498
FXUS63 KGRB 181811
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1211 PM CST Thu Dec 18 2025

Updated aviation portion for 18Z TAF issuance
and new information added to update section

.UPDATE...
Issued at 1150 AM CST Thu Dec 18 2025

Have made several updates to the forecast.

- Adjusted precipitation trends, with a significant gap between
  the current band of light rain and incoming snow showers with
  the short-wave/upper low tonight.

- Have removed mention of snow squalls in our messaging. Snow Squall
  parameters have decreased significantly since yesterday, as
  low-level winds (a component of the parameter equation) are
  lower.

- Slowed the cooling trend of the hourly temperatures a little
  bit, with flash freeze potential now occurring roughly 4-6 pm in
  NC/C WI, and 6-8 pm in eastern WI. Big question is whether roads
  will be wet, especially given the gap between the two periods of
  precipitation, and potential for drying due to incoming drier
  air and ongoing winds. Any wet or untreated roads will freeze
  and becoming ice covered and hazardous.

- Areas of fog have developed due to above-freezing dew points
  advecting over the expansive snow pack and frozen ground. Dense
  fog has been reported in many areas, but has been most
  persistent along the lakeshore. A Special Weather Statement has
  been issued to address travel concerns with the fog. The fog has
  also impacted our marine areas, so a Marine Dense Fog Advisory
  has been issued through 6 pm.

&&

.KEY MESSAGES...

- Mix of rain and freezing rain expected early this morning, then
  rain changing to snow this afternoon and evening. Snow squalls
  possible this afternoon and early evening, followed by a
  possible flash freeze. Significant travel concerns possible this
  afternoon and evening.

- Light snow possible Friday night into Saturday morning,
  especially over far northern Wisconsin.

- Periodic windy conditions and a chance of gales on Lake Michigan
  through Friday morning.

&&

.DISCUSSION...
Issued at 422 AM CST Thu Dec 18 2025

Short Term...Today...Tonight...and Friday

A strong low pressure system over northern Minnesota will track
over Lake Superior today then continue to western Quebec by
tonight. The region will initially be in the warm sector of the
low, with mainly liquid precipitation expected this morning. There
will be a window where freezing rain is possible this morning
before road temperatures warm up above freezing with probabilities
for freezing rain across north-central Wisconsin generally 10 to
30 percent. According to road temperature data and probabilities,
the greatest potential for this will be from 4 to 9 am north of a
line from Stevens Point to Athelstane. The rest of the region
should have warm enough road temperatures or a late enough onset
that freezing rain is not expected to be a concern.

As the low crosses Lake Superior this afternoon, an arctic front
will track across the area. Short-range guidance suggests
favorable conditions for snow squalls between mid and late
afternoon. Key ingredients include surface instability up to 75
J/kg, low-level frontogenesis, steep low-level lapse rates of 7-8
C/km, and strong forcing via the shortwave trough. Snow squall
parameter values between 2 and 6 in the NAM reflect this
potential. A flash freeze will also be possible as sub-freezing
temperatures spread west to east across wet roads through early
this evening. Light snow and minor accumulations will be possible
region-wide as the system departs tonight.

Long Term...Friday Night Through Wednesday

The extended forecast will bring another couple of fast moving
wintry systems to the region, the first of which will arrive
Friday into Saturday morning. This will be a fairly quick snowfall
for most of the region, the exception being across far north-
central in the lake effect snow, where on and off snow showers
could continue well into Saturday night. Snow totals with this
clipper are currently forecast to be relatively light, mainly due
to limited lift through the dendritic growth zone and the fast
moving nature of the system. Probabilistic guidance only brings a
50% chance of exceeding an inch of snowfall to areas north of a
line from Rhinelander to Iron Mountain, with a sharp cutoff
southwards. That said, a round of stronger winds does accompany
the snow, so some travel impacts will remain possible due to a
reduction in visibility through Saturday morning.

The next chance for snow will arrive along a warm front on Monday,
but there is some question on how much will reach the surface.
Soundings show upper levels saturate as the warmer air arrives
aloft but still a significant dry layer below, which may limit
snow reaching the surface. As a result, would currently expect
relatively low impact from the passage of this snow. Winds aloft
will be on the increase again however, but potential to mix to the
surface may be limited. Still, would expect a few gusts around 20
to 25 mph to be possible, with an increase likely if the
associated precipitation potential increases in subsequent
forecast cycles.

Temperatures will drop in the extended on Sunday behind the first
clipper, with highs generally in the teens, before coming back up
to more mild temperatures once the warm air arrives on Monday.
Highs are currently forecast to be in the upper 20s to middle 30s
in the middle of next week leading up to Christmas Day.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1150 AM CST Thu Dec 18 2025

Widespread LIFR and patchy VLIFR conditions have been reported due
very low ceilings and reduced vsbys in fog. A band of light rain
was moving through the forecast area, and should pull out of the
region later this afternoon. Vsbys should improve from west to
east during the mid to late afternoon.

A powerful arctic front will sweep through the region late this
afternoon and early evening. The arrival of much colder air could
cause a flash freeze on any wet or untreated airport runways. Snow
showers will increase from west to east this evening as a strong
upper trough moves into the region. Vsbys could be reduced to IFR
at time through late tonight, with accumulations around an inch
possible.

Low-level wind shear will shift east of the region early in the
TAF period. Surface winds will be gusty from the southwest this
afternoon, veering northwest and gusting to 25 to 30 knots behind
the front tonight into Friday.

&&

.MARINE...
Issued at 422 AM CST Thu Dec 18 2025

Models have come in more gusty this run, with winds at the top of
the boundary layer solidly in the middle to upper 30 knots with
a fairly shallow boundary layer of 950 mb. In addition, the wind
direction of southerly this morning means winds off the lake will
be unabated to the nearshore waters, especially along the Door
Peninsula where the direction is orthogonal to the shoreline.
Given winds are already gusting close to 30 knots just south of
the area, the potential is high enough for gales this morning to
go with a Gale Warning for the Lake Michigan nearshore areas given
the plethora of southward facing beaches.

Gales will again be possible tonight as winds turn northwest on
the backside of the departing low pressure system across the bay
and Lake Michigan nearshore waters. Although there will be a
window where Gales are not expected this afternoon, will continue
the Gale Warning through tonight instead of triple headlines along
the nearshore waters. Winds will then ease below gale force
Friday morning.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE.........Kieckbusch
DISCUSSION.....Kurimski/Uhlmann
AVIATION.......Kieckbusch
MARINE.........Kurimski

NWS GRB Office Area Forecast Discussion