Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 101008

Area Forecast Discussion
National Weather Service Sioux Falls SD
408 AM CST Sat Dec 10 2016

.SHORT TERM...(Today and Tonight)
Issued at 408 AM CST Sat Dec 10 2016

The obvious forecast concerns today and tonight are with this long
duration winter storm, now rapidly beginning to take shape this
morning.  Impressive warm advection/frontogenesis band quickly
lifting northeast this morning, and will overtake the area through
the morning hours today. Models all in good agreement through 00Z as
far as the track of this band of snow northward. Feel the NAM/GFS/EC
are a bit low on the QPF, so have blended in a bit more of the
higher resolution ARW/NMM especially for the afternoon/evening

By this afternoon a surface warm front will settle in around the
Missouri River valley, with a nose of drier air aloft feeding into
the Tri-State area. Have tapered off precipitation along the river
into the evening hours.  Further north, models all continuing to
show persistent focus for light snows, with pockets of
moderate mesoscale band snows from Huron to Brookings and Spencer.

The evening becomes much more interesting, as stronger PV anomaly
begins to enter the Plains. Frontogenesis actually begins to
intensify quickly in the afternoon across eastern SD, southwestern
MN and portions of Northern IA. Model cross sections and soundings
also showing a narrow corridor of instability lifting northward out
of Nebraska by the evening hours. Consecutive runs of the
ARW/NMM/NAM and even the latest HRRR runs showing this instability
being released into an intensifying frontogenesis band, and a
partial jet coupling result in a narrowing snow band over parts of
northwest Iowa and Southwest Minnesota after 9pm.  Should this band
develop, snow rates could become much more intense. The forecast
uncertainty is on the southern/western extent of the band and how
well defined it could become.

Snow Amounts: The general forecast continues to show a long duration
accumulation of 3 to 6 inches over many areas. The heaviest corridor
of snows may fall from Brookings to Pipestone, Worthingon, and
Spencer, where I wouldn`t be surprised to see 6 to 8" amounts (or
perhaps a bit higher should mesoscale banding develop)

Product Decisions: The long duration of this snow will likely keep
this to a long duration advisory.  Two areas to watch today will be
near Sioux City, where the initial band of snow may lift north by
mid-day and it`s possible the advisory could be ended early. I`ve
split that segment of the advisory apart from the remaining
portions. A second area to monitor is from Worthington to Spencer
where if signals for heavy snow continue to grow this morning, 24
hour warning criteria could be met. Through collaboration with
surrounding offices, will monitor the progress this morning and make
changes later this morning if needed.

.LONG TERM...(Sunday through Friday)
Issued at 408 AM CST Sat Dec 10 2016

Lingering snowfall on Sunday will occur ahead of inverted trough and
in advance of the strong mid-level trough digging from North Dakota
into eastern Minnesota late day. Snow distribution will initially be
defined by the approaching PV advection and lingering frontogenetic
boundary across southwest MN into far northern portions of IA. Still
indications of a bit of negative EPV lurking just south of the main
frontogenetic axis, so could yet be seeing some decent snow rates
along and east of a KMML to KSPW line, especially around the MN/IA
border.  Combination of cold advection west of the inverted trough,
along with the eastward surge of drying aloft should end a majority
of precipitation by 18z along with any need for advisory, with
additional accumulations limited to around a inch or less. There is
some potential for areas on back side of the precipitation zone to
lose deep enough saturation to involve ice processes west of I-29 in
the morning, but will not complicate things by adding in a mix with
freezing drizzle yet at this time.  Mildest temperatures will remain
along and ahead of inverted trough before cold advection kicks in,
with clouds also remaining prevalent through the day east of I-29.

With fresh snow cover on Sunday night and weak ridge sliding quickly
past, will have to be cautious on how low and quickly temps could
fall as lower clouds evacuate and areas of mid and high clouds in
westerly flow stream over. Southerly flow increasing from the James
valley west along with clouds likely will result in non-diurnal
trend to temps. Lows for now in the single digits above and below
zero, but expect that may need to drop further closer to mid and
late night ridge axis locations through northwest IA.

Monday will bring the second significant feature of the forecast, the
leading edge of the arctic blast which will move into the area
during the afternoon and evening hours. Could actually be relatively
mild ahead of the boundary, maybe even above freezing in the lower
Missouri valley, but a sharp change will spread across the entire
area during the afternoon and evening. Models averaging 3h pressure
rises of 7-10 hPa/3h behind the frontal zone, so even pushing winds
closer to the highest guidance value may not be enough with 30 to 35
knots available for mixing. There will likely be scattered flurries
and even some patches of light snowfall which accompany the boundary
for a couple hours, but any accumulation will be minor at best.

The remainder of the week will be battling temperatures as arctic
airmass becomes entrenched across the Northern Plains. Surface ridge
builds in quickly dropping into Nebraska by Tuesday morning, and
again opens with window for temps to drop quite a bit. Guidance
seems to be suffering from a climatological bias, or expectation of
more wind than might persist over fresh snow cover, and have nudged
lower again.  Another wave in cyclonic flow will push reinforcing
Arctic boundary southward Tuesday night.  Mixy and cold Wednesday
and would not go so far as to eliminate a flurry threat. Surface
ridge axis settles across Wednesday night, and again will have to
rely on an increase in higher clouds later night to keep temps from
free falling.

There are some greater discrepancies in models by the end of the
week, with ECMWF somewhat more progressive with waves in zonal flow
and GFS remaining more dominant with low level cold air and somewhat
slower spreading impact of positively tilted trough into the area.
Have stuck with chance pops for snowfall over the entire area with
deep dendritic temps likely as broad warm advection develops.
Certainly will be something to watch with any end of week travel


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1052 PM CST Fri Dec 9 2016

An area of very light snow or flurries with patchy MVFR conditions
will gradually be overtaken by prolonged light to moderate snow
which will spread into southeast South Dakota prior to daybreak
Saturday. The light to moderate snow will then expand east into
southwest Minnesota and northwest Iowa throughout the day Saturday,
while it diminishes through the Missouri River Valley. Visibility and
ceilings will both be reduced during snowfall. IFR conditions be most
likely, though periods of LIFR visibility will be possible at


SD...Winter Weather Advisory from 9 AM this morning to noon CST
     Sunday for SDZ040-055-056-062-066-067-071.

     Winter Weather Advisory until 9 AM CST Sunday for SDZ038-039-

MN...Winter Weather Advisory from 9 AM this morning to noon CST
     Sunday for MNZ071-072-080-081-089-090-097-098.

IA...Winter Weather Advisory from 9 AM this morning to noon CST
     Sunday for IAZ001>003-012>014-020>022-031-032.

NE...Winter Weather Advisory from 9 AM this morning to noon CST
     Sunday for NEZ014.



LONG TERM...Chapman
AVIATION...JH is the U.S. government's official web portal to all federal, state and local government web resources and services.