Area Forecast Discussion
Issued by NWS Duluth, MN

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FXUS63 KDLH 141213 AAB

Area Forecast Discussion...UPDATED
National Weather Service Duluth MN
613 AM CST Thu Dec 14 2017

Issued at 613 AM CST Thu Dec 14 2017

Minor update to adjust timing of the likely snow with shortwave
moving through the region. With limited radar returns over
Manitoba and northwest Ontario, opted to reduce the area of the
likely POPs and move them a bit quicker into northwest Wisconsin.
Model guidance continues to support an increase in coverage and
intensity of snow showers over northwest Wisconsin this afternoon
and have made only minor adjustments to the chance POPs in those
areas. Remainder of the forecast is on track.


.SHORT TERM...(Today through Friday)
Issued at 425 AM CST Thu Dec 14 2017

The main concerns for the short term this morning include
widespread light snow showers and flurries for the next 36 hours
and the potential for gravity wave and lake effect snow in
northwest Wisconsin tonight into Friday.

Northwest flow aloft and continued cold air advection with
cooperate with a weak shortwave trough located over northwest
Ontario and southeast Manitoba early this morning to bring a
period of light snow to the Northland this morning. Regional radar
mosaic and GOES-13 infrared satellite imagery have captured the
area of light snow very well. We expect the light snow to quickly
spread southeastward across our western zones and into the Twin
Ports through the morning commute. Lake effect snow showers over
northwest Wisconsin will taper off as winds turn westerly ahead of
the approaching trough. The area of light snow will persist
through this morning and gradually weaken over northwest Wisconsin
this afternoon. With low stratus and continued cold advection,
think we`ll see flurries continuing as the snow showers depart.

Another shortwave trough moves into the area from Manitoba and
Ontario this afternoon and evening along with another surge of
moisture. Another round of snow showers is anticipated with this
feature. Of particular interest is the potential for several hours
of moderate snow late tonight over Douglas and Bayfield Counties.
The high-resolution models feature a standing gravity wave
starting late this evening and reaching peak intensity after
midnight. The speed of the flow would suggest a band of light to
moderate snow. The standing wave lingers through mid-morning, but
drier air is forecast to advect into the wave train aloft. The
precipitation cascade will either weaken by 6 AM or dissipate
completely. The northwesterly winds in the boundary layer will
support lake effect snow showers in northern Bayfield, northeast
Ashland, and northern Iron Counties through the remainder of the
day. Light snow will move into the western and southern zones once
again Friday as isentropic ascent associated with warm air
advection interacts with divergence aloft. While the deterministic
models seem to highlight a narrow corridor for light snow, we
have expanded the light snow chances to account for the slightly
different precipitation placement in the guidance and general
uncertainty. The focus for the light snow will remain over the
western and southern zones, but the areal extent of the light snow
may be underdone by the models.

.LONG TERM...(Friday night through Wednesday)
Issued at 425 AM CST Thu Dec 14 2017

Northwest flow aloft Friday night covers the forecast area.
Embedded impulses in this fast flow, will work in conjunction with
a baggy area of low pressure drifting through the Dakotas into
MN, to bring some probability of light snow to the region. Highest
chances will be closest to the stationary thermal boundary from
the northwest corner to the southeast corner of the forecast area.
There is a very small chance of lake effect snow along the
Gogebic Range Friday evening. However, model differences in the
amount of cold air/delta T`s/wind direction over Lake Superior
question the potential and kept POPs small. On Saturday, upper
flow becomes quasi-zonal ahead of an upper trof digging into the
Rockies. Additional impulses will move along the Canadian border
and over southern MN. Have highest chance POPs over the northern
two thirds of the region nearest the international border impulse.
The Rockies trof begins to move out of the Rockies into the
Northern Plains Saturday night. This squashes the baggy low and
allows high pressure to build into the region from the West. As
the forcing moves east of the area, models point to a loss of ice
in the clouds and have a mention of light freezing rain on the
back end of the departing precipitation. Since this opportunity
will be short lived, not expecting any ice accumulation, and have
low chance POPs for the freezing rain and the departing snow.
Model differences reveal themselves with the timing/placement/
orientation of the upper trof as well as on surface features
Sunday. After some morning low chance POPs for light freezing
rain, in the southeast quad of the northwest Wisconsin zones, went
dry in the afternoon with high pressure arriving. Mainly dry
Sunday night with the upper trof becoming stretched out and
drifting south of the area. Small chance POPs for light snow late
to account for an impulse moving through south central Canada.
After a dry Monday morning, an upper level trof moves through the
region and shifts the upper pattern beck to fast northwest.
Impulses in this flow aloft will affect the region periodically
and bring chance POPs for light snow through Tuesday. Tuesday
night finds upper level and surface ridging over the area. On
Wednesday, the next upper trof pushes into the West Coast, with a
large surface low developing in the lee of the Rockies. Actual
placement of the low and its impacts on the region are in
question, which is reflected by model differences in timing and
placement of any precipitation.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 603 AM CST Thu Dec 14 2017

Embedded upper level impulses will move past the terminals through
the forecast. This will bring mainly MVFR conditions with some
periodic light snow showers or flurries, and isold IFR, through
the forecast.


DLH  17   2  13   5 /  70  30  30  30
INL  15  -1   7  -3 /  70  50  20  20
BRD  20   8  19  13 /  20  10  40  40
HYR  19   4  14   7 /  60  30  30  30
ASX  19   9  14   6 /  60  50  30  20




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