667
FXUS63 KGRB 152121
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
421 PM CDT Sat Jun 15 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- A round of showers and thunderstorms is expected to move across
  the area this evening through late Sunday morning. Heavy
  rainfall is likely, especially north and west of the Fox Valley.
  High rainfall rates may result in localized flooding and
  ponding on roads.

- Another round of thunderstorms is possible late Sunday afternoon
  into Sunday night. Some storms may become severe across central
  Wisconsin with damaging winds and hail as the main threats.

- Well above average temperatures are expected Sunday through
  Tuesday. Heat indices may reach the mid 90s at times.

- Periodic thunderstorm chances are possible through mid-week due
  to high heat and humidity. Some strong storms and locally heavy
  rainfall may be possible at times.

&&

.DISCUSSION...
Issued at 421 PM CDT Sat Jun 15 2024

Short Term...Tonight and Sunday

Precipitation/Thunderstorm Potential:

Upper-level ridging and a surface high will continue to depart the
region late this afternoon into this evening and make way for
widespread rainfall this evening through midday Sunday.

Outside of a few light showers or sprinkles falling out of a 12k ft
cloud deck this afternoon, an area of rain associated with an
embedded shortwave and increasing f-gen and WAA, was gradually
shifting into western WI this afternoon. This area of rain is
expected to spread and lift northeast across the forecast area this
evening through late this evening. Rainfall amounts during this time
should stay confined to west and north of the Fox Valley with about
0.10 to 0.30 inches for most locations and up to 0.50 inches in far
north-central WI. Thunderstorm potential during this time will be
very low due to limited instability.

A small lull in the rainfall is likely once the f-gen rain lifts
northeast out of the region, however, an isolated shower cannot be
ruled out before the next round of rain arrives from the west around
4am (09z) Sunday. Moderate to heavy rainfall is possible as it
tracks east across the region due to support from a strong southerly
LLJ (45 to 55 kts), low-level WAA, and PWATs of 1.50 to 1.75 inches.
By midday Sunday, additional rainfall amounts of 0.25 to 0.75 inches
are expected. Total rainfall amounts have decreased slightly from
the previous forecast, but central and north-central WI will still
see the highest total amounts, where locally higher amounts up to 1
inch are possible. Elevated instability will increase to a couple
hundred J/kg Sunday morning and bring the potential for a few
thunderstorms capable of some small hail, but severe weather is not
expected.

The rainfall will come to an end from west to east by early Sunday
afternoon. The remainder of Sunday will be a challenge to determine
when precip will occur or not occur as the surface warm front will
be residing over the area. Despite SBCAPE increasing to 3k-4k J/kg
Sunday afternoon, decided to go dry for the remainder of Sunday as
forecast soundings indicate a cap will develop between 700-800mb and
prevent thunderstorm develop. The first signs of the cap breaking
are during Sunday evening.

Temperatures:

The southerly flow will bring warm, moist air to the region for
Sunday. Sunday`s highs will be the first day will well above normal
temperatures climbing into the middle to upper 80s. Additionally,
humid conditions are also expected as dew points will rise into the
upper 60s to low 70s, resulting in heat index values in the upper
80s to middle 90s Sunday afternoon.

Long Term...Sunday Night Through Saturday

Focus for the extended remains on an active pattern bringing
several rounds of rain/storms to the forecast area followed by
building heat and humidity through mid-week. Some strong to severe
storms will be possible Sunday evening as a cold front sweeps
across the area. Periodic chances for storms will then continue
throughout the week as a high amplitude mid-level pattern brings
persistent southwest flow to much of the Midwest, maximizing
chances for diurnally-driven convection. This warm and moist
ridging regime has the potential to bring heat indices up into the
low to mid 90s across portions of central to east-central
Wisconsin Monday and Tuesday.

Sunday evening storm chances... Confidence is increasing in
potential for strong storms Sunday evening as a fast-moving
shortwave passes through the upper Mississippi Valley. Models
continue to show strong instability (3,000 to 4,000+ J/kg CAPE)
through Sunday evening as well as decent 0 to 6 km shear (25 to
30 knots). Damaging wind and hail would likely be the main threats
with stronger storms. Current thinking is that it would be
difficult to get severe hail due to the warm environment and wet
bulb zero heights around 12,000 ft, although non-severe hail will
definitely be possible. This being said, abundant moisture and an
open Gulf will place a corridor of 1.75" PWATs across central and
east-central Wisconsin, suggesting that any storms that do
develop would be efficient rain producers. There is still some
uncertainty regarding whether or not thunderstorm activity Sunday
morning will affect how the warm front behaves later in the
afternoon, which would consequently impact instability/storm re-
development due to airmass recovery time. Overall, thinking that
best chances for strong storms would be in central Wisconsin
overnight along a cold front approaching from the northwest. The
SPC currently highlights portions of central Wisconsin near
Marshfield and Wisconsin Rapids under a slight risk for receiving
severe weather.

Rest of the extended... On and off storm chances are expected to
continue Monday and Tuesday as the cold front exits to the
southeast. There are still some questions about timing and
location of storm development, especially since ridging across the
eastern US may keep most thunderstorm activity off to our west.
However, a persistent warm and moist airmass may be enough to
drive diurnal convection, especially on Tuesday. This airmass will
likely cause additional problems through mid-week as temperatures
potentially climb into the low 90s Monday and Tuesday. Portions of
east-central Wisconsin, mainly centered around the Fox Valley, are
under an extreme heat risk on Tuesday as heat indices soar into
the mid 90s.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 128 PM CDT Sat Jun 15 2024

VFR conditions are expected through at least early this evening,
with a few light showers at the central WI TAF sites this
afternoon. These light showers are expected to increase in
coverage and lift northeast through late this evening. A few of
the showers may reach the east-central WI TAF sites this evening,
but will remain light. A brief lull in precip is expected from
southwest to northeast as the showers lift across the region.
During this time, as surface winds will remain from the southeast,
LLWS will increase from the south ranging from 30 to 40 kts.

Later tonight into early Sunday morning, the next wave of rain,
moderate at times, will arrive from west to east, bringing MVFR
conditions and occasional IFR conditions. Elevated instability
will increase towards Sunday morning, leaving the small potential
for a few thunderstorms across all TAF sites as well. The precip
will come to an end from west to east mid Sunday morning through
the end of the TAF period.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from 1 AM CDT Sunday through late Sunday
night for WIZ022-040-050.
&&

$$

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