865
FXUS63 KGRB 310955
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
355 AM CST Fri Jan 31 2025

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Cooler today, but readings will stay slightly above normal.
  Breezy northeast winds from the Fox Valley to the lakeshore.

- Accumulating snow remains on track from Saturday afternoon
  through Saturday night, tapering off early Sunday morning. There
  is a 60-90% chance of 2 inches or more of snow north of a line
  from Wausau to Sturgeon Bay. There is a 50-70% chance of 4
  inches or more of snow north of a Tomahawk to Marinette line.

&&

.DISCUSSION...
Issued at 354 AM CST Fri Jan 31 2025

Short Term...Today Through Saturday Night

High pressure diving out of central Canada and low pressure
passing across the northern Ohio Valley will result in cooler
northeast low-level flow today. The high will lead to cool and dry
night tonight, then system currently coming ashore in the Pacific
Northwest will move in later Saturday, bringing a round of snow
through Saturday night, before tapering off early Sunday. A
plowable snowfall with at least a few inches still looks likely
north of highway 29.

Today and tonight...Winds have become northeast entire area as
cold front is south and low pressure slides across northern Ohio
valley. Precip from the system remains to the south, but could
see low clouds at times as low stratus tries to advect in from
north and east. May see patchy fog early over the north and east,
then a few flurries this afternoon far north-central. Highs stay
in the upper 20s north, but will reach mid to upper 30s elsewhere.
Sharply cooler than yesterday, but still above normal. As winds
turn southeast tonight, low clouds will break up. High clouds will
not arrive until late. Lowered mins as there is a window for temps
to tank, especially north-central where a few spots may dip toward
zero or even below, at least before high clouds thicken late. Lows
may even drop into single digits above zero for most areas along
and north of highway 29, including Wausau and Green Bay.

Snow late Saturday afternoon through Saturday night...Shortwave
trough and low pressure system both emerge across the northern
Plains by Saturday afternoon. Wing of mid-level warm advection,
isentropic ascent likely will support light snow as early as early
to mid afternoon over the north. Soundings completely saturate
after 21z far north and this is where up to one inch of snow will
occur late in the day. Light snow also makes it into central WI
late day, but it may remain dry with just a few flurries at best
from the Fox Valley to the lakeshore due to lingering dry air in
the low-levels.

Mid-level trough crosses the area Saturday night but primary low-
level low remains over the plains, so the system is kind of
disjointed. Strongest large scale ascent and warm air advection
occurs over the north early in the evening, before spreading over
the rest of the area in more progressive fashion late evening and
overnight. GFS continues to be on the higher end for qpf/snow
farther south on Saturday night, but is an outlier even compared
to its own ensemble system, so continue to stray from that
guidance. Instead, given steady, persistent forcing stays north,
prefer to lean on other deterministic models which are more in
line with overall model/ensemble consensus and in the area
highlighted by ECMWF EFI/SOT which has consistently pointed to
areas north of Wausau to Green Bay seeing most snow of at least a
few inches by late Saturday night. NBM probabilities of seeing 4
inches or more of snow set up north of a line from Tomahawk to
Marinette. Adding confidence is the spread from 25th percentile to
75th percentile up north is not really that high (3-5"). Think an
advisory will eventually be needed, either issued later today or
early Saturday morning.

Still think that this snow may end as some light freezing rain/drizzle
in parts of the area as soundings late Saturday night show mid-
level moisture decreasing while moist layer up to 700mb stays
below freezing. Majority of snow will have occurred by this point,
so that should not impact snow amounts as much as it looked like
it could earlier.

Long Term...Sunday Through Thursday

Starting at 12Z/Sun, will see the trailing bit of precip
associated with the overnight clipper system. Precip type could
be mixed for a brief window, as moisture strips out of the DGZ and
temperatures start to rise above freezing. This results in snow
mixed with or changing to rain/freezing rain. Additional snow
accumulations between 12-18Z would only amount to an additional
tenth or two, primarily across northern/northeast WI. Expect any
freezing rain impacts to be minimal as it will fall on top of
snow, and surface temps will be quick to rise above freezing on
Sunday. In general, slippery roads will be possible throughout the
morning from the overnight snow, especially in northern and
northeast WI where the highest snow amounts are expected.

For the rest of Sunday, a few breaks in the clouds, with
southwesterly winds boosting high temps into the upper 30s to
middle 40s.

There is a small chance for some light snow Monday morning (<20%),
with up to a tenth or two in central WI. The next period of focus
for any precip is Wed/Thu. Models are disjointed on the specifics
and therefore have fairly low confidence in the predictability--
both with timing and precip amounts. Stayed with a blended model
solution which generally features chance (~30%) PoPs throughout
this time period, but refinement of this will be necessary. NBM
probabilities show a 20-40% chance of getting 1" of snow or more,
but given the model spread, don`t have a lot of confidence in this
either.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 915 PM CST Thu Jan 30 2025

Clear skies and light winds were observed across the forecast area
late this evening, but low clouds were increasing over northern
Lake Superior and low stratus had edged into far southern WI.

Overall forecast confidence is lower than usual regarding low
cloud trends. Low clouds approaching from the south may get close
to the southern Fox Valley, but probably will not make it into the
forecast area. There is also some uncertainly about possible IFR
stratus developing in NC WI late tonight/early Friday. There is a
higher degree of confidence that the strengthening NE winds will
eventually bring more widespread low clouds (mainly MVFR) to
northern and eastern WI later Friday morning, though it is still
unclear how far SW these clouds will spread during the afternoon.
Will continue to forecast a period of MVFR ceilings at the
RHI/GRB/ATW/MTW TAF sites, and SCT-BKN low clouds at AUW/CWA.
Surface ridging later Friday evening may result in partial
clearing.

NE winds will increase on Friday, with gusts to 20 to 25 kts
likely in the Fox Valley and lakeshore areas.

&&

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

&&

$$

DISCUSSION.....JLA/KLJ
AVIATION.......Kieckbusch