183
FXUS63 KGRB 040927
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
427 AM CDT Tue Jun 4 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- A few strong storms are possible this afternoon into tonight.
  Gusty winds and small hail will be the primary hazards. Locally
  heavy rainfall is also possible with isolated amounts of 1 to 2
  inches. This may lead to localized flooding, especially in urban
  and low-lying areas.

- Gusty southwest to west winds up to 35 mph are expected on
  Wednesday. Locally higher wind gusts are possible in the
  afternoon and early evening associated with any shower or storm.

- Conditions hazardous to small craft will be possible at times
  from Wednesday through Friday.

&&

.DISCUSSION...
Issued at 427 AM CDT Tue Jun 4 2024

Short Term...Today...Tonight...and Wednesday

Active weather continues this afternoon into Wednesday with rounds
of showers/storms expected. Main forecast focus is on the potential
for heavy rain, localized flooding, strong storms and fog trends.

Lingering showers and possibly a rumble of thunder will remain
possible across eastern WI early this morning, then the rest of
the morning is looking dry. A weak shortwave/boundary and
increase in theta-E will work north/east across the area this
afternoon and evening, bringing isolated to scattered showers and
storms. Then a cold front will sweep across the state later this
evening into the overnight, bringing a round of more widespread
showers and storms. A secondary shortwave will rotate across the
state Wednesday morning, likely producing an uptick in activity,
before everything exits to our east by noon. Additional showers
and storms are expected later on Wednesday as the parent upper
low/shortwave drops south into Lake Superior, swinging another
cold front across the state.

Threat for widespread severe weather is low today due to the lack
of much shear (under 15 kts), but a pulse-type stronger storm
will be possible as CAPE increases to over 2000 J/kg this
afternoon. Gusty winds and small hail (high wet- bulb heights and
skinny CAPE will limit hail production) will be the main threat.
CAPE will wane in the evening as we lose daytime heating, so as
the front arrives, severe chances remain very low. But if the line
of storms has enough momentum, it could sneak into parts of
central and north- central WI with some stronger winds.
Instability will be down Wednesday afternoon, under 500 J/kg, but
shear will be better, closer to 35 kts. So a stronger storm will
be possible late in the afternoon, especially across northern WI,
with strong winds (inverted-V signature) and hail the main
threats. PWATs between 1.3-1.7" will provide for a heavy rain
threat in any storm. But due to the 1+ inches totals yesterday
being fairly spotty, the heaviest rain overnight staying south of
Fond du Lac and the progressive nature of the front, will hold off
on a flood watch. Most rivers are well below bankfull as well.
However, localized flooding will be possible where/if any storms
can move across the same location ahead of the front, especially
in urban areas. And rises in some area rivers is expected through
at least mid-week.

Fog and Trends: Patchy/areas of fog and low stratus will continue
north and west of the Fox Valley. The fog may be locally dense at
times, especially just south of the low stratus deck and north of
the mid/higher clouds to the south/east. The fog and low clouds
will mix out and lift through morning. If clouds can clear by
sunrise Wednesday morning, some fog could develop across central
and north-central WI, but current forecast has clouds hanging on
just long enough to avoid fog development. As dewpoints climb on
Lake Michigan, and lake temps in the low to mid 50s, some lake
fog will be possible as well.

Temps / Dewpoints: It will be a warm and humid day with highs
climbing into the upper 70s to mid 80s, except cooler readings
near the Lake Michigan shore line. Dewpoints will be in the mid to
upper 60s for most spots, with a 70 possible (currently 70 degree
dewpoints in far southern WI). This will push heat index readings
a couple/three degrees warmer. Behind the front, slightly cooler
and less humid air will arrive, but will remain on the mild/muggy
side in the Fox Valley. Highs mainly in the 70s are expected on
Wednesday, with less humid conditions expected as dewpoints drop
mainly in the 50s.

Long Term...Wednesday Night Through Monday

The long term forecast will be dominated by an upper-level closed
low progged to sit over the central Great Lakes region. This will
result in multiple chances for light showers and isolated
thunderstorms through this weekend and into early next week.

Wednesday evening through Thursday...Before the closed low reaches
the central Great Lakes region on Friday, it will gradually be
diving southeast across the Upper Mississippi Valley Wednesday
evening through Thursday. Meanwhile, a cold front will sweep across
the forecast area Wednesday evening. Anticipate showers and some
thunderstorms to accompany the front. While severe weather is not
expected, cannot rule out a few strong storms capable of strong
winds as forecast soundings indicate an inverted-V signature, aiding
to transport stronger winds to the surface. These showers and storms
will quickly exit by late Wednesday night, but additional showers
are anticipated for Thursday as the upper-level low will be moving
overhead.

Rest of the extended...With the upper-level closed low in position
over the central Great Lakes Friday through early next week,
additional precip chances will be determined by shortwaves
circulating around the low. However, at this time it is too
difficult to determine when the precip will occur due to weak
forcing and the light nature of the precip. Overall, anticipate on
and off chances for showers and isolated thunderstorms from Friday
through early next week.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1039 PM CDT Mon Jun 3 2024

Conditions will gradually begin to dry out tonight as showers/storms
exit to the east. However, lingering low-level moisture and
relatively light winds should allow for some low clouds and fog to
stick around north and west of the Fox Valley, with cigs and
vsbys dropping down into the MVFR to IFR range. Locally LIFR
conditions will be possible across central to north-central
Wisconsin.

Tuesday is expected to start out mostly dry with IFR and MVFR
cigs gradually lifting to VFR across the region by mid-afternoon.
GRB, ATW, and MTW may briefly meet the LLWS threshold early
Tuesday morning as 925 to 850 mb winds increase in the wake of a
shallow, transient shortwave. A cold front is then expected to
arrive late Tuesday, bringing additional chances for showers and
thunderstorms to the forecast area Tuesday evening. MVFR to IFR
cigs are expected to return from west to east during this time.
Some thunder may be possible at the end of the TAF period,
although opted not to include due to low confidence given timing
and a stabilizing effect from a lake breeze.

&&

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

&&

$$

DISCUSSION.....Bersch/Kruk
AVIATION.......Goodin