527
FXUS63 KGRB 152341
AFDGRB

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

Updated aviation portion for 00Z TAF 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 00Z TAF Issuance
Issued at 641 PM CDT Sat Jun 15 2024

VFR conditions will continue this evening, with mainly high-
based light showers and sprinkles spreading across parts of the
region. This activity will lift northward through the evening
hours, with AUW/CWA/RHI seeing the most persistent rain. A lull
in the rain is expected for a period later this evening and
overnight (shortest at RHI), with another push of WAA and strong
LLJ producing an additional round or two of showers (briefly
moderate to heavy) and embedded storms overnight into Sunday
morning. Ceilings are forecast to drop into the MVFR category
across central and northern WI, with some IFR conditions possible
at times. Mainly VFR ceilings are expected across eastern WI with
a period of MVFR possible Sunday morning. The chances for elevated
storms arrive after 04z, with chances increasing into Sunday
morning. Have continued with the small windows for thunder where
the best chances exist with the first wave of showers, but these
may need to be shifted/extended with a second wave possible later
in the morning. Storm chances on Sunday afternoon/evening are
dependent on if the cap can break, but it appears it won`t, with
maybe some elevated convection possible later in the day, but too
much uncertainty to include a mention.

South to southeast winds will continue tonight. Winds aloft will
be increasing, so a few gusts to 20 kts are possible, but an
inversion will likely limit the gustiness. When surface winds are
not gusting, LLWS is expected overnight as winds at 2000 ft
increase to around 40 kts. South to southwest winds are expected
to gust to 15-25 kts on Sunday.

&&

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

$$

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