm we are able to
get...for now kept consensus values of middle 70s to around 80 for
Thursday and Friday. However if the low does dissipate quicker as
the European model (ecmwf) shows...we would likely end up at least a couple degrees
warmer. The above normal temperatures still seem like they should
persist into the weekend.


Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1155 am CDT Sat Sep 20 2014

Winds will continue to remain west to northwesterly through the
afternoon hours. A potent upper wave will move through the
northern plains later this afternoon...bringing a stronger cold
front through the region. At the time...convection should remain
well northwest of the taf sites...but will have minor concerns
about brief gusty northwest winds behind the front into this

Once the front passes through...VFR conditions will remain into
Sunday morning.


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


Short term...
long term...chenard

National Weather Service Glossary of Abbreviations