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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
610 PM CDT Monday Sep 15 2014

Short term...(this evening through tuesday)
issued at 335 PM CDT Monday Sep 15 2014

Overnight period will be dominated by large ridge of surface high
pressure as it settles southeastward through the region. Winds will
diminish rapidly this evening...and lingering stratocumulus field
should be gone by around sunset. The core of ridge settles into
northwest Iowa later tonight...with light winds and clear skies. Dewpoints
both high enough to support condensation...and trending low enough
to allow for cooling temperatures to a point where some patchy frost would
be possible later night across mainly northwest Iowa where winds will remain
lightest...with some middle 30s in play for locations southeast of a
Jackson Minnesota to Sioux City Iowa line. Would also anticipate that some
near open water locations could find some shallow patchy...but not
enough to warrant mention.

Development of return flow will start to pull some of the moisture
which pools to the south back northward through the day on Tuesday.
Will likely see a few clouds early on spreading toward the western
County Warning Area...but a better coverage of clouds by middle afternoon across
southeast South Dakota. Despite this...will see temperatures moderate quite
a bit from the chilly Monday...with mainly upper 60s east to lower
to middle 70s west.

Long term...(tuesday night through monday)
issued at 335 PM CDT Monday Sep 15 2014

The main challenge with Tuesday night and Wednesday is figuring
out the potential for stratus. The various models continue to show
a stripe of very high 850-800mb moisture streaming northward into
our forecast area beginning Tuesday night. The moisture then
nudges slightly eastward through the night maximized near Interstate
29...then possibly lingers in our eastern zones on Wednesday at
least as broken cumulus ceilings. Between the southerly surface
flow and potential clouds...raised low temperature values along
the I 29 axis Tuesday night when compared to MOS/consensus values.
The warmer bias corrected values looked better. However
conversely...lowered the lows a bit from guidance values in our
far western and eastern zones where sky cover may remain clear to
partly cloudy. On Wednesday...relied a lot on the bias corrected
raw values which seemed to have the best handle on potential cloud
cover in our eastern zones...holding zones east of I 29 to near 70
and putting our south central South Dakota County areas in the upper 70s.
Left conditions dry for now as it appears the moisture depth is
not deep enough in our forecast area to produce measurable
rainfall...and winds will be pretty light Tuesday night and

Continued a dry trend Wednesday night and Thursday with similar
temperatures. Our eastern zones are still under the influence of
the cool high pressure system situated over the Great Lakes. But a
brisk southerly flow begins to take shape from the James Valley
and points westward which will keep them well into the 70s.

For Thursday night...not real confident about the slight chance
and chance probability of precipitation we have going east of the James River Valley.
Chances are certainly not Worth more than 30 percent and are based
purely on warm air advection return. At this time cannot totally
rule out a few elevated thunderstorms...but would also not be
surprised if future forecasts lower these chances even more. Dew
point values will be increasing dramatically by this time with the
brisk southerly flow...likely holding low temperatures in the middle
60s throughout locations west of I 29...slightly cooler in our

Then on Friday and Friday night...the deterministic GFS and European model (ecmwf)
continue to be in good agreement in moving the upper ridge off to
our east...and pushing a cold front through this area Friday
night along a strong short wave/upper trough. Friday looks very
warm and humid ahead of the front. For Friday night...raised the
probability of precipitation from the all blend chances for nearly all locations as these
two models have really locked onto this period for rain and
thunderstorm chances the past couple of days now. Also increased
the thermal low temperature gradient Friday night between our
northwest and southeast zones as the all blend more or less washed
everyone out into almost the same temperature. Therefore blended
some raw GFS and European model (ecmwf) lows into the all blend to come up with
these our western zones lows in the 50s...and our
eastern zones 60s.

Aside from some possible lingering showers in our eastern zones
with the frontal passage early Saturday...the rest of the extended
looks dry Saturday through Monday. That said...the deterministic
European model (ecmwf) and GFS are showing big differences by Sunday and Monday
with temperatures. The European model (ecmwf) is deeper and slower with the cold
upper trough movement to the and east and northeast of our area.
This of course in turn keeps cool high pressure draining down into
the northern plains from Canada...much cooler than the GFS which
actually begins a return flow by Monday. For now...compromised
between the two by cooling off the all blend highs and lows Sunday
and Monday as many times the GFS is too fast in exiting cool
air...witness today. But on the other is also quite
possible the European model (ecmwf) is too sharp with the Great Lakes upper trough
and producing too much cold air. Will re evaluate again Tuesday.


Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 609 PM CDT Monday Sep 15 2014

Could be some patchy shallow fog along rivers and streams around
daybreak...but otherwise VFR conditions expected through the


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


Short term...Chapman
long term...mj

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