541
FXUS63 KGRB 200353
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1053 PM CDT Sun May 19 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Thunderstorms are possible Monday. An isolated severe
  thunderstorm could occur during the afternoon south of Highway
  29. The main threats will be damaging winds and large hail.

- There is increasing confidence in the potential of a significant
  severe weather outbreak late Tuesday afternoon into Tuesday
  evening. Damaging winds, large hail and a few tornadoes are
  possible. Heavy rainfall and localized flooding is also
  possible.

&&

.DISCUSSION...
Issued at 246 PM CDT Sun May 19 2024

Short Term...Tonight and Monday

High pressure ridge that is bringing dry weather this afternoon
will give way to low pressure system and approaching warm front
late tonight. Wave of low pressure eventually makes it to western
Wisconsin on Monday while warm front attempts to lift south to
north across Wisconsin.

As low-level jet and elevated instability shifts into western
Wisconsin late tonight, swath of showers and some elevated
thunder will shift into central Wisconsin late tonight, even more
so toward daybreak on Monday. Coverage of these showers will be
enhanced by divergence aloft from upper jet across Lake Superior.
On Monday, showers and some thunder will shift west to east through
midday riding along the nose of low-level jet. Don`t expect any of
these storms to be severe, but with 1.25+ inch PWATs moving in
there could be heavy downpours. Most areas will see less than 0.50
inch of rain through early afternoon.

There is then loose agreement of a lull behind this initial wave
of showers and thunder from the subsidence before additional
showers and storms develop as sfc warm front and building
instability arrive from the southwest as highs most areas rise
into at least the lower 70s and dewpoints push toward the 60 mark.
Appears greatest coverage of the additional convection will stretch
from central Wisconsin to east-central Wisconsin. Though MLCAPEs
could rise above 1000J/kg, effective shear is weaker mainly 20 kts
or less, suggesting pulse type mode to any storms. However, the
lack of forecast shear may be offset by convective modulated
shortwave that moves through as well mid to late aftn. If that
wave is stronger (models differ on the strength), then at least
isolated severe storms would be possible over southern portion of
the forecast area where SPC currently has a marginal risk. Any
storms will have the potential to produce heavy downpours as the
PWATs stay in the 1.30 to 1.50 inch range.

Long Term...Monday Night Through Sunday

Thunderstorms trends, including severe and heavy rainfall
potential Monday evening and Tuesday afternoon and evening, will
be the main forecast concerns.

Depending on the timing of an MCV, there may still be a chance of
thunderstorms and a marginal severe threat over the southeast part
of the forecast area early Monday evening. This would occur as
the SE CWA briefly resides in the warm sector of a weak low
pressure system tracking through the area. If the MCV moves
through quicker, the severe threat would be confined to the
afternoon. Regardless, there does appear to be a lull in the
precipitation chances from 03z-12z/Tuesday, along with potential
for patchy fog. Lows should be in the upper 40s and 50s.

Confidence is increasing for a potentially significant severe
weather event Tuesday late afternoon and evening. A potent low
pressure system (990 mb low) is expected to track just west of the
forecast area late Tuesday afternoon and evening. Timing of this
will be critical, as an earlier arrival would help to maximize
instability in the warm sector over the southern part of the
forecast area area. Even if the system arrives in the evening,
advection of warm/moist air (dew points in the 60s) and steep
mid-level lapse rates (7-8 C/km) should support at least a narrow
tongue of modest instability into the SW part of the CWA. The
strength of the dynamics (negatively-tilted short wave trough and
low-level convergence with a 50-60 kt LLJ) and shear (0-6 km shear
of 50-60 kts, and 0-3 km helicities of 400-800) should help
overcome limited instability and bring a round of impactful severe
weather to the region. Supercells with damaging winds, large hail
and a few tornadoes will be possible. Given the dynamics and
anomalously moist air mass, heavy rainfall and localized flooding
will also be a concern, especially in any areas that received
heavy rainfall with earlier rounds of convection.

Wrap-around showers to persist over the northwest part of the
forecast area on Wednesday, followed by mainly dry conditions
Wednesday night through Thursday night. A cold frontal passage
will bring the next significant chance of rain later Friday into
Saturday, but timing issues are evident.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1053 PM CDT Sun May 19 2024

VFR conditions will continue late this evening and most of
tonight with mainly mid and high clouds spreading across the area.
A few sprinkles or light showers are are possible, but will have
no impacts to flying conditions.

An area of showers will arrive from southwest to northeast after
08z ahead of an approaching low pressure system and frontal
boundary, continuing at times into Monday morning. Weak
instability could allow for a few embedded thunderstorms. A break
is expected for much of the late morning and afternoon, with
additional showers and storms developing later in the day as the
warm front lifts across the area, especially across eastern WI,
with more spotty activity further west. Still some uncertainty
just how long the break will be and where the second round of
activity will be focused. VFR conditions look to last through at
least 11z then MVFR and possibly some IFR conditions are expected
at times on Monday, associated with the heavier showers and
stronger storms. The strongest storms could produce gusty/erratic
winds, hail and heavy rain. Tried to limit the thunder/ceilings/
visibilities conditions as best as possible to 2-3 hour windows
where the threat looks to be greatest, but those may need to be
shifted/changed as models hone in on exact timing of each round of
showers/storms. Another round of showers and storms looks to
arrive late Monday night (after 06z).

Fog may develop on Lake Michigan Monday into Monday night as
dewpoints climb ahead of the low, but should stay mainly over the
lake, so will not include at MTW. In addition, some fog will be
possible across most land areas Monday night as winds die off and
recent rain will provide moist low levels, but winds just off the
surface and cloud cover may limit this threat.

&&

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

&&

$$

DISCUSSION.....JLA/Kieckbusch
AVIATION.......Bersch