536
FXUS63 KGRB 210006
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
706 PM CDT Mon May 20 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Impactful strong to severe storms are anticipated across
  portions of the area Tuesday. The areas most likely to see
  severe weather will be central to east-central WI and
  southwards. Damaging winds, large hail, and a few tornadoes will
  be possible with stronger storms.

- Heavy rainfall on Tuesday may also pose a flooding concern.

&&

.DISCUSSION...
Issued at 331 PM CDT Mon May 20 2024

Short Term...Tonight and Tuesday

The upper trough that brought rain and some thunderstorms to areas
of central and east-central Wisconsin will gradually move
eastwards and out of the region by the evening hours today. A few
showers and some brief thunder may still cross portions of the
central to southern Fox Valley through Manitowoc, but most of the
area will see quieter conditions move back in to the region for
the rest of the day.

Tonight, the recent rainfall combined with lingering dewpoints and
light winds will allow for some ground fog across the area. The
densest fog will likely be limited to portions of central to
north-central Wisconsin.

Tuesday will be the main concern for the forecast period. A strong
negatively tilted 500mb trough will approach the area, aiding in
the rapid development of a surface low pressure system over Iowa
and Minnesota. As this fast moving low lifts towards the region, a
surface warm front will come northwards to central to east-central
Wisconsin. This late morning to afternoon time period will be the
first to watch, as most models bring some initial development
ahead of the surface warm front. Precipitable water content at
this time will be near their climatological max for this time of
the year, so very heavy rainfall will be possible, especially in
any convection. This initial round may also be accompanied by
strong gusty winds but other severe concerns are not expected for
this portion of the system.

As we get to the late afternoon to evening, the surface low will
continue to deepens and shift northwards lifting the warm front
further north. At the same time, the attendant cold front will
sweep across the region from the west. This will serve as the
trigger for a strong organized fast moving round of convection.
Given the strong ambient shear but more modest instability
(especially off the surface) suspect storm organization will take
the form of a quasi-linear system, potentially with some embedded
supercells. Any storms that develop will be fast moving, so wind
and large hail will be possible with any convection. With low LCL
heights and plentiful ambient helicity, tornadogenesis will be
possible with any embedded strong storms as well, with the
greatest threat likely being along the interaction between the
warm front and cold front across central to east-central
Wisconsin. In short, all severe weather threats will be of concern
Tuesday evening. The main question will be the northerly extent
of severe potential, as areas in northern Wisconsin may see fairly
limited severe weather while those along or south of a line from
Wausau to Green Bay will see the potential for severe weather.
This second round should develop to our west and cross the region
fairly rapidly, with drier conditions arriving around midnight,
plus or minus an hour or two.

Long Term...Tuesday Night Through Monday

The main focus in the forecast period will be the Tuesday to
Tuesday night system. The extended period is relatively quieter.

Quiet conditions follow the Tuesday system, with dry conditions
expected across the region for the middle of the week.
Temperatures during this period will primarily be in the middle
60s to middle 70s.

The next chance for active weather will be around the end of the
week as a low pressure system cross southern Canada and clips the
northern portion of the area. Timing remains uncertain for this
system so the overall thunderstorm strength potential remains
uncertain at this time.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 705 PM CDT Mon May 20 2024

Widespread shower activity has exited into Lake Michigan, with
only a stray shower or two expected the rest of the evening
possible. A mix of VFR/MVFR conditions are expected for much of
the evening, with some breaks in the clouds expected at times,
then low clouds and fog is expected to develop across much of
the area, with IFR/LIFR and possibly some VLIFR conditions
overnight into early Tuesday.

Models remain on different pages on how things will play out on
Tuesday, with a couple/three waves of showers and storms possible
across the area. A round of showers and isolated storms is
possible in the morning associated with the warm front, then a
another stronger round or two of showers and storms is expected as
the cold front sweeps east across the area. These storms are
likely just after the current TAF period, but may reach central WI
after 21z if things initiate earlier than expected. The storms
could produce very strong winds, large hail and heavy rain. Due to
the model timing differences, have not added TEMPO grounds for the
thunder just yet.

&&

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

&&

$$

DISCUSSION.....Uhlmann
AVIATION.......Bersch