550
FXUS63 KGRB 220549
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1249 AM CDT Wed May 22 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

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

- Heavy rainfall through this evening may also pose a flooding
  concern, especially as areas of central WI have already received
  an inch of rainfall.

- Showers and thunderstorms possible at times Wednesday and
  Thursday, mainly across northern Wisconsin.

- Better chance for widespread showers and thunderstorms Friday
  into Friday night.

&&

.DISCUSSION...
Issued at 258 PM CDT Tue May 21 2024

Short Term...Tonight and Wednesday

Initial convection ahead of the warm front

The first round of convection is on its way this afternoon, with
showers and a few thunderstorms developing north and ahead of the
warm front. These initial storms remain capable of producing
moderate to heavy rainfall, some small hail, and the occasional
wind gust up to 30 to 40 mph.

The bigger concern will be a second round this evening, as a
second round of strong storms is brought in along a surface cold
front. This line is already in development on radar in western
Iowa this afternoon, and is expected to steadily cross towards and
into Wisconsin by the early evening. Although instability remains
on the more modest side in our area, with MUCAPE around 1000
J/kg, the remaining dynamics remain impressive. Bulk shear and
helicity values remain incredibly high, and wind fields support
these storms to be quite fast moving once they arrive in the
evening. Therefore, one of the primary concerns will be strong
wind gusts reaching the surface. Additionally, low LCL heights and
high helicity values associated with these storms will support
tornado-genesis, especially in portions of western to central
Wisconsin. North and east of these areas the tornado threat
quickly drops off, but again wind is expected to be the
predominant concern, with gusts up to 60 to 70 mph possible at
times.

Finally, localized flooding can`t be discounted with these storms
either, as some portions of central WI have already reported near
an inch of rainfall from this morning and although the next round
of storms will be fast moving, they will also bring additional
heavy rainfall.

The main uncertainties between this afternoon and the evening
remain the norther extent of the warm front and whether any
additional development precedes the arrival of the second line.
Clearing on satellite evident in southern Minnesota and Wisconsin
do support this front to begin lifting towards central Wisconsin
by this evening, which would also support stronger storm
development. If any storms do develop ahead of the main line as
well, they too may ingest enough ambient shear and helicity to be
a severe concern, but these will be more localized than the
longer, stronger line.

Finally, as the system departs, dry conditions are expected to
return to the region fairly quickly overnight. Winds however will
remain gusty, with some gusts up to 25 to 35 mph still possible
through the overnight into Wednesday. Cooler conditions follow for
tomorrow.

Long Term...Wednesday Night Through Tuesday

Zonal flow is expected to persist over the region through this
forecast period with a few embedded shortwave/troughs bringing
chances for showers and storms at times. Timing out these
shortwave/troughs will be the main forecast challenge.

The greatest confidence for showers and storms appears to be Friday
into Friday evening from a cold frontal passage. Models suggest
marginally severe parameters will be present as the cold front moves
through, but it is too early to determine the full severe potential
at this time.

High pressure will follow this cold front for Saturday into Saturday
night, leaving dry conditions in the forecast. Models are not in
agreement with the next chance for showers and storms due to
disagreement with the evolution of two mid-level shortwaves riding
along the zonal flow late this weekend. This translates to chances
for showers and storms from Sunday through Tuesday, but anticipate
the timing to become more confined over the coming days.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1249 AM CDT Wed May 22 2024

Line of showers and storms will exit into Lake Michigan by 2am,
with lingering showers possible at times the rest of the night,
especially across northern WI. Look for VFR/MVFR ceilings to drop
to IFR across central and northern WI as low pressure tracks into
western Ontario. The low clouds will slowly lift Wednesday morning
then scatter out through the afternoon.

Winds will become southwest/west overnight into Wednesday and
become gusty. Gusts of 30-35 kts are expected, highest late
Wednesday morning and afternoon. LLWS will end across eastern WI
by 09z as the stronger winds aloft decrease.

&&

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

&&

$$

DISCUSSION.....Uhlmann/Kruk
AVIATION.......Bersch