895
FXUS63 KGRB 190340
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1040 PM CDT Wed Jun 18 2025

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Thunderstorms will be possible at times From Thursday afternoon
  through Saturday morning. The time periods with greatest chances
  for thunderstorms include Thursday afternoon and evening and
  then again from late Friday night into Saturday morning when
  some of the storms could become strong or severe at times.

- Prolonged period of dangerous heat is expected Saturday through
  Monday as hottest temperatures of the year are forecast. Heat
  index values may reach the middle 90s to around 100 degrees each
  afternoon. Low temperatures in the 70s will provide minimal
  relief to the heat and humidity.

&&

.DISCUSSION...
Issued at 148 PM CDT Wed Jun 18 2025

Short Term...Tonight Through Friday

The latest RAP analysis and satellite/radar imagery show a
plethora of shortwaves within mean troughing stretching from the
northern Plains to eastern Oklahoma. Surface low pressure is
centered over west-central Illinois. Surface based instability is
in excess of 3000 j/kg is ahead of the surface low, but is
considerably more modest over northern WI (600-800 j/kg) where
temperatures are approaching their convective temps. A few cells
starting to pop over far north-central and northeast WI in a
region of agitated cu. Think cells will remain rather isolated
until 3-4 pm when convergence becomes better defined west of the
Fox Valley. But because deep layer shear is only about 20 kts,
think cells will be rather pulsey in nature and could produce
brief gusty winds to 40 mph and small hail. Otherwise, an area of
rain and isolate storms will move from central to northeast
Wisconsin through the rest of the afternoon. Focus of this
forecast generally remains on thunderstorm trends through Friday.

Tonight: Showers and thunderstorms will be scattered across parts
of central, east-central, and far northeast Wisconsin this
evening. Most of the thunderstorm activity is forecast to remain
west of the Fox Valley and Bayshore where instability will be slow
to wane. A few strong storms with gusts to 40 mph, small hail, and
locally heavy rainfall will be possible through early this
evening. Most thunderstorm activity should diminish by late
evening or early overnight.

Over east-central WI, areas of rain and isolated thunderstorms
will exit by mid-evening as well.

Where thunderstorm activity occurs, conditions will be more
supportive of fog developing after midnight. Statistical and
numerical guidance is not too keen on fog development, so kept it
out of the forecast for the most part and will pass along concerns
to the next shift.

Thursday afternoon-night: Shortwave energy will be moving across
the northern Great Lakes within northwest flow aloft and weak warm
advection in the low levels. Winds aloft will be increasing over
the course of Thursday as jet energy arrives from the northwest.
Convective temps will be quite low and in the upper 70s, which is
forecast to be breached early in the afternoon. Modest instability
around 1000-1500 j/kg will be reached by this time which will aid
in widely scattered to scattered thunderstorm development by 2-4
pm. Guidance is generally focused on far northeast WI in a region
of surface convergence. Inverted V soundings and instability aloft
will support potential for isolated cells or clusters capable of
producing large hail and damaging wind gusts.

Later in the night, a 30-40 kt low level jet will push the leading
edge of strong instability into the region. A cluster of showers
and storms could therefore arrive into central and east-central WI
after midnight. Considering the time of day and projected elevated
instability around 500-700 j/kg, these elevated storms should not
be severe but could produce small hail.

Friday: Morning thunderstorm activity should quickly exit east or
diminish by midday. Some surface destabilization should take place
in the afternoon, but that will be beneath warming temps aloft as
mid-level capping moves into central WI. As a result, think the
area will be dry for the afternoon and precip chances will
eventually lower in subsequent forecasts.

Long Term...Friday Night Through Wednesday

The latest ensemble means show the average 5 day 500 mb height
anomalies configured with a trough over the western third of the
CONUS and a dominant sub-tropical ridge over the eastern CONUS.

Thunderstorm trends:

As the sub-tropical ridge strengthens and broadens, a potent 45-50
kt low level jet will advect a large reservoir of instability
(2000-3000 j/kg) into Wisconsin on Friday night. Combined with
deep layer shear of 35-50 kts, severe ingredients look mighty
impressive after midnight on Friday night when models have been
insistent on showing MCS activity crossing the region. Far
northern WI and the Upper Peninsula continue to have the bullseye
as the most probable location, but too far out into the forecast
to have high confidence in the location and timing of a potential
MCS. Given the dynamics and instability, there is potential
however for all hazards to be possible late Friday night into
Saturday morning.

After the MCS passes, strong capping is forecast to be present for
the rest of Saturday. In fact, 700mb temps will be near the
climatological max for the day. As a result, very difficult to
envision convection redeveloping during the heat of the day and
the medium range models are generally void of precip.

The capping aloft, although not as robust, figures to continue to
be present through Sunday. By Monday, minor height falls and a
cold front are forecast to move into Wisconsin. Plenty of
instability will remain present by this time (1000-2000 j/kg) and
deep layer shear (25-35 kts) will also be sufficient for organized
storms. The chance of storms will continue into next Wednesday
thanks to southwest flow aloft and a frontal boundary in close
proximity to the area.

Excessive Heat:

No significant changes to the potential excessive heat for the
weekend. This will be the first significant heat wave of the
season to impact the area and quite a departure from recent
sensible weather. Most of the area will be in the major category
for heat risk on Saturday and Sunday. The extreme heat risk even
pokes into the Fox Valley on Sunday. Low temps will be near record
high mins on Friday night, Saturday night, and Sunday night, make
relief at night difficult to come by. Max heat indices continue
to trend higher on Saturday and Sunday afternoons, with 100s
becoming more widespread over central and northeast WI. Therefore,
confidence is increasing for excessive heat headlines this
weekend.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1032 PM CDT Wed Jun 18 2025

The steady rain in eastern WI was gradually shifting east late
this evening, while scattered showers and isolated storms
continued farther west. Expect the decreasing trend to continue,
with mainly dry conditions overnight.

Developing light NW winds should keep marine fog out over Lake
Michigan overnight, but there may be enough clearing to support
patchy fog development over NC/far NE WI, especially where any
significant rains occurred this afternoon. The fog should quickly
dissipate early Thursday.

A short-wave trough will move through northern WI Thursday
afternoon as a weak front sags south into the area. This will
cause scattered thunderstorms to develop in an unstable air mass.
Have added Prob30 for thunderstorms at the TAF sites for the
afternoon. The threat of storms may continue in the southwest part
of the forecast area into the evening hours.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......Kieckbusch