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National Weather Service Sioux Falls South Dakota
256 PM CDT Sat Sep 20 2014

Short term...(this evening through sunday)
issued at 256 PM CDT Sat Sep 20 2014

Potent shortwave beginning to move across North Dakota this
afternoon as a strong 140+ knot upper jet noses into the northern
plains. Convection has broken out ahead of a stronger cold front
dropping southeast through the region. Further southwest...a weak pre-
frontal trough continues to edge eastward toward the James River
Valley ahead of the more potent front...with dew points dropping
into the 40s and temperature quickly mixing into the lower 80s.
Through the remainder of the afternoon...will continue to monitor for
additional development across mainly northern and northeastern zones
as the upper wave pivots through the area. Latest hi-res data
continues to indicate potential for tail end storms on the southwestern
flank of more organized mesoscale convective system moving through southwestern Minnesota late this
afternoon. Low instability and high shear setup would support low
topped convection...and even a few mini-supercells. The main risks
would be damaging winds given ambient flow behind the front and
steep low level lapse rates. Given thin cape profiles and progressive
nature of the activity...would lean against a large hail risk.

Any risk of convection will be limited to the very early evening
hours...with the effective front moving through the area by 7pm.
Anticipating a brief window of rather gusty winds as the front moves

High pressure will settle southward through the overnight hours and
Sunday will be very pleasant. Winds will remain from the north with
temperatures either side of the 70 degree mark.

Long term...(sunday night through saturday)
issued at 256 PM CDT Sat Sep 20 2014

Sunday night starts with high pressure and pretty clear skies over
the area. With light winds it should produce lows in the 40s and be
the coolest night in this stretch. Monday and Tuesday will bring the
slow approach from the SW of current Southern California trough cutting
through The Rockies ridge. This system is likely to bring half
decent middle level moisture but not too much lift and instability.
Timing puts the threat of showers and possible a very few
thunderstorms starting Monday night in southeast South Dakota...over the area
for Tuesday through Wednesday...and decreasing threat of
thunderstorms Wednesday night. Temperatures during this time of
plenty of cloud cover will be marked by a low diurnal range with
lows in the middle to upper 50s and highs mostly in the upper 60s to
low 70s.

The abating of the showers will take place with the passing of the
wave. GFS then shows a wave breaking off from the Canadian jet and
digging around rebuilding rockies upper ridge. The ec shows a broad
building of heights over the eastern two thirds of the USA with a
very weak inverted upper trough sliding slowly west across the
Central Plains south of our area. Because this feature is shown so
weak...this would tend to bring in dry air from the ridging to the
east. Will therefore go with dry weather especially since even the
GFS shows weak thermodynamics...including weak instability...with
its stronger system. Daytime temperatures Thursday through Saturday
would warm back fully into the 70s while lows would stay in the middle
to upper 50s.


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...dux
long term...

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