108
FXUS63 KGRB 170000
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
700 PM CDT Mon Jun 16 2025

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- A line or broken line of storms is forecast to impact the area
  from late evening through the middle of the overnight. Some
  storms could be strong to severe between 9 pm and 1 am across
  central and north- central Wisconsin. The main threats would
  include strong winds and hail.

- Thunderstorms will be possible at times from Wednesday through Saturday.
  There is some potential for severe weather on Thursday and late
  in the day on Friday. However, details concerning timing and
  location are uncertain at this time.

- A hot and humid airmass will move into the region this weekend
  into early next week.

&&

.DISCUSSION...
Issued at 208 PM CDT Mon Jun 16 2025

Short Term...Tonight and Tuesday

The latest RAP analysis and satellite/radar imagery show several
embedded shortwaves moving east across the northern Plains and
western Great Lakes early this afternoon. Small clusters of
showers continue over central WI and southern WI, but there has
been a diminishing trend over the past several hours after
dropping areas of heavy rain (1-2.5" inches) over northwest Wood
and western Marathon counties earlier. Other clusters of storms
are crossing the Minnesota/Wisconsin border within a region of
warm advection/moisture convergence. This activity is occurring
ahead of low pressure centered over west-central Minnesota. As the
surface low tracks east-northeast and drags a cold front across
the region, forecast concerns revolve around the thunderstorm and
severe potential tonight.

Thunderstorm trends through tonight: Low pressure will track from
west-central Minnesota to western Lake Superior this evening to
the north shore of Lake Superior overnight. This low will push a
cold front across the region from late evening into overnight.
Details of thunderstorm evolution is becoming more clear though
some details still need to get worked out. In particular, the
ingredients producing the cluster of storms over southeast
Minnesota is forecast to move into central WI early this evening.
The convective allowing models weaken this convection before
reaching central WI which matches the latest observational trends.
Will continue to show a low chance of storms over central WI
through early in the evening.

The main round of storms is forecast to come through as a swath or
broken line segment from about 9 pm through 2 am tonight. The
strongest storms should occur over central and north-central WI as
storms will be weakening as they move east.

Severe weather potential: Ingredients are modest and nothing
stands out as particularly impressive. Most unstable capes will
range from 1000-1500 j/kg and effective shear will range from
30-40 kts. Mid-level lapse rates around 6.5 C/km and downdraft
cape up to 1000 j/kg point toward a low end hail and wind threat
in mostly elevated storms. In general, a marginal risk describes
this event well as there could be an isolated severe storm that
could produce large hail or damaging winds. The threat of severe
storms will diminish as storms move east and the severe threat
should be mostly over by midnight.

Rest of the forecast...The chance of showers and storms have
diminished on Tuesday and Tuesday night as the region will be
north of the cold front. As the next low pressure system moves
towards the region, the chance of rain will increase late Tuesday
night, primarily over central and east-central WI.

Long Term...Tuesday Night Through Monday

The 5 Day 500mb geopotential height averages from the ensemble means
indicate show a relatively stable pattern that consists of a trough
over or around the Pacific Northwest and sub-tropical ridging
building over the southeast CONUS. An expanding/intensifying heat
threat for the start of Summer will unfold over the western Great
Lakes from Friday through next weekend.

But prior to this warmer airmass arriving, focus generally remains
on thunderstorm chances from Wednesday through Friday as the
unsettled weather pattern continues.

Thunderstorms Wednesday:  Shortwave energy will be traversing the
region on Wednesday which will push a surface low northeast across
southeast Wisconsin.  Given this track, the reservoir of instability
will likely remain south of northern Wisconsin meaning that severe
weather probabilities are very low.  However, as precipitable water
values climb to around 160% of normal, heavy rain will be the
highest risk.  There is a 30-50% chance of greater than 1 inch of
rain over east-central WI from this system.

Thunderstorms Thursday and Friday:  Shortwave and jet energy will be
moving into the northern Mississippi Valley on Thursday.  The
airmass will be considerably more unstable by this time as models
project 1-2k J/kg of cape on Thursday afternoon with convergence
along a weak cold front providing lift in the low levels.  With the
jet energy aloft, moderate shear 30-40 kts are projected by the
models.  Seems like a low end severe threat will be possible Thu
afternoon if these trends continue.

By Friday, sub-tropical ridging will be building northward into the
Great Lakes.  With an increasing low level jet, a large reservoir of
instability will be building into the region through the day.  These
ingredients look favorable for MCS activity and severe weather at
some point on Friday and Friday night.

Heat over the weekend:  Very warm temperatures are forecast as 925mb
temps soar into the middle 20s celsius.  These temperatures would
support highs in the lower to middle 90s, which is cooler than the
current forecast portrays.  Therefore wouldn`t be surprised if temps
warm in subsequent days.  Dewpoints will be rising into the middle
60s to lower 70s by this time, so heat risks could come into play.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 645 PM CDT Mon Jun 16 2025

Scattered showers and thunderstorms with locally heavy rainfall
were ongoing over NC WI early this evening, and could impact the
western TAF sites for the next hour or two.

A line of thunderstorms over eastern MN is expected to track
through the forecast area between 03z-07z, and could produce wind
gusts to 40 kts at the western TAF sites. Models strongly suggest
a weakening trend with the storms as they shift east into a less
unstable air mass.

The storms will help to push a cold front through the area
overnight, and post-frontal low clouds (MVFR/IFR) should push
into the region. The best bet for IFR conditions will be across
NC WI (including the RHI TAF). The low clouds will gradually rise
during the mid to late morning, and should scattered out by
midday, leading to VFR conditions. Little or no precipitation is
expected during the day.

Low level wind shear is forecast to develop in the Fox Valley area
late tonight, but should weaken with the passage of the cold
front.

&&

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

&&

$$

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