809
FXUS63 KGRB 241125
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
625 AM CDT Fri May 24 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Showers and thunderstorms are expected today into this evening.
  Some of the storms could be strong to severe this afternoon into
  this evening. Severity of storms will depend on how much
  instability builds up during the middle to late afternoon.
  Heavy rain is also possible with the storms.

- Memorial Day weekend starts off dry for Saturday, but rain
  chances increase by Sunday afternoon and continue into Memorial
  Day. A soaking rain could occur during this time.

- A few rivers over central and eastern Wisconsin could surpass
  minor flood stage early next week.

&&

.DISCUSSION...
Issued at 242 AM CDT Fri May 24 2024

Short Term...Today...Tonight...and Saturday

A line of showers and thunderstorms, currently developing across
western Minnesota early this morning along an axis of instability,
is expected to track through the upper Mississippi Valley and
eventually Wisconsin later this morning. Although this area of
showers and thunderstorms may hold together, which isn`t a given
as some hi-res models do break this line apart as it tracks
through northern Wisconsin, it will outrun the axis of instability
as it tracks east. Therefore, any storms that make it through
Wisconsin this morning are not expected to be severe.

Attention then turns to the potential for additional showers and
thunderstorms this afternoon and into this evening as a warm front
lifts north and convection fires along the occluded front tracking
east through the western Great Lakes region. There are several
concerns and questions regarding the severe weather potential and
the associated types of severe weather that will be possible for
the second round of convection. The main concern is how much
destabilization can occur in the wake of the first round of
convection that tracks through this morning. There are some models
that keep showers and thunderstorms ongoing for most of the day,
which would inhibit any development later today as there would not
be enough time for instability to recover. The next concern is
where the warm front will end up today. Recent model runs indicate
the warm front is forecast to lift more north than previous
forecasts and makes a surge into central Wisconsin. This would
obviously increase severe weather chances overall and especially
the tornado threat as storms would likely be surface-based
instead of elevated as they would be north of the warm front. Of
all the parameters in play the shear is the least in question as
models agree that there will be 40-60 knots of bulk shear for any
storms to work with.

If enough destabilization can occur this afternoon, as is
forecast by a few of the CAMs models, with MUCAPEs of 1000 to
1500 J/kg there is a threat for damaging winds and large hail (5
to 14 percent probability) this afternoon and evening, mainly
south of Rhinelander to Sturgeon Bay. There will also be a threat
for a few tornadoes (2 to 4 percent probability) in the warm
sector, which is currently forecast to lift north somewhere along
a line from Tomahawk southeast to Manitowoc as 0-1km SRH values
soar to 200 to 300 m2/s2 in this region.

In addition to the severe weather threat, heavy rainfall will also
be a concern and as a result some flooding is possible. PWATs
during this event will hover around 1.5 inches, or 175 percent
above normal, along with long skinny CAPE profiles which means the
storms will be efficient at producing rain. Although the storms
are expected to be fast moving, around an inch of rain is expected
to fall through this evening with some areas receiving upwards of
2 inches in the areas that get hit the hardest from both rounds
of rainfall. Area rivers are already near bankfull given the
recent rounds of heavy rainfall, with any additional rain likely
to push several rivers into flood stage.

The storms will push east of the area as the occluded front
tracks through the area, with the severe potential waning late in
the evening with the loss of daytime heating as instability falls
to just a few hundred J/kg by midnight. Dry weather is expected
behind the low pushing off to the east late tonight, which is
expected to last into Saturday as high pressure builds over
Illinois with a ridge extending north through the western Great
Lakes. Mostly sunny skies and generally pleasant weather are
expected for the first part of the holiday weekend.

Long Term...Saturday Night Through Thursday

Models are in pretty good agreement with the movement of a
shortwave trough/closed upper low that will impact northeast WI
Sunday into Memorial Day. Models are also pointing at potentially
heavy rainfall with this system which may lead to localized
flooding and rising river levels. A modest upper ridge will bring
a couple of days of dry weather mid-week before the next system
approaches late next week. Temperatures appear to be below normal
through mid-week before warming closer to normal late week.

