FXUS63 KGRB 181757
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1257 PM CDT Mon May 18 2026

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Strong to severe storms and locally heavy rainfall possible at
  times through tonight. The greatest coverage of storms will
  occur across the northwest half of the forecast area in the 
  late evening and overnight hours tonight.

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

&&

.DISCUSSION...
Issued at 239 AM CDT Mon May 18 2026

A cluster of strong thunderstorms was ongoing over north central 
WI early this morning, in advance of a short-wave trough. This 
activity had marginal instability (MUCAPE 1000-1500) and strong 
deep layer shear (45-50 kts) to work with, and a few storms were 
pulsing to near-severe levels and exhibiting mid-level rotation at
times. A marginal severe threat will continue over north central 
and perhaps far northeast WI through about 5 am. Otherwise, low 
clouds and areas of fog (locally dense) covered much of the 
region, and temperatures were stuck in the upper 40s to middle 
50s. A warm front was situated across southern WI, where 
temperatures were in the lower to middle 70s.

The warm front will lift north today, but there is some question 
if it will reach northern WI this afternoon. As the front lifts 
north and daytime heating commences, the foggy conditions should 
improve this morning. Have lowered max temperatures several 
degrees, especially over northern WI. Still expecting decreased 
coverage of storms today due to cloud cover, capping, a weakening 
low-level jet and weak mid to upper level ridging. Decreasing deep
layer shear should also result in a lesser threat of severe storms
during the day. Will keep slight chance to chance pops through 
the day and into the early evening hours. 

A surface wave will lift into north central WI later this evening,
followed by a cold frontal passage during the overnight hours.
CAMs show an uptick in thunderstorms over central and north 
central WI later in the evening, with a linear MCS moving through
mainly north central and far northeast WI during the overnight 
hours. The late night timing of this is not especially favorable 
for severe storms, though MUCAPE of 1000-1500 j/kg and deep layer 
shear increasing to 35-50 kts supports severe potential.

The cold front sweeps through eastern WI Tuesday morning with 
lingering showers and a few storms, but dry weather returns by 
afternoon. Tuesday will be a windy day with west to northwest 
winds gusting to 25 to 35 mph. Temps over eastern WI will remain 
above normal with highs in the low to mid 70s, but cooler readings
in the 50s and 60s are expected farther west.

High pressure will bring dry and cooler conditions for the middle
of the week, with potential for frost or freeze headlines Tuesday
night and Wednesday night. Warmer temperatures and small 
precipitation chances return for the weekend.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1256 PM CDT Mon May 18 2026

Low pressure and a modified warm front lift across WI through 
tonight. Cold front sweeps through on Tuesday morning. Subsidence
in wake of cluster of storms over Lake Michigan and lower Michigan
will restrict pop-up showers and storms this afternoon. As the warm
front lifts through, could be isolated showers and storms late this
afternoon into this evening. Another line of showers and storms will
move into central and north-central WI late this evening and overnight.
Not clear whether this line makes it into eastern WI. A few lingering
showers and storms on Tuesday morning, then it dries out with gusty
northwest winds developing behind the cold front. 

Cigs MVFR to VFR today, lowering to IFR to MVFR late tonight into early
Tuesday. Cigs improve to VFR in all areas on Tuesday afternoon.

Southerly winds gust to 15-20 kt into this evening, with winds shifting
southwest into the overnight. Winds shift to northwest Tuesday morning,
with gusts to 25 kt.

&&

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

&&

$$

DISCUSSION.....Kieckbusch
AVIATION.......JLA