Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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777
FXUS63 KFSD 091125
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
525 AM CST Fri Dec 9 2016

.SHORT TERM...(Today and Tonight)
Issued at 435 AM CST Fri Dec 9 2016

Mid-level clouds rapidly increasing across portions of southwest
and south central South Dakota this morning. Temperatures ahead of
the cloud cover have plummeted near or below zero in many areas, as
sign of the intensity of the airmass given a lack of snow across the
Tri-State area.

For today, a bit of a forecast deviation from the previous forecast.
Models continuing to show broad isentropic lift forming and
spreading eastwards through the afternoon as a minor shortwave
drifts across the eastern Dakotas later today. Despite nearly
saturated soundings by mid-late afternoon and lowering DGZ, most
models (excluding the NAM and NAMNest) suggest very little QPF.
However, with minor warm front forming along or south of the I-90
corridor this evening, lift should be sufficient enough to produce
fairly persist light snows. What`s more interesting is that the
soundings continue to show ice present through the overnight
hours with weak warm advection signals within the soundings along
the I-90 corridor. Shortwave energy is moving away however, but will
continue to keep a mention of light snow or flurries through the
night. In general have raised the precipitation risk upwards of 40%,
which could end up trending higher depending on development later
this morning.  Could even see upwards of a half inch of snow by
midnight especially along the I-90 corridor.

Temperatures will be more difficult to pinpoint tonight with the
increase in cloud cover.  Likely a bit of a non-standard diurnal
curve, with temperatures holding in the teens.

.LONG TERM...(Saturday through Thursday)
Issued at 435 AM CST Fri Dec 9 2016

Setting up for a longer duration snowfall event this weekend with a
series of waves pushing through a zonal to northwest flow pattern
across the northern plains. NAM has relaxed the solution for banding
southward, especially with quick look at the 06z run which continued
the trend. GFS further south with main precipitation, with Canadian
furthest south. QPF also showing a fairly significant disjointed
distribution within ensemble sets, suggesting that while the larger
scale ideas are correct between converging larger scale models, the
subtle differences in band location could bring drastic QPF/snow
differences.

Precipitation Saturday morning will break out fairly rapidly as
right entrance region of upper jet drags into the region, enhancing
the low- to mid-level isentropic lift in broad theta-e advection.
Fairly strong frontal forcing in place, with even some indication of
a small layer of instability around 650-550 hPa poised just above
frontogenetic layer.  Much of the lift is expected within the
dendritic growth zone, so likely that will see areas of decent
snowfall rate within the band. 2 to 4 inch snowfall by early evening
likely northeast of a Mitchell to Storm Lake line, at which time the
main thermal packing will lock in across southwest Minnesota.

Initial jet entrance will pull away and yield to a stronger wave
with decent PV signature digging east and southeast through the
northern plains by later Saturday night and Sunday. Ahead of this
feature, warm advection will continue in a somewhat lesser mode
suggesting persistence to light snowfall overnight and into Sunday,
the latter period finding mainly along and east of I-29 ahead of
inverted trough.  By 18z Sunday, much of the precipitation will be
winding down, lingering in parts of southwest MN and northwest IA as
mid-level trough axis and inverted trough slip eastward.

Total snowfall is likely to vary from 4 to 6 inches from east
central SD and near I-90, east and southeast through northwest Iowa.
Snowfall will be drawn out, and likely that areas to west and south
of southwest Minnesota experiencing a lull in snowfall from later
afternoon through the Saturday evening period.  Messaging has been
fairly consistent and strong with system, and with no more than a
marginal threat of reaching warning criteria of 6" in 12 hours or 8"
in 24 hours, not much could be gained in issuing a broader watch at
this point.  System does appear to be more one-dimensional as well,
with winds not expected to create any significant issues on top of
the snowfall itself.  At this point, a consensus was reached with
surrounding offices to hold off issuing an advisory, which would
appear to be most likely evolution to the forecast.

The other big story of the medium and long range period will be the
bitter cold push, expected to arrive later Monday and Monday night.
This air mass is even colder than the current one, and will likely
bring temperatures no better than the single digits above for most
areas from Tuesday through weeks end, along with lows below zero.
Blended guidance does not seem to be adapting to expected snowcover,
and local climatology would suggest highs in the mid single digits
above and lows as cold as some teens below zero if could lose the
wind, the best chance around Thursday morning. Winds largely
underforecast once again. Behind leading arctic boundary later
Monday will likely come a burst of winds from 20 to 30 mph, along
with flurries and scattered snow showers.  Brisk winds will again be
featured with core of coldest air settling in by Wednesday and
Wednesday night. Dangerous wind chills from 15 below to 30 below
zero will be common from Tuesday through Thursday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 521 AM CST Fri Dec 9 2016

The primary concerns today will be focused on lowering ceilings by
this afternoon and an increased risk for light snow through the
late afternoon and evening hours. Latest guidance suggests MVFR
ceilings to form gradually later today, with occasional light snow
at HON/FSD. visibility restrictions may be possible, but for now
will maintain only a tempo for 5 miles at FSD.

More uncertainty tonight as a sfc warm front hold near the area,
could see light snow or flurries linger towards 06z, with an
increased risk for MVFR ceilings should winds remain light.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Dux
LONG TERM...Chapman
AVIATION...Dux



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