512
FXUS63 KGRB 150831
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
Issued by National Weather Service Milwaukee/Sullivan WI
331 AM CDT Thu May 15 2025

.KEY MESSAGES...

- Marine Dense Fog Advisory for the nearshore areas of Lake
  Michigan through the early evening.

- A Dense Fog Advisory has been issued for the eastern Lake
  shore counties through the early morning to a little bit after
  sunrise.

- A broken line of strong thunderstorms will cross northern WI
  between 4 and 9 PM today. Some severe storms are likely with
  the passage of a cold front. All hazards possible over central
  WI and large hail possible over eastern WI.

- Another period of showers and weak storms expected Friday with
  some potential for near severe level wind gusts with some of
  these showers/storms.

&&

.DISCUSSION...
Issued at 330 AM CDT Thu May 15 2025

Today through Friday night:

Early this morning fog and low stratus continues to slowly slide
westward with most of the dense fog located closer to the
lake/bay. A dense Fog Advisory remains in effect for Door, Brown
and Calumet counties and east. Its possible this may need to be
extended westward some but the current thought is that the fog
has largely seem relegated to areas near the water with
improvements in visibility inland. Either way as the sun rises
this morning, similar to Wednesday we expect the low stratus and
fog to quickly dissipate and by mid morning we are mostly clear.

Concern will then turn to the afternoon and evening with the
potential for a significant severe weather event across the CWA.
The EML coming into the region this morning, originally
presented the area with the concern that capping might be strong
enough to potentially prevent convection in parts of the region.
However, models, especially CAMs, have largely put this
potential to bed with the forcing from the upper low and surface
front likely helping overcome the cap and allow for widespread
convection to develop in the western fringes of the CWA by the
mid to late afternoon. Severe storms have become increasingly
likely with this event as the potential limiting factors have
been mitigated with the latest model guidance.

This event continues to feature very high instability in the
range of 2000-3000 J/kg of MLCAPE. As storms develop and push
east instability will decrease somewhat but remain in the
1000-2000 J/kg range. This is largely due to the midlevel lapse
rates around 7.5-8.0 C/km. Deep layer shear will increase into
the early afternoon before steadily decreasing into the late
afternoon and early evening but will still be plenty, hanging in
the 40-50 kt range ahead of expected convection. 0-3km SRH will
be in the 200-300 ms/s2 range with the 0-1km SRH 50-100 m2/s2
initially but will gradually increase as storms push east. The
supercell composite will be in the 10-15 range in the southern
half of the CWA with decreasing values further north where the
better likelihood for storms becoming more linear exists. The
HREF STP shows values in the 2-4 range which is suggestive of
some higher end tornadic potential. There are a few limiting
factors that may limit some of the higher end severe potential.
The first is that while CAMs indicate the mode may largely be
discrete there will be concerns for the proximity of storms to
be close so as to cause storm to storm interactions that may
ingest rain-cooled air from each other and thus help weaken
storms overall cutting off the ingest of more warm moist air
ahead. This is some ways could help the cool temperatures a bit
and lower LCLs as LCLs will likely be fairly high around
1000-1500m which would make tornadoes a bit more difficult. All
hazards will be possible with this event as very large hail,
tornadoes (potentially strong) and damaging winds (DCAPE > 1000
J/kg). The higher end tornado risk does appear more limited
toward southern portions of the CWA

The timing for this event is the mid to late afternoon in the
far southeastern parts of the CWA and push out of the CWA by the
mid evening hours. The prime area of concern seems to line up
primarily in the southwestern parts of the CWA with the
strongest storms likely from around Marathon county down to
Waushara and Winnebago counties. As storms push east of there
they could begin to interact with the lake breeze boundary.
While initially this could add some enhanced stretching
potential storms will become elevated as they overrun the
boundary. This would lessen the tornado and wind risk though the
hail risk may remain. Into the evening storms will gradually
weaken as they lose instability as we lose our diurnal heating
but there will be severe risks continuing throughout the CWA
until storms exit by the late evening hours. Quieter conditions
are expected Thursday night.

The closed upper low and associated weakening surface low will track
through central and northern MN during the day Friday. Some
shortwave energy is expected to slide across the eastern half
of WI Friday out ahead of that upper low. In addition, the low
level jet will also be streaming across the region and there
may be some upper divergence present from the left exit region
of an upper level jet to the south.

