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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
608 PM CDT Wednesday Sep 17 2014

Short term...(this evening through thursday)
issued at 306 PM CDT Wednesday Sep 17 2014

Picture perfect weather conditions across the region this afternoon
as temperatures have risen into the 70s and winds remain light.
Only minor forecast concerns through the daybreak hours. Minor
shortwave over eastern Montana will drop southeast later this
evening. Latest hrrr and hopwrf runs indicating this wave should
begin producing widely scattered showers as it moves into the area.
Generally these will be high-based showers...and given a rather
substantial dry sub-cloud layer...anticipate a considerable
amount of virga or a few sprinkles mainly west of I-29 through

The passage of this upper wave will allow a weak surface boundary to
drift northward early tomorrow morning..with low-level moisture
steadily advecting northward. Should see stratus development towards
daybreak...and cannot rule out patchy fog especially along edge of
advancing stratus. Low level isentropic plots show very weak lift
within the cloud layer...but feel lift too weak to support drizzle.
Clouds are expected to hang around for much of Thursday...gradually
lifting and scattering in the afternoon. Temperatures will remain
in the 70s.

Long term...(thursday night through wednesday)
issued at 306 PM CDT Wednesday Sep 17 2014

Little doubt that extensive stratus will expand/develop Thursday
night into early Friday with enhanced low level jet and strengthening
inversion. Fog a bit more problematic...likely favoring western
edge of the cloud area...and near surface southeast to south flow
providing increasing moisture. Sref probabilities of low clouds
are extremely high...but modest in the realm of lowest
visibilities...generally supporting this expectation. Not too
enthusiastic about precipitation chances...especially in the
evening. A little enhancement of lift on leading edge of the
warming temperatures aloft occurs across northeast Nebraska by around
06z...and could be just enough elevated forcing and weak
instability to shake out an isolated thunderstorm heading through
southwest Minnesota and northwest Iowa along elevated Theta-E
advection zone.

If not for a minimal chance of a shower or thunderstorm lingering
early Friday in the far east/northeast...Friday will be a capped
day as temperatures 800-750 hpa temperatures warm to +15 to +22c...with the
main battle of the day over maximum temperatures set between the
stratus and the southwesterly flow aloft. Trend has been to slow
down the eastward progression...with early day clouds at least
back to west of the James River...and perhaps holding tough into
very early afternoon toward I 29. Should eventually clear with
good push from westerly component ahead of advancing frontal zone
reaching the far western County Warning Area by late day. Enough interference
early in the day east to hold readings closer to 80 degrees...but
some areas to the west toward lower James and Lower Brule will
give a good run at upper 80s to near 90.

Precipitation should be exclusively Post frontal on Friday
night...somewhat scarce early on...but gradually taking hold with
deeper Post frontal qg lift...and perhaps a little middle level
weak contribution to moisture by the time deformation reaches the
southeastern half of the County Warning Area later at night. Not looking for any
severe storms with instability limited...and effective shear
reduced by nocturnal decoupling and the degree of westerly
prefrontal component.

Behind any lingering showers/thunder Saturday morning...looks to
be a well-mixed day...and with main cooling holding off until
secondary wave passage to the east on Saturday evening... will
likely be a fairly warm day...and last above normal for a few
days. Mixing would favor mainly upper 70s to lower 80s...along
with brisk midday and afternoon winds of 15 to 25 miles per hour.

For Sunday through Wednesday...looks like a fairly quiet time
as flow amplifies a bit into early week with western ridge and
eastern trough...then weakens and translates eastward as several
solutions indicate a western trough progressing into the area by
later Tuesday or early Wednesday. Would not be surprised to need
to add in a threat for shower/thunder for Tuesday night into
Wednesday...but timing and location of upper trough in flux at
this point and not Worth cluttering up the forecast...especially
considering the run to run significant changes in models over last
24 hours. Stretched out diurnal range just a touch on Monday and
Tuesday with dry surface ridge slipping through the area...and
maintaining a trajectory into the east on Tuesday.


Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 602 PM CDT Wednesday Sep 17 2014

Expect cumulus field to continue to dissipate through the early
evening...otherwise light southeasterly winds through the
overnight. Possible MVFR visibilities toward morning with
potential for stratus.


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


Short term...dux
long term...Chapman

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