Area Forecast Discussion
Issued by NWS Duluth, MN

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403
FXUS63 KDLH 101725 AAB
AFDDLH

Area Forecast Discussion...UPDATED
National Weather Service Duluth MN
1125 AM CST Sat Dec 10 2016

.UPDATE...
Issued at 1125 AM CST Sat Dec 10 2016

Arctic high pressure area is centered over the Red River Valley
as of late this morning, and will continue to settle slowly
eastward for the remainder of today/tonight. A couple areas of
very persistent arctic stratus/stratocumulus clouds continue to
hang on this morning, one over the Minnesota Arrowhead, and the
other more expansive area covering most areas along and south of
the U.S. Highway 2 corridor. These clouds continue to produce a
few very light flurries, but perhaps more importantly are having a
profound effect on temps. As is typical this time of year in
arctic air, with less than 9 hours of sun and a very low sun
angle, snow cover and light winds, hourly temps are largely
governed by the insulating effects of cloud cover - or the lack
thereof. This was clearly demonstrated early this morning when an
area of clearing developed over interior northeast MN allowing
Embarrass to drop over 15 degrees in less than 2 hours. These
effects will likely persist through this evening as areas of
clear/cloudy skies remain under the ridge axis. Thereafter,
thicker clouds from the next approaching wave will spread across
the entire region, and should allow for slowly rising temps in low
level warm advection for later tonight through most of Sunday.

The other primary issue that has our attention this morning is the
evolution of lake-effect snow showers tonight/Sunday morning
before the primary shield of synoptically-forced light snow
spreads across the region. The center of the arctic high pressure
will move to a position over western Lake Superior early tonight
with very light winds. With water temps still in the mid 40sF,
drainage flow from land to water is likely to provide a near-ideal
set up for the possibility of lake-effect snow in the form of
warm-core swirls or eddies. These are a fairly rare form of lake-
effect snow, especially on western Lake Superior, and forecasting
their precise development and eventual movement is exceptionally
difficult - akin to trying for forecast the precise location and
movement of a single thunderstorm in the summer 18+ hours in
advance. However, large-scale conditions will be favorable
tonight/Sunday morning, and if any of these eddies/swirls were to
develop out over the open lake, they eventually would likely be
drawn westward into somewhere along the north shore late
tonight/Sunday morning as the larger scale low level flow becomes
east/northeast. Adding at least a bit of confidence, many of the
hi-res models do depict some sort of solution along these lines
with lake-effect eddies forming over the western arm and then
coming ashore in the Two Harbors/Silver Bay/Tofte/Lusten areas
between 3-9 AM. If this scenario were to play out, these small
lake effect swirls are notorious for producing very heavy snowfall
rates over very small areas as they are often very slow moving as
they come ashore.

&&

.SHORT TERM...(Today through Sunday)
Issued at 329 AM CST Sat Dec 10 2016

An upper level short wave trof was clipping the Minnesota Arrowhead
at 08Z, while at the surface, high pressure was building in from the
Dakotas. Plenty of low level stratus was over the forecast area with
a few flurries. An area of snow showers was occurring ahead of the
trof over the tip of the Arrowhead. Only trace amounts of snow are
expected. Some holes/clearing was already taking place along the
western edges of the region as the high moves closer. The high will
drift over the region today allowing for a partly sunny to mostly
cloudy sky. There will be weak warm air advection through the day
which is indicative of the partly sunny to mostly cloudy sky.
However, max temps will not benefit much from the warmer air with
highs from around 5 above to around 10.

The high will drift into Ontario tonight as low pressure gets
organized in the lee of the Rockies. Model differences begin to
reveal themselves on the timing of the onset of snow and the
location of the low pressure. The ECMWF is the farthest
north/fastest, the GFS/NAM and GEM are slower and not as far north.
This also in agreement with the ARW/NMM for snow placement. Have
opted for the slower approach and lowered pops in the evening.
Models are a bit closer together late tonight with the location of
the low pressure and coverage of the snow. They are also in
agreement with the approach of potent short wave trof into North
Dakota by 12Z Sunday. Have kept the area along the international
border with the lowest pops as dry air from the high lingers. Higher
pops over the southern edge of the forecast area closer to the low.
Lower QPF amounts tonight have resulted in lower snow amounts
tonight with around one inch along the southern edge of the region,
with light amounts elsewhere.

