223
FXUS63 KGRB 261939
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
239 PM CDT Wed Mar 26 2025

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Freezing drizzle or light freezing rain could bring icy travel
  to north- central and far northeast Wisconsin from late Thursday
  night through Friday morning.

- There is a 20-30% chance of thunderstorms across the region
  from late Friday afternoon through Friday evening. Some of the
  stronger storms could produce small hail.

- A wintry mix consisting of rain, freezing rain, sleet, and snow
  will impact the region from Friday night through Sunday night.
  Areas north and west of the Fox Valley have the highest chance
  of impactful snow and ice accumulations.

- There is a 30-60% chance of precipitation totals over an inch
  through Sunday night. The widespread precipitation could lead to
  fast flows and minor flooding along the shorelines of area
  rivers and streams.

&&

.DISCUSSION...
Issued at 239 PM CDT Wed Mar 26 2025

Short Term...Tonight and Thursday

The latest RAP analysis and satellite/radar imagery show high
pressure stretching across the central Mississippi Valley and into
the Great Lakes. Weak mid-level warm advection is generating areas
of clouds between 6000-9000 ft across Wisconsin, but otherwise,
quiet weather continues. Looking upstream, stronger warm advection
is occurring over the Plains where frontogenesis in the mid-levels
is producing light rain into northwest North Dakota. As this warm
advection moves towards the northern Mississippi Valley late
tonight and into the western Great Lakes on Thursday night, light
precip and potential impacts are the main forecast concerns.

Scattered to broken areas of clouds will continue to move across
the region through this evening but the weather will remain
quiet. By late tonight, clouds become more widespread again as a
moisture-starved cold front moves into far northern WI and warm
advection increases over southern Wisconsin. Models continue to
point towards a band of light rain developing in the warm
advection zone from west-central WI to southeast WI from about 4
am to noon Thursday. The northern edge of this light precip could
graze central and east-central WI. Temperatures look to be in the
middle 30s, but forecast soundings indicate there is a deep enough
above freezing layer for precip to fall as mainly rain. Therefore
no impacts are expected.

Clouds depart for a time on Thursday afternoon and Thursday
evening, before moisture returns northward late on Thursday night.
This moisture then gradually deepens after midnight from south to
north. Saturation depths look to become sufficient for drizzle
from central to east-central Wisconsin after about 3-4 am, and
across northern WI in the 6-8 am timeframe. There appears to be a
small window for minor ice accumulations before road temps warm
into the 40s by 9-10 am across the far north.

Long Term...Thursday Night Through Wednesday

The forecast gets considerably more complicated for the rest of
Friday and through the weekend. As a weak shortwave impulse moves
east across the northern Plains, a warm front will lift north into
central Wisconsin on Friday afternoon and evening before a cold
front surges south across the area in the wake of a surface wave.
Light precip will likely occur for much of Friday north of the
warm front while the heaviest precip will occur in an axis from
the Arrowhead of Minnesota to the north shore of the Upper
Peninsula. Some thunderstorms will be possible at times on Friday
afternoon and evening with the approaching surface wave thanks to
strong fgen along a very impressive baroclinic zone, most unstable
capes up to 1000 j/kg and mid-level lapse rates of 7.5-9.0 C/km.
Potential for hail will be a concern with any storms.

One of the primary uncertainties on Friday revolves around how
quickly surface temperatures warm through the day. While the
deterministic NAM/GFS show temperatures remain below freezing all
day along the Upper Peninsula border, probabilities show a rapid
decrease in chances of freezing rain (changing to rain) in the
10am-1pm Friday timeframe. Easterly flow at the surface (off Lake
Michigan) rather than northeasterly trajectories makes me think
that probabilities are on the right track in terms of precip
types.

Once the low passes to the east, probabilities increase for
freezing rain and snow late on Friday night into Saturday morning.
Probabilities of 6 hourly ice accumulations greater than a 0.10"
inches increase to about 50-60% over far northeast WI by 7 am
Saturday morning.

Another period of concern is Saturday night into Sunday when low
pressure tracks along the tight thermal gradient over the southern
Great Lakes. The ensemble means generate a widespread 0.50" to
0.75" of qpf within strong mid-level frontogenesis and upper level
divergence. With the 850mb freezing line bisecting northern
Wisconsin from southwest to northeast and northeast wind
trajectories north of the low track, uncertainty is very large
regarding potential snow amounts (0-10") and freezing rain amounts
(0.00-0.60") across the forecast area. Therefore, the greatest
impacts are poised to occur during this period but confidence is
too low to mention anything specific.

Below normal temperatures will follow this potent system on Monday
and Tuesday. Then another wintry system could impact the region
around midweek.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1220 PM CDT Wed Mar 26 2025

Mainly VFR conditions are forecast through the taf period.
Scattered to broken clouds between 5000-8000 ft are forecast to
develop over the forecast area this afternoon before dissipating
early this evening.

A narrow band of light rain will graze past central WI early on
Thursday morning. A brief period of MVFR ceilings are possible
south of a ISW to OSH line from about 10z to 18z where there is a
chance of light rain. Otherwise, ceilings could lower to around
5000-6000 ft across central and east-central WI before ceilings
rebound Thursday afternoon.

No significant impacts from winds or visibilities are expected.

&&

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

&&

$$

DISCUSSION.....MPC/MPC
AVIATION.......MPC