Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1018 PM CST Sat Mar 8 2014
Short term...(this evening through sunday)
issued at 402 PM CST Sat Mar 8 2014
Still some high-end stratus across the northeast portion of the
forecast area at middle afternoon. Has been slowly diminishing in
coverage and thinning per visible satellite...and this trend is
expected to continue into the evening per forecast soundings/time
sections of moisture. Opting to hang onto some higher sky cover in
our far northeast at the start of tonight period...trending toward
mostly clear by middle-late evening...though possible the clearing may
occur a little faster than currently forecast.
Aside from that...temperatures and winds are main focus for the next
24 hours. Surface ridge axis moving through the southeast half of
the area and just starting to see more widespread southwesterly low
level flow organize across the west. This will become more dominant
through the evening as the ridge moves away from the area...and with
35-40kt flow developing at 925mb through the overnight hours...will
likely see strong elevation signature in winds/temperatures by late evening
through just after daybreak. Have been showing this in our forecast
in recent days and no reason to alter much at this point. Strongest
winds will be atop and just downslope of the Buffalo Ridge...in the
area which unfortunately also still has the deepest and loosest snow
cover across the region. Thus will continue mention of blowing snow
overnight as well...though will tempered somewhat late in downslope
areas where temperatures expected to climb near or above freezing
after evening lows in the lower 20s. Remainder of the area should
likewise see somewhat non-diurnal temperature trend...though not as
dramatic as downslope warming in our far northeast...with daybreak
temperatures largely in the 20s from the James Valley into northwest
Iowa...with lower 30s west of the James Valley.
Relatively mild start to what should be the warmest day of the week.
Given recent trends on warmer days...will continue to lean toward
warmer guidance across western half of the County Warning Area with no snow cover.
Overall little change here from previous forecast...with just a
slight bump upward in areas near western fringe of existing snow
cover...as well as in the downslope area around kmml/ktkc. Should
begin day with mostly clear skies...but models generally agree on
increasing middle-high level moisture across our north through the day.
Have largely kept sky cover in higher end of partly cloudy range...
but if more extensive than expected...this could be only thing to
potentially hamper warming a bit across our northern areas.
Long term...(sunday night through saturday)
issued at 402 PM CST Sat Mar 8 2014
Most mild night of the period likely to be Sunday night as will
have a few more middle to high clouds to deal with...and also the
passage of a weak boundary which will aid to keep things a bit
better mixed. Clouds may thin a bit late through the North/East...
but looking for a return and greater thickening of clouds during
the day Monday. Greater area will be snow free...but with some
taken off the temperatures aloft with passage of boundary and
clouds...expect that temperatures will be several degrees cooler than
Sunday in snow free areas...and fairly close to Sunday readings
for snow covered locations.
Most significant feature in the longer term of the forecast will
evolve on Monday night and Tuesday. Northern stream jet pushing
into the Great Lakes which helped drive the cold front southward
on Monday will also take up residence to the northeast...setting
up in strong confluence zone. Very strong frontogenetic forcing
develops on later Monday night and sags southeastward on Tuesday
across the County Warning Area. Unstable and dry to the south...and enough
instability in proximity to frontal boundary that have very high
level of concern for significant and intense banding. Thermal
profiles have very strong lift in the less stable and dendritic
portions of the column...and a longer portion of the profile in
the lesser shear portion down low where temperatures are highly in an
aggregation window. This could lead to the potential for some very
high rates...but fortunately for a somewhat brief window as band
is transient to the southeast. Thermodynamics continue to be
riding the edge of liquid/solid once again... gradually trending
over to mention of snow with time by later Monday night and
especially Tuesday morning. Some difference in low level air mass
noted between models in terms of saturation... and troublesome
handling of boundary layer will have impact on how to determine
change. A lot of dry air poised between saturating middle levels and
the surface layer...so will get some fairly effective evaporative
cooling with the persistent north to northeast flow as low
pressure develops in the Central Plains. For the most part...it is
the lowest 40-50 mb that is keeping precipitation liquid in the model
world for as long as it does. Have tended to side somewhat toward
the cooler side as a result of the persistent northerly flow and
intense vertical motion...and have forecast a band of two to
little more than three inches through The Heart of the County Warning Area.
For the remainder of the forecast period...some cooler air slides
in behind the system for Wednesday...and have shaded a bit cooler
for eastern areas on Wednesday morning and afternoon. Should do a
great deal of damage to any new snow on Wednesday with the high
moisture content. Rapid rebound to temperatures Thursday...which should
be the warmest day after Monday. Caveat will be potential stratus
for Thursday...but at this time with as much westerly component...
do not expect there to be an issue. Northwest flow aloft will keep
systems mainly diving toward the Great Lakes...and keep things in
a more mixy environment overall through Friday. Another system
digs and splits a bit more to the west on Saturday...and this
could create a bit more murky setup in terms of lower clouds by
the weekend with flow picking up an easterly component.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1017 PM CST Sat Mar 8 2014
VFR through the period. A small chance for a little low level wind shear late
tonight and early Sunday morning as winds at about 1200 feet above ground level
increase to 30 to 35 miles per hour from the southwest while the surface
winds should stay around 8 to 12 miles per hour. Should be enough wind at the
surface however to avoid any official low level wind shear.