461
FXUS63 KGRB 170412
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1112 PM CDT Sun Jun 16 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Thunderstorms are possible tonight. A few storms may become
  severe, especially over north-central Wisconsin. Large hail,
  gusty winds and heavy rainfall will be the main threats.

- Well-above average temperatures are expected through Tuesday.
  Heat indices may reach the lower to middle 90s at times Monday
  and Tuesday across central, east-central, and portions of far
  northeast Wisconsin.

- Periodic thunderstorm chances are possible through much of the
  upcoming week. Some strong storms and locally heavy rainfall
  may be possible at times.

- There is a marginal risk of excessive rainfall across portions
  of north-central and central Wisconsin from tonight through
  Tuesday. This may lead to localized flash flooding and rapid
  rises in river levels.

&&

.DISCUSSION...
Issued at 445 PM CDT Sun Jun 16 2024

Short Term...Tonight and Monday

Precipitation/Thunderstorm Potential:

This afternoon`s surface analysis had a surface low pressure system
located over central MN with its warm front extending over central
WI. Conditions remained dry under partly to mostly cloudy skies this
afternoon due to capping in place. However, thunderstorms began to
develop at a triple point north of the surface low center in far
northern MN. How these thunderstorms evolve with the movement of the
surface low and its cold front over the next couple of hours will be
the deciding factor for the forecast area to see thunderstorms.

Based on the latest guidance, thunderstorms will remain tethered to
the front and have it progged to sag southeast into north-central WI
this evening. As the front reaches north-central WI, it will enter
an area of 3000 J/kg of MUCAPE, steep mid-level lapse rates, and
deep-layer shear of 30-35 kts. This will produce thunderstorms
capable of large hail, gusty winds, and heavy rainfall. Confidence
for severe storms decreases across the remainder of the forecast
area as the front will move into an area where deep-layer shear is
less supportive for long-lived storms. However, the potential for
showers and thunderstorms will continue across the remainder of the
forecast area through the night, but coverage will become scattered.
Any showers or thunderstorms will also contain heavy rainfall as
PWATs will remain between 1.50 and near 2 inches.

The front is progged to settle somewhere across the central part of
the state Monday morning. Another surface low will then move across
the Central Plains and cause the front to lift north as a warm
front. Additional showers and thunderstorms may develop along the
warm front as a result, but where they develop is still in question
due to the uncertainty where the front will stall over the state
Monday morning. Limited PoPs to chance wording Monday afternoon
because of this.

Temperatures:

Despite the cold front progged to move over the region, an
abnormally warm airmass will remain over the area and keep above
normal temperatures in the forecast. A mild night is expected with
lows ranging from the low 60s to low 70s. Temperatures will climb
into the 80s for most locations on Monday, in addition to dew points
in the middle 60s to low 70s. This will make for another humid day
with heat indices reaching into the low 90s for some locations
across east-central WI Monday afternoon.

Long Term...Monday Night Through Sunday

Models are in good agreement with the placement of a strong upper
high just off the mid-Atlantic states and an upper trough that
extended from south-central Canada to the Pacific Northwest at the
start of the period. The resultant southwest flow into WI will
bring periodic chances for showers and thunderstorms as several
shortwaves move across the area where plenty of moisture to
reside. This upper high eventually weakens and shifts south, thus
allowing for the upper trough to shift east and bring more rain
chances to northeast WI next weekend. Temperatures will run above
normal for much of the upcoming week, including humid conditions
through Wednesday.

Monday night and Tuesday...
A warm front initially over central WI early Monday evening is
forecast to gradually lift north into northern WI late evening and
into Upper MI by daybreak. Expect showers/thunderstorms to
accompany this front, thus highest pops to be placed over northern
WI. PW values of at least 1.5 inches with dew points in the 60s,
could bring locally heavy rain, so will need to watch for
localized flooding across the north. On the flip side, parts of
central and east-central WI may not see any rain, just a warm and
muggy night in the warm sector. Min temperatures to range from
the middle to upper 60s far north/near Lake MI, lower to middle
70s south. Northeast WI to reside in this warm sector on Tuesday
with the warm front to the north and an approaching cold front
into the Upper MS Valley. While no trigger is apparent, increasing
instability with MUCAPES > 2K J/KG could kick off random
thunderstorms during peak heating. The vast majority of the
forecast area will be dry Tuesday and quite warm/humid. Look for
max temperatures to be in the lower 80s along Lake MI, middle to
upper 80s north and upper 80s to lower 90s south.