On Sunday, the low pressure center moves to the southern end of Lake
Michigan. Meanwhile, the upper trof digs into western Lake Superior.
Plenty of lift associated with the trof and being on the northern
periphery of the surface low leads to a bit more snow. Highest
pops/snow amounts will be along the southern tier of zones, with
lighter amounts farther north. 1 to 2 inches is expected over the
northern half of the region, with 2 to 3 inches over the southern
portion. The wind will be fairly light throughout and will not have
an impact. Highs will be warmer with teens north to lower 20s in
northwest Wisconsin.

.LONG TERM...(Sunday night through Friday)
Issued at 329 AM CST Sat Dec 10 2016

The focus remains on the very cold, Arctic air that will blast into
the Northland this upcoming week. There still seems to be pretty
good agreement through the at least the middle of the week, with the
coldest temperatures of the season likely coming for Wednesday with
subzero daytime high temperatures. The latest models in general
trended colder, so trended a bit colder in the forecast.

There will likely be lingering light snow across the Arrowhead and
northwest Wisconsin Sunday evening, from more substantial snow
across the Northland Sunday daytime, as an area of low pressure
moves across the Upper Midwest and Great Lakes region. The snow will
gradually taper off and shift east through the night. Additional
snowfall will generally be less than 1 inch, but parts of the North
Shore could get up to 2 inches.

A one-two punch of Arctic air will hit the Northland early this
week. The first cold front will come Monday afternoon and evening,
followed by another front (with even colder air) Tuesday afternoon
and evening. Expect a nose-dive in temperatures to about 25 degrees
below normal by Wednesday. Wednesday should be the coldest day, with
daytime highs up to several degrees below zero and wind chill
temperatures of about -15 to -25 degrees. Parts of the Northland
will likely see wind chills of -25 to -30 degrees during the early
morning hours Tuesday, Wednesday, and Thursday, possibly needing
the issuance of wind chill advisories.

The Arctic cold fronts will be accompanied by light snow,
especially Monday`s front. The strong-cold air advection and
synoptic lift from the falling heights with an approaching upper-
level trough will result in about a few-tenths of an inch of
widespread snow late Monday, and another dusting is likely Tuesday.
Also, the prolonged period of the very cold west-northwest flow over
western Lake Superior will result in some lake enhanced/effect snow
downwind over parts of the South Shore in northwest Wisconsin. The
wind direction is not ideal for substantial snowfall for northwest
Wisconsin (not enough of a northerly component), but the best
chances of snow will likely be for northern Bayfield Peninsula,
where the wind direction will favor the best fetch across the Lake.

There is less agreement between the GFS and ECMWF late this week
than earlier in the week. The GFS maintains the very cold west-
northwest flow on Friday, but the ECMWF has warmer southerly flow
and synoptic snow spreading into the region from a system farther
south in the US. Leaned on a blend of the two for now.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 624 AM CST Sat Dec 10 2016

The cold west-northwest flow across northeast Minnesota and
northwest Wisconsin has brought widespread MVFR ceilings and
pockets of IFR conditions because of -SN. High pressure will build
into the region this morning, though, and likely result in a
return to VFR conditions by early this afternoon due to
clearing/scattering and/or lifting of the ceilings.

Ceilings will again lower tonight as a system in the Plains
spreads -SN into Northland. There will likely be widespread
IFR/MVFR conditions by late tonight.


&&

.PRELIMINARY POINT TEMPS/POPS...
DLH   7   4  20  11 /  10  60  60  30
INL   6  -5  13   5 /   0  30  30  20
BRD   8   7  18   6 /  20  90 100  10
HYR  11   7  21  12 /  10  70  80  30
ASX  11   7  22  16 /  30  50  70  50

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
LS...None.
&&

$$

UPDATE...Miller
SHORT TERM...GSF
LONG TERM...Grochocinski
AVIATION...Grochocinski



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