Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
618 am CDT Wednesday Sep 2 2015

Short term...(today and tonight)
issued at 359 am CDT Wednesday Sep 2 2015

Showers and thunderstorms in our far east overnight have
moved well east of our area early this morning...and looking for a
hot and humid day across the area with upper level ridging
dominating throughout the central Continental U.S.. with 850 mb temperatures
nearing 30 degrees c over central South Dakota...will have highs in
the middle/upper 90s through our south central South Dakota zones where
mixing will be optimal. While still very will not be quite
as hot back to the east with highs in the upper 80s from the
Interstate 29 corridor and eastward. Dewpoints close to 70 over much
of the County Warning Area will only add to the discomfort of the very warm
temperatures...making it feel like lower to middle 90s across the
eastern three quarters of our County Warning Area. The heat index will be even
higher in our far west where deep mixing will bring drier air to the
surface...yielding dewpoints in the 50s through our western James
River Valley and south central South Dakota zones...but keeping the
heat index just below advisory criteria.

With the very warm airmass in place across the area...tonight will
remain warm and humid with lows well above normal for this time of
year...falling to around 70 overnight.

Long term...(thursday through tuesday)
issued at 359 am CDT Wednesday Sep 2 2015

Warm and humid conditions will continue to be the story into the
Labor Day weekend...with more unsettled pattern setting up by late
Friday as upper ridge begins to shift east...placing US in conveyor
belt of weak short waves traversing through southwest flow aloft.
Timing the various waves and associated precipitation chances difficult at
this range...but generally looks to favor nighttime convection with
somewhat drier conditions prevailing during the day. Friday through
Saturday...focus looks to remain across northwest portions of the
forecast area with proximity to ridge keeping northwest Iowa on the
drier side. Expect Thursday to the warmest of the late week period
with highs similar to today in the upper 80s to middle 90s. Warmest
temperatures expected in our southwest corner...but this area will
also see slightly lower dew points in the middle 60s which should hold
heat index values just shy of 100 degree advisory criteria. Farther
east in areas along/east of the James Valley...dew points likely to
be a degree or two either side of 70...but with temperatures topping
out in the upper 80s/around 90...heat indices also hold in the lower
to middle 90s. Temperatures expected to be a few degrees cooler Friday
and Saturday with increased cloud cover and slight precipitation threat...
but little change in dew points through this stretch. The humidity
and persistent southerly flow will keep overnight lows around 70
through Saturday night...or just a few degrees shy of normal high
temperatures for early September!

Precipitation threat will push east Saturday night through Sunday night as
western Continental U.S. Trough lifts out into the northern plains/southern
Canadian prairies. Cold front associated with this trough will push
through our area sometime in the late Sunday/Sunday night time
frame...though models still lacking consensus on timing of the front
with European model (ecmwf) the fastest...GFS in the middle but trending faster than
previous runs...and Canadian on the slower end of the spectrum.
While operational GFS is slower than European model (ecmwf)...many of its ensemble
members support the faster solution so will favor this over the
slower Canadian solution. Severe threat is questionable through this
period as forecast soundings are fairly moist with modest middle level
lapse rates and stronger shear lagging west of the more favorable
instability. May have to watch potential for heavy rain closely both NAM and GFS push precipitable water values back to
1.5+ inches...more than 150 percent of normal and into upper Echelon
of climatology for early September.

Cold front will bring an end to the rain threat for most by Labor
Day on Monday...with temperatures returning to near seasonal normals
of highs in the 70s and lows in the 50s.


Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 615 am CDT Wednesday Sep 2 2015

Patchy MVFR visibilities this morning...otherwise VFR conditions
expected through the remainder of the taf period. A low level jet
will develop across portions of the area late tonight...with low
level wind shear at ksux after 06z.


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


long term...jh

National Weather Service Glossary of Abbreviations