Area Forecast Discussion
Issued by NWS Duluth, MN

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FXUS63 KDLH 182109

Area Forecast Discussion
National Weather Service Duluth MN
309 PM CST Mon Dec 18 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 223 PM CST Mon Dec 18 2017

On-going light to moderate snow will continue to fall across our
northern counties this afternoon, and will continue this evening,
especially over the Minnesota Arrowhead region. This is all due to
a weak warm front and mid-level shortwave that continues to
advance eastward along the International Border. An associated
surface cold front will then move through later tonight. Webcams
this afternoon show aggregate dendrites falling near International
Falls, due to saturation within the dendritic snow growth, and
isentropic lift within that saturated layer. Due to the transient
nature of this system, snowfall amounts should remain limited, but
could still cause some hazardous travel conditions. Generally,
total snowfall amounts with this system should range between 1 to
3" north of Highway 2 in northeast Minnesota. The concern then
turns to the potential for some freezing drizzle overnight
tonight, mainly along and north of the Iron Range. Both the NAM
and GFS model soundings indicate that once the cold front moves
through this evening, there will be a loss of ice crystals aloft
as drier air moves in. However, there remains a shallow saturated
layer in the low-levels which look to contain supercooled liquid
water. The layer appears too shallow to support freezing rain, so
thinking hydrometeors will fall as drizzle. It appears that
surface temperatures will be cold enough over our northern
counties to support the freezing drizzle, while areas in northwest
Wisconsin will be above freezing for a period tonight, so
freezing drizzle should be limited there. Ice accretions should be
fairly small, so not thinking a Winter Weather Advisory will be
needed at this time. However, will issue a Special Weather
Statement to address the potential for freezing drizzle on top of
freshly fallen snow, which should lead to some travel

Chances of precipitation should fall fairly quickly once the
shortwave moves through, but there could be some lingering light
snow/freezing drizzle north of the Iron Range Tuesday morning.
Soundings support a transition back to snow in this area as a colder
air mass moves in behind the surface cold front as ice crystal
growth aloft increases. A channel of sheared vorticity should linger
through the day Tuesday, but lift is looking very weak with it, so
not expecting much in terms of precipitation. In fact, skies look to
be partly sunny in most locations in the Northland, except for over
our north. 925 mb temperatures Tuesday at noon quickly drop down to
between 4 and 10 degrees below zero, so Tuesday looks to have a
return to below freezing temperatures for high temperatures. Highs
Tuesday will dip into the middle 20s north to the lower 30s over
northwest Wisconsin, which is still a little bit above seasonal
averages. Gusty northwest winds will also develop as the surface
pressure gradient tightens due to high pressure building over the
Canadian Prairies. Gusts may reach up to 20 to 25 knots, with even
some gusts near 30 knots possible along the North Shore.

.LONG TERM...(Tuesday night through Monday)
Issued at 223 PM CST Mon Dec 18 2017

High pressure will be over the forecast area Tuesday night. However,
there is a small chance of some lake effect snow showers over
northeastern Iron County. There is some question on the wind
direction over Lake Superior in the evening, per the various models,
resulting in this small chance. Some light snow may begin to
encroach the southwestern corner of the area overnight in the warm
air advection ahead of developing low pressure in the lee of the
Rockies. There is still some uncertainty on how this low will evolve
as it pushes east of the Rockies, the track of and strength of the
low, and its affect on the region. The GFS and ECMWF are in fairly
good agreement, the NAM is too far north, and the GEM is too far
south. If the GFS/ECMWF are correct, the track of the low is farther
south for the 12Z runs than the previous forecast which would lead
to the bulk of the higher snow amounts being further south as well.
The snow will spread north and east with time Wednesday and
Wednesday night, then begin to diminish from northwest to southeast
on Thursday. By 12Z Friday, the GFS begins to speed up the surface
low and has it eastern Illinois, while the ECMWF is slower and has
the low center near the Quad Cites along the Mississippi River. The
GFS has high pressure over the region Friday, while the ECMWF has a
lingering trof and more light snow. A blend was used to resolve
these differences. Models do come back into better agreement Friday
night through Sunday, with the dropping of a cold core upper low
from the Arctic into northwest Ontario. This drops cold high
pressure into the region. Minor differences in 850mb temps, but they
will be largely around -30C. Some lake effect snow showers will be
possible, but direction is in question and pops reflect this


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1145 AM CST Mon Dec 18 2017

A warm front was moving through western Minnesota at the start of
this forecast. A mix of mid and high clouds were streaming over
the terminals ahead of this front. Some low clouds were hanging
tough over the Arrowhead through DLH, and into portions of
northwest WI. Cigs were mainly MVFR with the low clouds along
with some BR, and periodic IFR vsbys. Some light freezing rain was
occurring along the Canadian border. Confidence is low on this
pcpn affecting INL and have a VCSH mention. Elsewhere, some
improvement to VFR cigs and vsbys where low clouds are currently
and the warm air scours out these clouds. Still anticipating some
light snow to spread over INL and HIB this afternoon with MVFR
conditions returning to all terminals. After the warm front and
pcpn move past the terminals, look for the wind to turn northwest
and become gusty through the end of the forecast.


DLH  24  27   5  17 /  60  10   0  40
INL  23  23  -1  10 /  70  20   0  10
BRD  24  28  10  19 /  10   0  10  70
HYR  26  31   8  21 /  20   0   0  40
ASX  28  31  11  20 /  60  10  10  30


LS...Small Craft Advisory until 10 PM CST Tuesday for LSZ121-140>148.



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