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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
550 am CST Sat Feb 6 2016

Short term...(today and tonight)
issued at 433 am CST Sat Feb 6 2016

Most immediate concern this morning is fairly widespread dense fog
which developed over the middle-section of the County Warning Area overnight. Seeing
some improvement as of 09z across northwest sections of the fog working into the Sioux Falls area around a
slightly stronger southwest flow begins to work into that area.
Looking at latest rap/hrrr/sref visibility projections...fog may
persist beyond current 14z advisory expiration. However...with same
slight increase in southwesterly winds expected there shortly after
sunrise...will stick with the 14z timing for now and monitor trends
to see if extension in time will be necessary.

After fog Burns off...expect relatively quiet period through the
rest of the today and tonight. Main concern shifts to winds...first
in southwest flow across higher elevations of southwest Minnesota
today...then general increasing low level flow later tonight as
strong low begins to approach northern Minnesota. For today...high
resolution models indicate enhanced pressure gradient in along the
Buffalo Ridge...resulting in localized stronger area of winds along
and downslope of the ridge. Cannot rule out some areas of drifting
snow at times...but do not anticipate visibility reductions in
blowing snow...especially with temperatures expected to approach or
exceed freezing by the time the winds reach their strongest this

For tonight...see stronger west-northwest flow begin to develop
above the surface...with 45-50kts indicated across the northwest
half of the forecast area by 06z-09z. However...forecast soundings
from both GFS and NAM show that the boundary layer remains a bit
decoupled from the stronger flow...with better connection remaining
to our north/west where pressure rises and broader cold advection
begin to aid in mixing. Thus while winds will be increasing through
the early not see speeds approaching headline criteria
prior to 12z Sunday. Will be quite a mild night though...with winds
and fair amount of boundary layer moisture holding temperatures in
the Lower-Middle 30s most areas through the night.

Long term...(sunday through friday)
issued at 433 am CST Sat Feb 6 2016

Most significant period of the forecast starts Sunday and continues
into Monday...associated with a period of increased winds as upper
level wave deepens rapidly dropping toward the western Great Lakes.
The pattern of pv wrapping on Sunday is especially concerning
heading into later Sunday afternoon and evening...indicative of an
extremely dynamic wave. Winds atop the mixed layer increase to 45 to
55 miles per hour by afternoon. A bit toward the stable side to start Sunday...
but this conditions will not last too long with tendency for
increasing lapse rates. Battle of two biases in low level wind
forecast...the NAM and its tendency to overly restrict mixing by
maintaining the cold pool over snowfall...and the GFS which can do
quite the opposite with an overforecast of off surface winds. Not
overly impressed at daybreak Sunday with potential for High Wind
Warning criteria...which will likely need this better near surface
mixing to initiate stronger winds aloft reaching the surface. This
does gradually become more favorable by later morning and especially
afternoon...with lapse rates approaching dry adiabatic even over
deeper snowcover. Shallow steep lapse rates will also contribute to
the development of banded snow showers. More favored area for snow
will sweep down across the northeastern half of the County Warning Area during the
afternoon and evening hours...but even the west should destabilize
enough for isolated to scattered snow showers. There had been some
concern that rain would be possible across southwestern areas...but
at this time appears as if temperatures aloft will cool enough to produce
ice...and the warmth of near surface layer will likely not be enough
to melt.

Deep snowpack in places...along with very strong winds and the
threat of new snowfall brings likelihood of some severely reduced
visibility...and even blizzard conditions. Much of current snow is
likely of limited use to degrade visibility...especially where temperatures
will reach well above freezing today. Also am expecting there to be
some need for a new snow aid any turbulent breakup
of snow crust. Visibilities certainly would be lowest during times
of snowfall...potentially worst during that period Sunday night
which times out with a secondary cold advection surge focused
especially through the eastern County Warning Area as intense low pressure drops
southward through Wisconsin.

Have maintained current watch status for high winds west and
blizzard east where snowfall potential is greater...but did extend
the blizzard watch through Monday morning as will continue to find
some scattered snow showers...and gusty winds 25 to 40 miles per hour cycling
through the region. Will have to keep an eye on the areas which
received heavy snowfall from last blizzard near lower Missouri
Valley...but am counting on compaction and lesser snowfall to keep
from reaching a true ground blizzard. Fairly inevitable that some
type of advisory will exist between the two watch areas.

Tuesday to Friday am generally dealing with existence in the tight
lower level baroclinic zone...which will waver some back and forth
as a couple waves traverse the northwest flow into the northern
plains. As a result...several pulses of warm air advection noted
across this thermal ribbon which could be enough to generate areas
of light snowfall...but models really exhibit little in the way of a
focus on either timing or location. Fortunately no latter week
system would appear to have any great not important to
try and hammer out details at this point. Some consensus of a much
colder push again Friday into Saturday.


Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 541 am CST Sat Feb 6 2016

Dense fog with LIFR-vlifr visibility/ceilings persisting mainly
east of I-29 corridor and along/south of I-90 at the start of the
period. Continued improvement from west to east is expected this
morning...but with VFR conditions expected to dominate by 19z.

Thereafter primary aviation concern for this taf period will be
increasing winds. Higher elevations of southwest Minnesota will
see frequent wind gusts above 25kt from the southwest by midday.
Increasing winds begin to spread over much of southeast South
Dakota and southwest Minnesota this evening...with westerly wind
gusts of 25-30kt prevailing after 06z. Stronger winds of 40+kt
at/above 1kft above ground level will also create areas of low level wind shear
for parts of southeast South Dakota tonight.


Fsd watches/warnings/advisories...
South Dakota...dense fog advisory until 8 am CST this morning for sdz071.

High wind watch from Sunday morning through Sunday evening for

Blizzard watch from Sunday afternoon through Monday morning for

Minnesota...dense fog advisory until 8 am CST this morning for mnz089-090-

Blizzard watch from Sunday afternoon through Monday morning for

Iowa...dense fog advisory until 8 am CST this morning for iaz001>003-

Blizzard watch from Sunday afternoon through Monday morning for

NE...dense fog advisory until 8 am CST this morning for nez014.



Short term...jh
long term...Chapman

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