Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
637 PM CDT Tuesday Sep 30 2014
Short term...(this evening through wednesday)
issued at 341 PM CDT Tuesday Sep 30 2014
Potent vertically stacked upper low continues to lift north
northeastward across west central South Dakota. Warm frontal
boundary is currently draped near the James River Valley area while
a cold front extends southward across south central South Dakota into central
Nebraska from a triple point to the north of Chamberlain. Narrow
band of light to moderate rain just east of Interstate 29 continues
to trudge slowly to the east. Some weak instability has developed
over eastern Nebraska and should lift northward into the area later
this afternoon through the evening. Per the latest radar
loop...shower activity has weakened across southwest Minnesota.
However...a more robust area of showers with embedded storms is
lifting into northwest Iowa and should sustain continued
precipitation in our east through much of the overnight hours as the
warm front nudges northward. With rather limited instability...will
go with isolated thunder mention and no severe threat.
Further west...dry air continues to stream into south central South
Dakota along the dry slot. Dewpoints are rapidly dropping into the
upper 40s and lower 50s just behind the front. Clearing has allowed
temperatures to reach the 70s west of the James River with some
cumulus developing near the front in south central South Dakota. If
cumulus can get going further north near the triple point along the
moisture gradient...scattered thunderstorm development will become
likely. Any storms that develop in this small corridor where very
steep lapse rates and the greatest instability exists - mainly in
Beadle...Sanborn and Jerauld counties - low topped severe supercells
will be possible. Hail to half dollar and gusts to 60 to 65 miles per hour will
be the main threats. Near the boundaries/triple point
particularly...shear profiles suggest isolated tornadoes will be
possible. Again...with such a narrow area of concern...any storms
that develop this afternoon and evening will exit the area quickly.
Have also issued a Wind Advisory for our south central South Dakota
counties...with gusty winds working into Gregory...cm and Brule
counties early this afternoon. Chamberlain gusted to 44 miles per hour
earlier...so expect some gusts around 45 miles per hour in this area through
Cold front will make very little progression to the east overnight.
On Wednesday it should slowly sag across the forecast area through
the day. Ahead of it...mostly cloudy skies will persist. Scattered
to occasional rain showers will remain possible in northwest Iowa
into south central Minnesota throughout the day. Not much
convergence or support along the front for thunderstorms...but will
continue to mention isolated thunder with this activity. Look for
highs on Wednesday in the low to middle 70s.
Long term...(wednesday night through tuesday)
issued at 341 PM CDT Tuesday Sep 30 2014
Wednesday night will generally start out dry...with increasing rain
chances later at night into the day Thursday. Models have trended
north with the next wave pushing across the plains...and have thus
increased probability of precipitation across the area. Followed a blend of the European model (ecmwf)....GFS
and Gem...with the 12z NAM an outlier. Through 12z Thursday...most
shower activity should stay near the Missouri River...spreading
northeast on Thursday. Highest probability of precipitation are across southeast
locations...but will need to continue adjusting up if trends
continue. Mainly expecting just rain...although could see some
isolated thunder southeast of a Yankton to Windom line...where some
weak instability may exist. With the increased rain chances and cloud
cover...lowered highs Thursday a few degrees...with 60s expected.
One more strong upper low will impact the area on Friday...diving
southeast out of Canada. This will bring colder and windy
conditions. Winds will likely pick up Thursday night as cold air
advection kicks in...and remain strong into Friday. Still some
uncertainty on the exact track and evolution of the upper low. GFS
is an outlier with this system...and thus favor the more similar
European model (ecmwf)...Gem and NAM solutions...which bring the upper low further
southwest towards our area. So some uncertainty on where strongest
winds set up...but seems probable that Wind Advisory criteria could
be met over at least a portion of the area...with winds of 25 to 35
miles per hour gusting 40 to 50 miles per hour. With the upper low closer...will have more
cloud cover to deal with as well...with isolated to scattered
showers also a possibility. All of this combined with highs only in
the low to middle 50s...will result in an unpleasant Friday.
Trough axis pushes east of the area Friday night allowing for
clearing and lighter winds. Will be a cold night...although
currently thinking winds stay up just enough to prevent temperatures
from really tanking...and thus some of the guidance seemed too low.
Either way 30s are likely...but assuming winds stay up...most of the
area probably will not hit freezing...but something to watch.
Saturday into early next week will see ridging slowly begin to build
back in. Although a few waves passing to our northeast in the
northwest flow will likely slow its progress. This means that
warming will be slower than earlier thought...with Saturday in the
50s and 60s...and Sunday and Monday mainly in the low to middle
60s...with lows in the 40s. Precipitation chances are generally low
over the weekend into early next week...although Sunday could see a
few showers depending on how close the wave to our northeast gets.
At this time looks like highs return to slightly above average
levels by Tuesday.
Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 638 PM CDT Tuesday Sep 30 2014
Lingering rain may continue to move northeast in the far eastern
part of the area in southwest Minnesota and northwest Iowa through 12z.
Otherwise low level moisture over Minnesota and Iowa looks set to back up
slowly west with low cloud deck solidifying in the 1k to 2k foot
range. This should solidify the low cloud deck at fsd and sux
before 03z...with the low clouds...preceded by VFR...likely
getting into Hon for a few hours about 09z. This backing up of the
low level moisture seems already evident with the stalling of the
dry air boundary as the main short wave moves north away from the
western part of the area. At this time do not expect any
visibility restrictions in fog as the low level flow should hold
up enough to prevent this. A combination of warming with some
drying from the south should break up the low ceilings over most
of the area back to VFR during the 15z to 18z period.
South Dakota...Wind Advisory until 7 PM CDT this evening for sdz050-057-063.