184
FXUS63 KGRB 271912
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
212 PM CDT Thu Mar 27 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 late tonight
  through late Friday morning.

- There is a 25-35% chance of thunderstorms across the region
  from late Friday afternoon through Friday evening. Some of the
  stronger storms could produce small hail, although a few reports
  of large hail one inch or greater can not be ruled out.

- A 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 20-50% chance
  that the area between Wausau and Iron Mountain will see 0.25"
  ice accumulation Saturday night and Sunday.

- There is a 50-80% chance of precipitation totals over an inch,
  and 10-30% chance over two inches across northeast Wisconsin
  through Sunday night. The widespread precipitation could lead to
  fast flows and minor flooding along the shorelines of area
  rivers and streams next week.

&&

.DISCUSSION...
Issued at 212 PM CDT Thu Mar 27 2025

Short Term...Tonight and Friday

The latest RAP analysis and satellite/radar imagery show weak
surface high pressure centered over the area early this afternoon.
Warm advection is increasing over the northern Plains where there
is a modestly tight temperature gradient in the mid-levels. Weak
frontogenesis along this thermal gradient contributed to light
showers over the southern Fox Valley, but these showers have
exited leaving behind an elongated area of clouds with bases
5000-8000 ft. As a warm front lifts northeast into the southern
Great Lakes late tonight, potential for freezing drizzle will be
the first area of focus. Then attention turns to thunderstorm
potential on Friday afternoon and evening as a surface wave moves
across the state.

Freezing Drizzle Potential Late Tonight into Friday Morning: As
warm advection increases through tonight, saturation below 7000 ft
will gradually deepen primarily after 1-2 am across the region.
In generally, saturation depth appears to have lessened over
northern WI, where the greatest concern is for freezing drizzle
due to surface temperatures forecast to be in the upper 20s to
lower 30s. Think saturation could be sufficient for light freezing
drizzle to develop after 08-09z, which is slightly earlier than
forecast yesterday. However, not confident about coverage and
intensity (due to saturation concerns) prior to when surface road
temps warm into the 40s after 14-15z. Given the low confidence
forecast, prefer to handle potential freezing drizzle with SPS`s
rather than issuing a Winter Weather Advisory.

Otherwise, CAMS show an arcing band of light rain lifting
northeast across the area after about 09-10z Friday.

Thunderstorm Potential Friday Afternoon/Evening: A wave of surface
low pressure will track across west central to northeast Wisconsin
late Friday afternoon through Friday evening. Increasing moisture
transport via a 50-55 kt low level jet will shift into northern
Wisconsin ahead of the surface low. Modest to strong mid-level
fgen north of the warm front will have potential to interact with
elevated instability in excess of 1000 j/kg to produce
thunderstorm activity. Given effective shear in excess of 40 kts
and mid-level lapse rates from 7.5-8.5 C/km, concern remains for
large hail over far northern WI. The strong inversion north of the
warm front will severely limit and damaging wind potential.

Icing Potential Late Friday Night: Once the low passes, low level
cold advection will resume in the low levels thereby creating
potential for freezing rain/drizzle. However, the deformation zone
behind the surface wave is projected to remain north of the Upper
Peninsula border and models have backed off QPF in the 06z-12z Sat
time frame. As a result, it appears the potential for significant
icing has diminished in this forecast cycle.

Long Term...Friday Night Through Thursday

The complicated, potentially high impact weather will continue
from Saturday through Sunday night. As the cold front stalls over
southern Wisconsin or northern Illinois, as secondary surface low
will travel along the stalled boundary and across the southern
Great Lakes late on Saturday night and for much of Sunday. Track
of this surface low (and thermal profile) varies with each ensemble
group. Therefore, confidence remains low where the greatest
impacts from winter weather will occur.

As low pressure draws closer to the region on Saturday and
Saturday night, mid-level fgen will increase again north of the
stalled front. Within this fgen zone, the ensemble means indicate
qpf amounts from 0.70" to 1.10" from Saturday night through
Sunday.

Given the 1030mb high over northern Ontario and a continues feed
of colder via northeast winds, the synoptic setup (which ensembles
and deterministic models also show) will be favorable for a
corridor of freezing rain. Significant freezing rain is possible
in this corridor given the 90th percentile of freezing rain is in
excess of 0.50". Also think heavy snow will be possible north of
the mixed precip band with amounts over 6 inches possible. This
period has potential to be a high impact winter event, but
confidence is too low in the details for any sort of headlines.

After a cool down on Monday and Tuesday, yet another significant
system could bring impactful weather on Wednesday. Yet it is too
early to determine whether the impacts will come from winter
weather or thunderstorms, or both.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1216 PM CDT Thu Mar 27 2025

Mainly VFR conditions will prevail through this evening. Light
rain or sprinkles will exit the southern Fox Valley by 18z taf
issuance. Mid level clouds between 6000-8000 ft will linger behind
over central to east-central Wisconsin through the evening.
Meanwhile, northern Wisconsin will remain mostly clear.

As a warm front lifts north into the southern Great Lakes late
tonight and Friday morning, expect ceilings to deteriorate from
south to north into IFR/LIFR range. As saturation deepens below
7000 ft, the chance of drizzle and light showers will increase
generally after 08-09z. With drizzle chances increasing, the
probability of lower visibilities into MVFR will increase after
09z as well.

As drizzle becomes more widespread after 09z, temperatures will be
near or below freezing over north-central and far northeast
Wisconsin. Light icing will be possible prior to 14-15z as
pavement temperatures warm above 40 degrees and air temperatures
approach the freezing mark. Thereafter, temperatures should warm
slightly above freezing limiting potential icing.

Little to no improvements in ceilings are likely through Friday
morning, but some improvements should occur in the afternoon over
central and east-central WI as the warm front lifts north.

&&

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

&&

$$

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