Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
623 am CDT Monday Sep 1 2014
Short term...(today and tonight)
issued at 428 am CDT Monday Sep 1 2014
Upper low over the County Warning Area early this morning generating isolated to
scattered showers. These are in a weakening state...and thinking
should generally be dry by 12z. Will likely see some cumulus and
eventually some cirrus moving in from the west today...but should
generally be a partly to mostly sunny day. Based on thermal fields
and expected sunshine...did bump up highs a bit...looking for middle to
upper 70s over the area. A wave currently entering Wyoming will
quickly push east today...moving over our area later this afternoon
and evening. Pretty good upper level forcing with this wave...but we
lack a low level forcing mechanism. Cap is relatively small...but
without this forcing mechanism...think it will probably hold over
most of the area this afternoon...keeping it mainly dry.
There does appear to be an area of locally enhanced convergence
across western Minnesota...and hi res models are all showing
development in this area. Think most of the showers and storms in
this area will probably stay northeast of our County Warning Area...but included
some low probability of precipitation near Marshall. Other area where storms may develop
this afternoon is across the Western Plains. These will track
east...possibly approaching Chamberlain and Gregory by around 0z.
Thus also will carry some probability of precipitation in this area. Given minimal
cape...not much of a severe threat...although could be some strong
winds with the activity across the Western Plains...possibly getting
into our south central South Dakota counties. Think most of the
activity will dissipate after sunset...and thus will keep the
forecast dry in between the two areas mentioned above. Although if
we heat enough...not out of the question we could see an isolated
thunderstorm just about anywhere. Skies clear tonight...but we may
hold on to just enough of a breeze to prevent temperatures from falling too
low...though still expecting middle 50s.
Long term...(tuesday through sunday)
issued at 428 am CDT Monday Sep 1 2014
Tuesday will overall be a pleasant day across the area...as
surface ridge slips southeast...with zonal flow working toward
weak ridging. Potential for middle level boundary to get caught up
just to the south of the area during the day...so could get a few
harmless middle clouds across the south...and a spillover of a few
higher clouds into the northern areas. Highs will be seasonable in
the upper 70s and lower 80s.
On Tuesday night...increase in middle level warm air advection and
push of the elevated mixed layer into the southwestern County Warning Area. Just
enough convergence and weak qg forcing to be concerned toward the
latter half of the night...as boundary pushes slowly northeast.Added
in an isolated level pop for elevated showers north of a Huron to
Sioux Falls to Spencer line...but looks like any thunder in a 50
j/kg maximum cape environment would be even more rogue than the showers.
Kept a small lingering chance during the morning across southwest
Minnesota and east central South Dakota...but boundary should not wait
around for long as weak wave moves past.
Will likely have some stratus building northward on Tuesday night
across Nebraska...with a late night pulse in coverage as higher
dewpoints sneak in below warming temperatures aloft. This stratus is
almost unanimously underdone in model solutions...and have some
concern that it will not go away quite as quietly as NAM/GFS/
Canadian/European model (ecmwf) suggest. Potential is there to mess up what will
likely be the final of the warmer days for a while...at least
across SW Minnesota and northwest Iowa...while looks a bit more certain with
veering low level flow that the west will break out more readily.
For now...have some readings approaching 90 in the west...and have
lower to middle 80s in the east. With the warmer and much more humid
air mass...thermodynamics profiles are concerning given the
3000-3500 j/kg mean cape between operational models...until
considering that middle level boundary pushed northeast during the
morning...and there is a good solid 75-150 j/kg cap on things...
along with next to no larger scale lift forcing.
In fact...should be capped enough that as strong wave crashes
into the Flat Ridge on Wednesday night...initiating cyclogenesis
to the west and north and forces a strong cold front through the
Dakotas...this front should remain largely inert overnight and
even well into Thursday as it pushes into northwest Iowa by
afternoon. NAM remains consistent with track record as slowest...
GFS fastest...and others varying pace between. Only by Thursday
afternoon in the southeast County Warning Area as more organized area of Post
frontal q vector support swings out across the boundary. Should
get a good increase in precipitation along and Post frontal during
the evening...which will then build south as strong low level
northerly drying with ageostrophic support of jet entrance.
For the most part...have left the remaining three days of the
extended dry as surface ridge to the north wedges a good dose of
low level dry air into the region. Several solutions attempt to
bring another Post frontal wave with a little middle level moisture
pooling around the start of the weekend...but with models
uncertain on timing on top of this low level dry air...did not
make sense to add in any lower chance probability of precipitation at this time. Period of
Friday through Sunday will remain cool...with highs in the upper
60s to middle 70s.
Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 620 am CDT Monday Sep 1 2014
Generally expecting VFR conditions through the period. Could be
isolated to scattered thunderstorms this afternoon and
evening...greatest coverage west of the James River. Confidence on
coverage and timing too low to include mention in the tafs at