FXUS63 KGRB 180538
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1238 AM CDT Mon May 18 2026

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Strong to severe storms and locally heavy rainfall possible at
  times through Monday night. Timing and coverage of severe storms
  still uncertain beyond tonight.

- Drier and cooler weather returns mid to late week. Frost or 
  freeze headlines may be needed Tuesday night and Wednesday 
  night.

&&

.DISCUSSION...
Issued at 232 PM CDT Sun May 17 2026

Round of strong to severe thunderstorms are main focus through
Monday night. What a difference a day makes. Surface warm front
still to the south and with northeast winds, temps are stuck in
the 40s to lower 50s. Springtime in the Great Lakes. Despite near
stable cold layer, elevated instability into WI has supported 
organized storms that are now as close as southern Fox Valley to
the lakeshore. These storms are being forced by MCV that came out
of IA earlier this morning. Based on radar trends and near term
CAMS, expect initial showers/tsra to impact the Fox Valley and 
lakeshore areas through late this afternoon (4-5p). Then as 850mb
front lifts north with approaching low-level jet and elevated
instability, could see additional development of showers and
storms through the evening. Lack of MCV/velocity enhancement and
sfc warm sector still well to the south suggests that hail would
be the primary hazard with this activity. 

Attention this evening eventually turns to developing cluster of 
severe storms along NE/SD border as many CAMS indicate this will 
grow upscale and ride eastward, arriving over western WI after 
midnight. If this line of storms hold together they would be 
supported by increasing low-level shear and could pose a damaging
wind threat as they push into western and north-central cwa overnight.
At the least, wind gusts over 40 mph and heavy downpours can be 
expected. As this moves through, sfc warm front will finally be
arriving into our area from the south. If this front is a little
faster moving to the north, would be a better shot at sfc based
severe storms/damaging winds. 

Still will ride with working theory supported by good portion of
near term guidance that in wake of the overnight showers and
storms and in a more capped warm sector, there could be a min in
showers and thunderstorms during the day on Monday. Will be warmer
day and the most humid day in a while (dewpoints well into the 
60s). Going to have to watch if there is an outflow layed out by
the overnight convection as this could lead to additional
convective initiation (CI). Also, some models still graze 
southeast area (OSH, MTW) with round of showers and storms late 
morning through early afternoon. Given the extent of instability 
that will be present by that time (MLCAPEs up to 2000J/kg), strong
to severe storms could occur. Chance pops cover the scenario on 
Monday for now with details to still be resolved.

Primary cold front shifts through late Monday night or even
Tuesday morning. Still hints of pre-frontal sfc trough, waves of
low pressure moving through ahead of the front, perhaps modulated
by upstream convection. Cannot rule out severe storms late Monday
evening or even overnight Monday night as any lift from these 
mechanisms will be interacting with MUCAPE of 1500-2500J/kg and
0-1km shear over 30 kts. The stronger low-level shear ahead of the
main cold front could offset the typical downward trend in severe
potential that typically is seen deeper into the night. No qualms
with SPC Day2 slight risk. 

Cold front sweeps through Tuesday morning with residual chance of
showers and a few storms. All this comes to an end by Tuesday
afternoon. Tuesday will be a breezy day as southerly winds shift 
to the northwest. Temps over eastern WI will remain above normal
with highs in the low to mid 70s. Readings will stay in the 40s
and 50s in Vilas County. 

Small chances of showers and storms return late week into the 
weekend. Main issue this week will be potential for frost or 
freeze headlines Tuesday night and Wednesday night.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1238 AM CDT Mon May 18 2026

Scattered to widespread rain and thunderstorms are expected
overnight as a line of convection moves in from the west. Isolated
severe storms are possible in central and north-central WI within
this line through ~09Z, with hail and wind gusts of 40 to 60 mph.
However, the threat for severe storms at any one location is low.
The more likely hazard is brief, heavy rain. Timed out best 
window for TSRA at each TAF site and included PROB30 groups. 
Meanwhile, mainly LIFR conditions are expected through the night
due to low clouds and fog. Some improvement from south to north
late tonight as a warm front lifts north. 

Thunderstorm potential should diminish by ~12Z Monday morning, 
with some lingering showers through mid-morning. Uncertainties 
arise after this, but the general consensus shows a cap in place 
for Monday which would limit any thunderstorm develop during the 
afternoon and evening, along with MVFR and VFR flying conditions.
There is a greater threat for storms after 03Z/Tue as another 
line develops to the west and moves into the area.

LLWS develops continues overnight and diminishes around 12-13Z. 
Easterly surface winds will veer to the south overnight, becoming 
southwest on Monday. Gusts up to 25 kts are possible at times.

&&

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

&&

$$

DISCUSSION.....JLA
AVIATION.......KLJ