331
FXUS63 KGRB 272352
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
652 PM CDT Thu Jun 27 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Showers and thunderstorms may bring periods of moderate rainfall
  to the forecast area Friday evening and overnight. Small hail is
  possible with some storms Friday night, but the overall severe
  weather threat is low.

- Increasing winds and waves will create hazards for mariners and
  beach-goes on Friday. 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 351 PM CDT Thu Jun 27 2024

Short Term...Tonight and Friday

Cooler weather and partly sunny skies will persist through the
late afternoon before cloud cover builds back in from the
southwest this evening. Low temperatures for tonight are expected
to be slightly more mild than previous nights given overcast
skies, ranging from the low 50s in far northeast Wisconsin near
the Upper Peninsula border to the upper 50s in the Fox Valley.

Precip chances will move through the forecast area in two waves,
with the first round expected to arrive late tonight into tomorrow
morning as a surface low treks across the upper Mississippi
Valley into Ontario. Light to moderate rain during this time will
be driven by a combination of weak shortwave energy and isentropic
ascent from warm air advection ahead of an approaching warm front.
QPF continues to look unimpressive due to meager forcing and the
lingering influence of low-level dry air from departing high
pressure to the east. Probabilities for exceeding a half inch of
rainfall remain on the low end, with at most a 10 to 20 percent
chance across central Wisconsin through Friday evening. Some
elevated storms will be possible mainly tomorrow afternoon during
peak heating, although the axis of strongest instability will
remain off to our west along the cold front. This being said, best
potential for any strong to severe storms won`t be until Friday
evening and into the overnight when the cold front provides a more
robust forcing mechanism. Southerly winds will begin to ramp up
Friday afternoon as a nocturnal LLJ propagates across Wisconsin
and mixes down to the lower levels, creating conditions hazardous
to small craft and beach-goers on the shore of Lake Michigan.

Long Term...Friday Night Through Thursday

The main focus is on severe thunderstorm potential Friday night,
followed by rain and thunderstorm chances early next week.

Friday night...A cold front will be located across eastern MN at
00Z/Sat with convection expected to be ongoing along and ahead of
this front. Additional support from a mid-level short wave is
coming in behind the front. Instability along the front in MN is
anywhere from 1700 to 2300 J/kg, but as the front moves east
instability wanes with only 600 to 900 J/kg in central Wisconsin
at 06Z/Sat. Thunderstorms will remain possible area wide Friday
night, but the severe threat will be decreasing by this point.
SPC maintains a small section of the GRB forecast area in a
marginal for this time window, which is reasonable. With storms
also becoming elevated, hail looks like the main threat. Small
chance for some winds, but inversion around 850mb will make it
more challenging for severe level winds to reach the surface.

Meanwhile, pwats rise to near 2.0 inches, but due to the
relative quick movement of this system event total QPF values only
range from around 0.25 to 0.50 inches, with a less than 10
percent chance of exceeding 1.0 inch. Therefore, excessive
rain/flooding impacts should be minimal. However, with a number of
rivers in or near bankfull any rain will maintain this near-flood
state a bit longer.

Saturday...A break in precipitation for the rest of Saturday
morning, with some widely scattered instability showers or
thunderstorms on Saturday afternoon and early evening. Severe
potential is low. High pressure should keep conditions dry and
quiet for Sunday and most of Monday.

Early next week...Another system organizes in the Plains with
frontal boundaries and coinciding shortwaves impacting the area
in the Monday night to Tuesday night time frame. Pwats climb close
to 2.0 inches, with convective parameters climbing as well. Parts
of the GRB forecast area are already highlighted in the Day 5 ERO.
Will need to monitor this time frame for potential heavy rain and
thunderstorms.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 652 PM CDT Thu Jun 27 2024

VFR conditions will continue this evening and much of tonight as
the daytime cumulus clouds fade toward sunset and mid and high
clouds continue to lower and thicken. A few light rain showers or
sprinkles will be possible tonight, but most/all of what is on
radar in central and west-central WI is virga and not reaching
the ground as there will be some dry air to overcome. On Friday,
look for clouds to lower and flying conditions to deteriorate into
the MVFR/IFR categories as rain spreads across the area. A few
LIFR conditions will be possible in central and north-central WI.
While a thunderstorm cannot be ruled out before 00z Saturday, the
better (mainly elevated) instability arrives Friday evening, so
will not include any thunder at this time. Some low-end LLWS
will be possible on Friday, especially across central and north-
central WI, as a LLJ of ~50 kts pushes over the area.

&&

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

$$

DISCUSSION.....Goodin/KLJ
AVIATION.......Bersch