Saturday night and Sunday...
Clouds will be on the increase/thicken across the area Saturday
night as high pressure moves farther away and a new system pushes
across the central Plains. Min temperatures to be in the middle to
upper 40s north, upper 40s to lower 50s south. Conditions are
looking better for a soaking rain to overspread the region on
Sunday. Low pressure with attendant cold front will lift north
and east into WI, while a trailing mid- level shortwave trough
over the Upper Midwest sends a strong vorticity max into WI in the
afternoon. A diffluent flow aloft to also exist, adding to the
lift from the cold front. Strong low- level moisture advection
into the area will bring PW values to around 1.5", thus heavy
rainfall is a distinct possibility. Instability appears marginal
as precipitation arrives in the morning, but bulk shear of 35 to
45 knots could produce some stronger storms. Max temperatures
Sunday to mainly be in the middle 60s to around 70 degrees.

Sunday night and Memorial Day...
The showers/thunderstorms will likely carry over into Sunday
evening before diminishing later Sunday night as the cold front
shifts east and the shortwave trough pushes across WI. If this
heavy rain does occur (models are showing 1-2"), will need to
watch river levels as the ground is saturated from rains earlier
in the week. Otherwise, mostly cloudy skies to persist with min
temperatures in the middle to upper 40s north, upper 40s to lower
50s south. There are still some model differences on Memorial Day
regarding a secondary shortwave trough diving southeast across the
Upper MS Valley. There is enough evidence between a cyclonic flow
behind the departed system and this approaching trough to keep
pops in the forecast through the day. Max temperatures for
Memorial Day will be in the lower to middle 60s north/near Lake
MI, middle to upper 60s south.

Monday night and Tuesday...
Chance of showers/isolated storms would continue into Monday night
until this latest shortwave trough exits the area. As an upper
ridge enters the Rockies on Tuesday, a prevailing northwest flow
into WI to exist and models are showing one more shortwave trough
to move southeast within this flow into WI. Cool air aloft
combined with daytime heating should create enough instability to
generate additional showers/some rumbles of thunder, especially
during the afternoon. Increasing mid-level Q-G forcing associated
with the shortwave trough will also aid in precipitation
development. Max temperatures Tuesday to be around 60 degrees
north-central WI, middle to upper 60s elsewhere.

Tuesday night and Wednesday...
Any precipitation will end Tuesday evening with the loss of
daytime heating, the departure of the shortwave trough and high
pressure which will be building across the Upper Midwest. This
high pressure is forecast to move into the western Great Lakes on
Wednesday and finally provide for a dry day across the region.
Under mostly sunny to partly cloudy skies, look for max
temperatures to be in the middle 60s north-central WI/near Lake
MI, upper 60s to lower 70s elsewhere.

Wednesday night and Thursday...
The high pressure to slide toward the eastern Great Lakes on
Thursday, but still have an influence on our weather through the
day. A return of southwest winds will pump up max temperatures
Thursday into the upper 60s along Lake MI, lower to middle 70s
inland.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 625 AM CDT Fri May 24 2024

High clouds will continue streaming in across the area this
morning as an area of showers and thunderstorms tracks through the
region later this morning through the early afternoon. There
should be a break before another area of showers and thunderstorms
sweeps through the area from west to east coincident with an
occluded front expected to move through the area. This second line
of showers and thunderstorms could produce strong to severe storms
depending on how much of a break there is between the two waves
and if enough instability can build. Conditions are expected to
fall to MVFR/IFR with the waves of showers and thunderstorms with
worse conditions expected with the second line. Skies will then
clear out from west to east overnight as the occluded front and
attendant showers and thunderstorms track off to the east.

East winds will slowly increase early this morning to 10 to 20
knots then turn southeasterly this afternoon. Winds will then turn
westerly behind the occluded front tonight.

&&

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

&&

$$

DISCUSSION.....Kurimski/Kallas
AVIATION.......Kurimski