Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
1214 PM CDT Monday may 25 2015

issued at 1214 PM CDT Monday may 25 2015

Interesting afternoon in store with pretty aggressive clearing
working northward toward the southern County Warning Area...toward location of a
weak boundary which will anchor in for the afternoon from southern
Minnesota to northern Nebraska. As wave across northern Nebraska
wanders slowly eastward...should get a bit better push of storm
development along/north of the boundary than have seen thus far
this morning which developed on edge of drying aloft. Not much
to stop convection from developing once can get a bit of surface
based destabilization...which is somewhat concerning to keep
storms more isolated and allow a bit better instability to
develop. On a most optimistic case...can perhaps squeak out 1000
j/kg ml cape...and with the largely unidirectional shear of 30 to
40 knots...should be in the realm of getting some organized storm
structure. Watching location of boundary for maximum shear
potential and slightly enhanced convergence...and should define
better lower end severe threat today. Main threat northeastern
Nebraska into northwest Iowa...but may sneak a bit more northward
depending on progression of boundary.


Short term...(today and tonight)
issued at 324 am CDT Monday may 25 2015

Very challenging and problematic forecast in the near term today and
tonight. For starters we have fog...some of it rather the
eastern sections of southwest Minnesota and also in Gregory County.
Basically the fog in southwest Minnesota is due to stratus on the ground
from their higher terrain...while the fog in south central South Dakota was
originally aided by an upslope flow which is now beginning to wane.
But accounted for these areas by just going widespread fog into middle
morning...with only patchy or areas of fog elsewhere. With a dry middle
level layer...there could be some patchy drizzle almost anywhere
through middle morning. But for now decided to leave out as the
forecast already has a light shower and fog mention and did not want
to clutter up the grids and wording with too many elements.

Otherwise...our ever present large upper low is centered where the
Nebraska Panhandle...eastern Wyoming and northeast Colorado meet...with a secondary
circultion close to northeast South Dakota. This second circulation is causing
quite a bit of light shower activity at this time primarily north of
our forecast area. So all told for the morning hours...really not
expecting a lot to happen in our forecast area as we are between
short waves. A surface and low level wind shift oozes down in our
northern and western zones this afternoon behind the weak northern
upper low. This wind shift could provide a focus for thundertorm
development throughout our southeast zones late this afternoon and
early evening before dissipating. Therefore kept likely probability of precipitation going
generally along and south of a Tyndall South Iowa Great Lakes line
during those hours. Some of the cells could be marginally severe...
especially from Sioux City to Spencer and southward...with NAM ml
cape values between a 1000 to 2000 j/kg and 925mb lifted index
values -4 to -7c. Wind shear is not too impressive...especially in
the low levels...but 0-3km bulk shear vectors are averaging 30 to 40
knots in that area coupled with 0-3km helicity values of 100 to 120.

Tonight...the upper low opens up and moves into southern Nebraska and Kansas.
A general drying trend will take place from north to south with only
small probability of precipitation lingering after the evening hours generally along and
east of a Sioux Falls to Marshall Minnesota line. But honestly
from 06z to 12z Tuesday there will likely be very little going on in
our forecast area except for a few light showers in those areas.
Cloud cover could stubbornly hang on even in our northwest zones as
the various models are showing stratus. With the low level air
fairly dead...this could certainly be a possibility. Therefore kept
lows on the mild side...warmer than many consensus readings.

Long term...(tuesday through sunday)
issued at 324 am CDT Monday may 25 2015

Complicated and somewhat murky forecast continues into the extended
with subtle model differences leading to fairly substantial changes
in the forecast. Overall...appears to be a fairly activle period
with upper level low shifting to the east and a second system
working into the western US.

On Tuesday...cold front associated with the vacating systems moves
across the eastern County Warning Area. Models have slowed the progression of the
front. Focus for precipitation is mainly along and ahead of the front...and
have concentrated probability of precipitation across northwest Iowa into far southwest
Minnesota. Tall thin cape profiles leading towards effective
hydrometor development as opposed to any severe weather potential.

Tuesday night into Wednesday appear to be fairly quiet with a
surface ridge moving across the area.

By Wednesday night...trough to the west starts to approach as low
level jet develops across the plains leading to thunderstorm
potential across central South Dakota. This threat continues into
Thursday and Friday ahead of a cold front working south. Model
discrepancies continue...with the GFS stalling the front across the
area...while the European model (ecmwf) showing very strong front that would result
in well below normal temperatures for the weekend. For
now...trending lower with temperatures across the north....while
what to do with temperatures further south is more uncertain.


Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1214 PM CDT Monday may 25 2015

MVFR ceilings continuing to rise up toward lower VFR...with strong
clearing push into ksux area. Should be somewhat self destructive
with VFR cumulus expected to build in areas which clear to the
south. Thunder mention will be limited to kfsd and ksux...with
greatest in the latter. Very slow moving boundary and lingering
lower level moisture could produce another IFR fog/stratus episode
for mainly the ksux area later tonight.


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


short term...mj
long term...

National Weather Service Glossary of Abbreviations