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FXUS63 KFSD 080451

National Weather Service Sioux Falls SD
1051 PM CST Sat Dec 7 2019

Issued at 926 PM CST Sat Dec 7 2019

Temperatures struggling to fall this evening with strong southwest
flow just above the surface. Expect temperatures to fall, mainly
after midnight as trough moves through the area and winds decrease
aloft. Have raised forecast lows a few degrees, otherwise forecast
is on track. 


.SHORT TERM...(This Evening through Sunday)
Issued at 407 PM CST Sat Dec 7 2019

High clouds have thinned for much of the area this afternoon, and 
as a result, a very mild day on the upper end of the guidance 
distribution due to slightly deeper mixing into the sharp inversion 
and more wind, a predictable and common feature of warm days of late.
Unfortunately, that tidbit will be of little use over the next week 
as we transition into a much colder regime.

For tonight, breezy winds will continue into the evening, but will 
gradually give way to a decreasing surface gradient with pre frontal 
troughing, leading to decoupling favoring lower elevation locations 
especially with snow cover later night. Will have to see how thinner 
times in clouds play into these complex trends. Otherwise, a weak 
wave will move through this evening, and will start the southward 
push to a frontal boundary which will be getting close to Highway 14 
by mid morning.

This boundary will really act more like a density current than a 
dynamically driven boundary heading through the day, to the 
progression to the colder air will be somewhat slower, and likely 
not a great factor to the diurnal trend through the lower Missouri 
river valley. A widespread stratus field will accompany the stronger 
push of low-level cooling expected to start by midday in the 
northern areas, with a steady and slow progress through the day. The 
saturation depth is fairly meager at 2-3kft at most, so even as 
there gets to be some warm advective lift developing toward evening 
atop the moist layer, not really a high threat to get much of any 
freezing drizzle going in the shallow layer through at least the 
daytime period. 

.LONG TERM...(Sunday Night through Saturday)
Issued at 407 PM CST Sat Dec 7 2019

The development of precipitation will come in two ways heading 
into Sunday night, 1) potentially some light freezing drizzle with
the increasing lift in the shallow near-surface layer, and 2) 
light snowfall as the depth of saturation increases with the 
approaching mid-level frontal boundary and incorporates ice 
bearing temps. The better forcing continues to trend slower, along
with the deep-layer cooling, so will have slower onset to 
snowfall most areas compared to previous forecast. There may be 
some spotty light freezing drizzle that develops ahead of the 
deeper saturation, but should not be too significant. The main 
forcing associated with the developing snow band will accompany a 
sharpening wave diving into western SD toward midnight, and 
exiting southwest MN before midday Monday. Not the most efficient 
thermal profiles for snowfall with lift somewhat outside the best 
DGZ, so kept ratios somewhat lower than initial blend. Winds will 
become stronger later into the night and especially Monday morning
with 25-30 kts in mixed layer, so could be a few spots of 
blowing/drifting snowfall as well.

Amounts continue to trend lighter in terms of snowfall, and 
ensembles seem to be settling in on the better risk area with a 
slight northward shift. At this point, accumulating snowfall of an 
inch or more should be limited to north of I-90, with up to 2 inches 
near Highway 14.

Following the system, the main message is the colder air through the 
midweek period.  The initial bubble of arctic air settles into the 
area by Monday evening, but will soon be followed by another 
reinforcing wave/front Monday night. Would not be shocked to at 
least see some flurries or even brief very light snowfall on Monday 
night into early Tuesday with the secondary wave/frontal zone. The 
main chunk of cold air settles in Tuesday into Tuesday night, and 
will likely be the coldest air in place through at least Wednesday.
Wind chills occasionally from -15 to -30 across the area, and would 
not be shocked to need an advisory in some areas around Tuesday 

Still remain some differences in timing of waves/reinforcing pushes 
of colder air by Wednesday, so hesitant to make too many changes to 
initial grids beyond Wednesday. Does appear to moderate temps late 
week, and in the transition could easily see a couple periods which 
favor some light snowfall or flurries with such cold air in place 
during periods of warm advection, but timing precludes mention at 
this point. Perhaps a better chance for snow by the weekend, as flow 
becomes more zonal.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1045 PM CST Sat Dec 7 2019

VFR conditions through 15Z Sunday. Trough moving through the area
overnight. Gusty southwest winds ahead of the trough continuing at
KSUX. If the gusts drop off over the next few hours, could have
low level wind shear at KSUX. Winds rapidly decrease aloft after

MVFR to IFR stratus will spread south through the day tomorrow.
Have added mention of freezing drizzle as stratus thickens and
dendritic layer remains dry. Once ice is introduced in the column,
lowered visibility and introduced snow. 




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