The main concern with this will be the very strong winds available
to mix down to the surface during the day Friday. Gusts from 30
to as high as 45 mph are looking likely for the early afternoon
hours in the Fox Valley. A Wind Advisory may be needed for this
region. In addition, a few thunderstorms are possible Friday
afternoon. Continued gusty winds and a few showers are possible
over the rest of the area Friday evening. There is some concern
with these showers and weak storms to mix down some stronger
winds aloft. With the gusty winds aloft some models are
indicating that winds from some of these weak storms/showers
could be borderline severe with soundings suggestive of this
potential as well given unstable conditions to 700mb and even
areas where DCAPE > 1000 J/kg.

Saturday through Tuesday:

The closed low will continue east across the Great Lakes Region
Saturday. The associated occluded and weakening surface low pressure
system is expected to move northeast from northeastern WI and the UP
of Michigan to Quebec and Ontario Canada. What remains of the
cyclonic flow should be able to provide enough lift to generate some
showers across the forecast area. Rain showers are likely to be
diurnally driven with coverage increase a bit during the afternoon
vs the morning and evening hours. Regardless of this coverage is
expected to be scattered to isolated. As winds become northerly, dry
air will also move in across northern and central Wisconsin. POPs
will be around 20 to 40% across the forecast area Saturday. By
Saturday night, POPs drop off quickly as dry air moves in and the
cyclonic flow moves off to the northeast.

By Sunday, an upper level ridge and high pressure begin to move into
Wisconsin. With the center of the high passing to our north across
Manitoba and Quebec Canada, a component to the northerly winds
are expected to stay in place through Monday. North to northeast
winds will usher in some CAA leading to a cooling trend with
highs in the mid 50s to mid 60s and overnight lows in the low
30s to mid 40s. Sunday night looks to be the coldest with
temperatures near 30 across north central Wisconsin. This will
be the area to keep an eye on for an frost potential. Quiet
weather will continue Monday.

The next chance for rain and storms will be late Monday night into
Tuesday as another trough approaches the state from the Plains.
Winds should shift to southeasterly ahead of this approaching trough
leading to some warmer temperatures and WAA for Tuesday. The showers
and storm potentially is likely to extend into the overnight
hours Tuesday with some broad lift across the region. Its likely
that the POPs of 50-80% are likely to tighten up in time as we
get closer to early next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1147 PM CDT Wed May 14 2025

Low CIGS and VSBYs are gradually pushing west early this morning
off the lake/bay again. While this is not expected to be as
widespread in terms of low VSBYs we will still see the
widespread VLIFR CIGS potential for much of northeastern WI. As
you get further away from the lake we will likely see fog become
less of a concerns with the denser fog mostly right near the
lake or bay. In addition as you move west CIGS will likely lift
to some degree as well with CIGS becoming more IFR to MVFR.
Similar to Wednesday morning expect the sun to quickly help
dissipate the low CIGS and VSBYs within a few hours after
sunrise.

After this concern will turn to the strong storm potential
later this afternoon into the evening. Currently storms are
projected to develop and push into central and north central WI
by the late afternoon with storms quickly pushing through
northern WI and likely exiting northern WI by the mid to late
evening. Storms, especially earlier on, will likely be strong
with large hail, damaging winds and even tornadoes possible.
However, this will not be prolonged for any location with storms
likely in and out of an individual spot within an hour or so.
Into tonight there should not be much in the way of VSBY or CIGS
concerns with the exception of perhaps far northeast WI into
the evening ahead of the storms. Once the storms push through
we should expect a return to VFR conditions.

&&

.MARINE...
Issued at 330 AM CDT Wed May 15 2025

Given that the environment will continue to remain favorable
(moist air over the cool open waters) for dense fog until this
pattern breaks, the Marine Dense Fog Advisory is in effect until
7PM today. We may be able to expire that a little earlier on
Thursday as southeast winds kick in, so there is uncertainty in
the end time. This includes the Bay area, although we may see
less foggy conditions during the daylight hours there.

Cronce

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until 10 AM CDT this morning for WIZ022-039-
     040-050.
MARINE...Dense Fog Advisory until 7 PM CDT this evening for LMZ521-522-
     541>543.

&&

$$

DISCUSSION...
AVIATION...
MARINE...