978
FXUS63 KGRB 030331
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1031 PM CDT Sun Jun 2 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Showers are likely with a chance of thunderstorms on Monday.
  There is a marginal risk of severe thunderstorms over parts of
  east-central Wisconsin in the afternoon. Gusty winds, small hail
  and brief heavy rain would be the main threats.

- More strong storms are possible Tuesday afternoon into Tuesday
  night. Gusty winds, small hail, and heavy rain will be the
  primary hazards.

- Conditions hazardous to small craft will be possible at times
  from Wednesday through Saturday.

&&

.DISCUSSION...
Issued at 216 PM CDT Sun Jun 2 2024

Short Term...Tonight and Monday

Main forecast challenge to be timing/extent of showers and
thunderstorms on Monday with the small possibility of stronger
storms in the afternoon.

The 19Z MSAS surface analysis showed high pressure that extended
from northern Lake Superior to the mid-MS Valley. An area of low
pressure was situated over central SD with a warm front that
stretched southwest into southwest IA. Visible satellite showed
high clouds streaming over WI with fair weather cu that had
developed in the heating of the day.

A mid-level shortwave trough will move across the northern Plains
into the Upper MS Valley tonight. Clouds are expected to steadily
be on the increase through the night as south winds pull moisture
northward. This moisture transport, coupled with increasing
isentropic lift, will bring a chance of showers/thunderstorms into
north-central and central WI toward daybreak. There could be some
patchy fog to develop near Lake MI as moist air moves over the
cooler lake waters. Min temperatures tonight to range from the
lower 50s near Lake MI, to the upper 50s across north-central/
central WI.

The shortwave trough sweeps into the western Great Lakes on Monday
and bring showers/chance of thunderstorms to all of northeast WI.
The precipitation will begin to diminish in the afternoon across
central WI as the better forcing shifts east with the trough. Some
of these storms will have the capability of becoming strong as
shear reaches 30-40 knots in the afternoon, especially over east-
central WI. Models indicate MUCAPES of 500-1500 J/KG, but much of
this instability will depend on how fast showers/storms move east.
Latest guidance from SPC has moved parts of east-central WI into
the marginal risk for severe thunderstorms based on some models
developing a weak low pressure on the backside of the shortwave
trough. Gusty winds and small hail would be the main threats.
Another factor to watch will be for locally heavy rains as PW
values climb to around 1.5 inches and the forecast area to be
under the favorable right entrance region of the upper jet for
lift. Max temperatures Monday to be in the middle 60s near Lake
MI, lower to middle 70s inland.

Long Term...Monday Night Through Sunday

An energetic weather pattern will bring periods of showers and
storms from Monday night through Wednesday thanks to southwest flow
aloft and occasional shortwave energy traversing the region.
Thunderstorm chances and potential impacts are the primary forecast
concerns during this period.

Thunderstorm Chances:  Shortwave energy will be exiting the region
by the start of Monday evening.  Modest elevated instability will
remain in place behind the shortwave, but large scale ascent will
depart with the wave and nocturnal stabilization should lead to only
low chances of showers and storms.  Most models generate light qpf
over far northeast Wisconsin and this looks appropriate.

Northern WI will be in a weak warm advection pattern for the rest of
the night into Tuesday morning.  Relatively steeper mid-level lapse
rates near 7 C/km will remain over southern WI and northern IL
during this time and a relative lull in forcing should occur. Will
likely see a break in the precip coverage during this time.

This may change on Tuesday afternoon if shortwave energy arrives
from the southwest and gets to interact with daytime instability.
Not all guidance shows this shortwave moving into northern WI, so
confidence is relatively low with the details.  But if it does,
dewpoints will be creeping up into the middle and upper 60s by this
time and modified forecast soundings indicate instability upwards of
2000 j/kg. With temps approaching their convective temps by midday,
conditions support scattered thunderstorms developing through the
afternoon. Localized heavy rain will be possible given the
relatively slow storm movement.

Additional thunderstorms are likely to arrive from the west during
the evening when a potent shortwave and associated cold front move
through the region.  Forcing looks robust for widespread
precipitation, but instability will be at its diurnal minimum so
thunderstorm chances will be waning through the night.  Better
chance of strong thunderstorms will reside west of the region.

The region will reside under upper troughing on Wednesday.  Models
depict weak instability developing with the heat of the day that
could lead to scattered storm development in the afternoon.  Far
northern WI will have the highest chances.

Severe Potential:  The main period to watch will be Tuesday
afternoon into Tuesday night.  As mentioned above, ample instability
(2000 j/kg cape) will be in place as dewpoints climb into the mid
and upper 60s.  However, deep layer shear is weak and roughly around
20 kts.  Seems like a recipe for pulse storms and perhaps an
isolated severe threat given the magnitude of instability and dcapes
around 700 j/kg.  Outflow boundaries and lake breezes could also
augment the forcing arriving aloft (if it does arrive).  Strong
winds and hail will be the primary threats.

Prospects for severe storms on Tuesday night look low as timing of
the front coincides with minimal cape of 200-400 j/kg.  While deep
layer shear will be increasing aloft, storms are likely to be
elevated, thereby reducing the severe risk.

Rest of the forecast:  Beneath upper troughing, showers look
possible on Thursday and Friday.  There will be a cooler airmass in
place by this time, so thunderstorm potential looks low.  Flattened
troughing looks to hang around through next weekend which will keep
temperatures near to slightly below normal.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1029 PM CDT Sun Jun 2 2024

Conditions will start to deteriorate late tonight into Monday
morning as a system approaches from the west. A line of showers is
expected to arrive early Monday morning, affecting RHI, CWA, and
AUW by around 10Z to 13Z. MVFR to IFR conditions are expected to
build in from west to east throughout the morning, becoming more
widespread by early afternoon.

As storms move through the area Monday morning, western TAF sites
are likely to exceed the LLWS threshold as a nocturnal low-level
jet propagates to the east. Onshore flow may also result in some
early morning lake fog making it inland to MTW. As a result,
visbys may briefly drop to MVFR.

Some strong to severe storms may be possible across east-central
Wisconsin early Monday afternoon through the evening. Best
potential for stronger storms would be from the southern Fox
Valley to the lakeshore. Opted not to include thunder in the TAFs
due to low confidence given timing and the effect of a stable
lake breeze shunting most instability north and west of the Fox
Valley.

&&

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

&&

$$

DISCUSSION.....Kallas/MPC
AVIATION.......Goodin