371
FXUS63 KGRB 281747
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1247 PM CDT Fri Jun 28 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Thunderstorms are possible from late this afternoon through
   tonight which will bring an isolated chance for rainfall
   amounts greater than 1 inch, gusty winds, and hail. The
   overall severe thunderstorm potential is low.

 - Increasing winds and waves will create hazards for mariners and
   beach-goes into tonight. A Small Craft Advisory and Beach
   Hazard Statement have been issued.

 - Some rivers and streams will remain above bankfull or near
   minor flood stage into the middle of the week.

 - The next period to watch for heavy rain or stronger storms
   will be late Monday through Tuesday.

&&

.DISCUSSION...
Issued at 335 AM CDT Fri Jun 28 2024

Short Term...Today...Tonight...and Saturday

Precipitation will likely occur in two waves today and tonight.
The first wave, currently shown on doppler radar making its way
into central and north-central Wisconsin, will track through today
as a string of weak shortwaves tracks through the western Great
Lakes region. The surface low, currently analyzed over eastern
North Dakota, will slowly track east towards northeastern
Minnesota by this afternoon. This initial round of rain will be
fairly light, with rainfall amounts well below an inch.
Probabilistic rainfall amounts for today bear this out as the
probability for an inch is only around 10 percent and mainly
focused on western portions of central and north-central
Wisconsin. Some thunderstorms are possible this afternoon as
MUCAPEs rise to 300 to 600 J/kg. These initial storms are not
expected to be severe given the limited instability, forcing, and
effective shear of only 20 knots. Highs today are expected to
range from around 70 across the north, to the lower 70s across
central and east-central Wisconsin.

The second wave of rainfall will occur tonight as the main low
continues tracking east through the northern Great Lakes as the
attendant cold front makes its way into Wisconsin. The heavy
rainfall threat still isn`t high, but higher than the
precipitation today as the probability for an inch or more of
rain rises to 20 to 40 percent across the region. Therefore, while
rainfall amounts of 0.50 to 0.75 are generally expected tonight,
amounts up to 1 inch are possible. Thunderstorms will likely be
more likely tonight, especially this evening, with better forcing,
instability and shear. MUCAPEs rise to around 1000 J/kg along with
effective shear of 40 knots and a potent mid level shortwave
tracking through the western Great Lakes coincident with the cold
front. Some storms could be strong to severe with gusty winds and
hail the main threats. Lows tonight are expected to mainly be in
the lower to middle 60s.

Although the initial cold front will be through the area late
tonight into early Saturday morning, a secondary cold front will
track through on Saturday. This will bring additional chances for
showers on Saturday (20-30 percent) mainly across central and
northern Wisconsin. Some modest SBCAPE values of 300-600 J/kg will
mean thunderstorms will be possible with the shower activity but
should remain below severe levels. Highs on Saturday are expected
to range from around 70 across far north-central Wisconsin given
the earlier fropa from the second cold front, with highs in the
lower 80s across east-central Wisconsin with the later arrival of
the front.

Long Term...Saturday Night Through Thursday

Focus of this forecast revolves around the thunderstorm and heavy
rainfall potential from Monday night through Tuesday night.

Saturday night...A potent shortwave trough will be passing across
northeast Wisconsin in the evening.  With such strong forcing, could
see scattered showers lingering along the Lake Michigan shoreline
early in the evening. Instability is negligible by this point, so
the risk of strong thunderstorms is low.  Then clearing will occur
overnight.

Monday night through Tuesday night...On the backside of a large high
pressure system, strong warm advection will occur on Monday night
thanks to a 30-40 kt low level jet, pushing precipitable water
values (pwats) over 1.50 inches. Models generate widespread
precipitation in the elevated moisture gradient/convergent zone from
late evening into the overnight hours. Despite the strong ascent,
instability is very weak at 100-200 j/kg, so severe weather chances
look low.  But some showers could generate a moderate rainfall with
probability of 1/2 inch of rain from 25 to 50%.

After a relative lull in the precip on Tuesday morning, a second
round of precip is expected on Tuesday afternoon and Tuesday evening
along the cold front.  Most unstable cape values remain modest and
up to around 1000 j/kg, likely because of cloud cover and
intermittent light precip through the day keeping the boundary layer
cool.  With pwats climbing above 2.00 inches, heavy rainfall and
strong storms appear possible, so will need to monitor trends in
subsequent forecasts.

Scattered showers look to continue on Wednesday beneath flattened
troughing.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1244 PM CDT Fri Jun 28 2024

Showers will increase late this afternoon through the evening. The
greatest potential for thunder will occur 00z-04z. Showers taper
off late tonight with another round of isolated to scattered
showers and possible thunder Saturday afternoon north central to
east central WI.

Conditions have trended down to mainly MVFR already and will
settle into widespread IFR to low-end MVFR into this evening.
LIFR conditions remain possible for central and north-central WI
this evening and across the rest of the area overnight tonight.
Low-end LLWS will remain possible into early this evening as a
low-level jet of 40-50 kts crosses the area.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement through Saturday morning for WIZ022-040-
050.
&&

$$

DISCUSSION.....Kurimski/MPC
AVIATION.......JLA