Tuesday night and Wednesday...
The cold front is progged to slowly move east and reach north-
central to west-central WI by late Tuesday night. Even though the
air mass will not be as unstable due to the loss of daytime
heating, shear will be on the increase (25-35 knots). These
factors, combined with lift from both the cold front and the right
entrance region of the upper jet and hints of a mid-level
shortwave moving northeast into the region all point to another
round of showers and thunderstorms over the northwest half of WI.
Right now, the strongest storms are expected to our west, but will
need to watch the potential of stronger storms in later forecasts.
Eastern WI should stay dry Tuesday night with min temperatures to
range from the lower to middle 60s north-central WI, to the lower
70s east-central WI (cooler near Lake MI). This cold front will
move into northeast WI on Wednesday, but the question becomes how
far south and east this front can get before stalling as it
encounters the mid-Atlantic upper high. As long as this front is
is in our vicinity, we will have chances for additional showers
and storms. More clouds, more rain and a wind shift to the west
behind where the cold front passes will take temperatures down a
bit with readings on Wednesday in the middle to upper 70s north-
central, middle 80s east-central WI (inland from Lake MI).

Wednesday night and Thursday...
The nearly-stalled frontal boundary to remain parked over central
(or southern) WI depending on which model you buy into. Either
way, this boundary will be the focal point for more showers and
storms Wednesday night and Thursday. PW values of 1.5 to 2.0
inches dictate that more heavy rains will be possible and
depending on how much rain had fallen earlier in the week,
flooding concerns (both urban and river) will need to be
monitored. Max temperatures Thursday to be in the lower to middle
70s north/near Lake MI, upper 70s to lower 80s south.

Thursday night and Friday...
The frontal boundary to languish across WI through Friday,
although there a signs that the boundary may lift north a bit into
northern WI on Friday. Have no choice but to keep a chance of
showers/thunderstorms in the forecast for both Thursday night and
Friday. Max temperatures Friday to be in the middle to upper 70s
north/near Lake MI, lower to middle 80s south.

Friday night and Saturday...
Precipitation chances may taper off a bit headed into next
weekend, assuming the frontal boundary does become a warm front
and lifts north of the forecast area. This would leave northeast
WI in the warm sector and with plenty of moisture still over the
region, cannot rule out pop-up showers/thunderstorms, especially
on Saturday. Max temperatures to be in the middle to upper 70s
north/near Lake MI, lower to middle 80s south.

Saturday night and Sunday...
By the latter half of the weekend, the Pacific Northwest upper
trough to have traveled east and is forecast to be moving across
the Upper MS Valley, accompanied by a cold front and more rain
chances. Max temperatures for next Sunday to be in the middle to
upper 70s north/lakeside, upper 70s to lower 80s south.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1112 PM CDT Sun Jun 16 2024

VFR conditions will continue late this evening. Models showing
some lower ceilings (MVFR and possibly IFR for a time) developing
across central and northern WI overnight into Monday morning and
possibly again Monday afternoon/evening. VFR conditions look to
prevail for most/all of the TAF period at GRB/ATW/MTW. We continue
to monitor upstream for shower/storm develop as a cold front
approaches. Storms continue to have a hard time firing as a cap is
in place holding things in check, along with weaker lapse rates.
With instability decreasing, chances for any widespread
shower/storm activity is fading. While a spotty storm may still
occur, will remove thunder as chances are under 20%.

Additional rounds of showers and storms are expected on Monday as
instability will be in place. One round looks to be in the morning
with whatever develops across MN/IA and tracks northeast into the
region. Another is possible later in the day. There are still plenty
of unanswered questions as to when and where the storms will
actually form as details regarding where the boundaries will end
up remains illusive. Have added some PROB30s to get the ball
rolling on targeting the best chances for showers/storms.

Surface winds will remain under 10 kts for most of the night, but
winds aloft could allow for a few gusts to 20 kts, mainly over
eastern WI. LLWS is expected, mainly through 10-12z, as winds at
2000 ft will remain near 35 kts. Winds could be a little erratic
on Monday due to overnight boundaries/storms and questions on
where the cold front will stall, but should eventually become
south/southeast. An additional period of LLWS is expected Monday
evening/night.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 4 AM CDT Monday for WIZ022-040-050.
&&

$$

DISCUSSION.....Kruk/Kallas
AVIATION.......Bersch