273
FXUS63 KGRB 031139
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
639 AM CDT Mon Jun 3 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Showers and thunderstorms this morning mainly north-central.
  Chance of showers and storms all areas this afternoon. Heavy
  downpours possible, but the greatest risk of isolated severe
  storms will be south of Highway 10.

- Scattered showers and storms tonight with heavy downpours the
  main hazard.

- More strong storms are possible late Tuesday into Tuesday
  night. Gusty winds, small hail, and heavy rain will be the
  primary hazards.

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

&&

.DISCUSSION...
Issued at 311 AM CDT Mon Jun 3 2024

Short Term...Today...Tonight...and Tuesday

Regional radar shows subtle MCV with convection producing heavy
rain from central MN into northwest WI. This initial MCV will
shift into northwest WI this morning pushing showers and embedded
thunder into areas northwest of AUW to IMT. As the morning
progresses, primary H5 shortwave over the Dakotas shifts northeast
and main H85 jet shifts north versus east. Result is this initial
round of showers and storms will slide more north than east and
tend to diminish as they work toward northeast WI and east-central
WI. Even so, isolated to scattered showers will be moving across
the area.

By late morning into early afternoon, area of more concentrated
showers and storms will begin to arrive from the southwest. This
convection will be from a stronger embedded H7 shortwave within
broad mid-level trough that is helping to generate storms over
southwest MN to northwest IA this morning with additional activity
emerging out of this after daybreak this morning. These rounds of
showers and storms with enhanced effective shear peaking over 30
kts eventually shifts east and will have increasing instability to
work with into southern CWA mid to late afternoon. Appears at
least MLCAPES will reach 500J/kg, if not more toward 1000J/kg.
This is when greatest risk of severe storms will occur, though
think better chances for more organized storms will be farther
south. SPC has nudged the Slight risk (level 2 of 5) into Waushara
County with a marginal risk farther north generally south of a
line from Wausau to Sturgeon Bay. Generally limited instability
and effective shear on the northern edge of this zone will keep
severe risk to minimum that far north with greatest risk in our
area south of highway 10. Gusty winds and heavy rain could occur
though, especially on the northern fringe of the expected stronger
storms that will be clipping southern CWA.

Temps are tricky today. Most confident that lowest temps upper
60s/near 70 will be over north-central with morning rain and
upslope SE winds. Elsewhere, H85 temps up to 15c would yield highs
well into the 70s, but readings that high will only be realized
with substantial clearing mid morning onward, a dicey proposition
today with incoming decaying showers and developing showers and
storms from the southwest. Went mid 70s for top end, which is
below MOS guidance. If it ends up staying partly to mostly sunny
into early afternoon east-central, then temps in the upper 70s,
if not near 80, would occur just inland from cooling influence of
Lake Michigan.

Tonight, cannot rule out some showers or storms at times though
generally coverage will be lower than what occurs this afternoon.
Main drivers for convection tonight will be warm front in the
area and right entrance jet forcing along with an increasing low-
level jet. Best chances for rain over northeast and east-central
WI. PWATs stay well above 1.5 inches, so heavy rain could occur.
Small MBE vectors for a time tonight and warm cloud heights over
11kft support the slow moving, heavy rain idea as well. Mild and
muggy night with lows mainly 60-65F.

Tuesday will start off generally quiet as the low-level jet from
tonight shifts north as does sfc warm front and stronger forcing
doesn`t arrive over the far west until late in the day ahead of
sharp mid-level trough from scntrl Canada to the northern Plains.
Cold front tied to the trough will remain well to the west of
Wisconsin through late afternoon. The day will have a summery feel
with highs reaching around 80 most areas and dewpoints in the mid
to upper 60s. Broad lift ahead of the trough spreads into Wisconsin
during the afternoon and will combine with building instability
(MLCAPES 1000-1500J/kg) and convective temps that will be achieved
fairly early (early to mid aftn) to result in blossoming showers
and thunderstorms with greatest coverage over central WI. As it
has looked like for last couple days, severe potential is lower
and pulse type given weak effective shear less than 25 kts and
marginal mid-level lapse rates near 6.5 to 7.0C/km. Heavy rain
potential continues with PWATs pushing 1.75 inches which is near
the top range for early June and well above the 90th percentile.

Long Term...Tuesday Night Through Sunday

The extended weather pattern will see a large upper low slow down
and "park" over the Great Lakes, keeping periods of active weather
in the region in the area through the early weekend. That said,
after Wednesday the cooler and drier airmass brought in by this
same low will mainly only support light rain and rumbles of
thunder but no stronger systems.

Tuesday night through Wednesday...
Active weather will be on its way across the region during this
period of the forecast as a cold front crosses the area in the
late evening to overnight hours. While thunder does seem likely
at times, especially earlier on in the night, severe weather for
our area does not seem likely as the front will passage after
instability has already dropped off across the area. That said,
lingering weak upper instability plus low level forcing could
provide the impetus for some heavy rainfall in the overnight
period through Wednesday morning. Precipitable water values up to
1.5-1.7 inches would support another inch or so of rainfall in
this overnight period alone, which combined with the rain from
Monday and during the day Tuesday could bring some river rises or
local ponding on roads. Wednesday afternoon could be bit drier
behind the frontal passage, but the next fast moving shortwave
could quickly bring in the next round of rainfall Wednesday
evening.

Rest of the forecast...
The upper low is expected to establish itself around the border
between northern CONUS and southern Canada by Wednesday night
while slowly drifting eastwards over the next 3-4 days. This
pattern will make for a gradual shift to more northwesterly flow,
which will bring some cooler and drier air along with periods of
light rain each day in the forecast. The exact exit speed is not
certain, but if you`re looking for a drier forecast there is some
hope towards the end of the upcoming weekend and early next week.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 635 AM CDT Mon Jun 3 2024

Widespread showers and thunderstorms have moved into north-
central Wisconsin (RHI) and will fill in over central Wisconsin
(AUW/CWA) this morning. Majority of showers and storms will wait
until the afternoon hours at GRB/MTW/ATW. Expect MVFR conditions
this morning at RHI while borderline MVFR to VFR conditions occur
at CWA/AUW. Farther east, VFR this morning will become MVFR at
times this afternoon as showers and storms develop into the area.

Showers and some storms will be around this evening, though
coverage will be lower than what occurs today. Most areas will
dry out for a time late tonight though some IFR/MVFR fog could
occur at RHI/AUW/CWA. Otherwise, as a warm front lifts across the
area, plan on MVFR conditions due to cigs 010-030.

&&

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

&&

$$

DISCUSSION.....JLA/Uhlmann
AVIATION.......JLA