139
FXUS63 KGRB 240003
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
703 PM CDT Thu May 23 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Scattered showers and some thunder is possible into early this
  evening. Isolated strong storms could produce gusty winds and
  small hail.

- Showers and thunderstorms are expected Friday into Friday
  evening. Some of the storms could be strong to severe Friday
  afternoon and Friday evening. Severity of storms will depend on
  how much instability builds up mid to late afternoon on Friday.
  Heavy rain is also possible with the storms.

- Memorial Day weekend starts off dry for Saturday, but rain
  chances increase later Sunday into Memorial Day. A soaking rain
  could occur during this time.

- A few rivers over central and eastern Wisconsin will remain
  above bankfull through Friday.

&&

.DISCUSSION...
Issued at 223 PM CDT Thu May 23 2024

Short Term...Tonight and Friday

Convection trends through Friday are the main focus.

High pressure brought a dry morning but seeing signs that some
will see some rain this afternoon. A weak wave shifting through
in westerly flow aloft combining with MLCAPES up to 500j/kg is
helping to trigger some showers as we have already breached our
convective temps of 68-70F observed on the 12z GRB sounding. Have
kept highest pops and better thunder chances over southern tier of
CWA to the south of highway 29. Though shear is weak, 20kts or
less, steep low-level lapse rates lower wet-bulb zero heights
could support gusty winds and small hail in isolated stronger
storms into very early evening. Otherwise, any lingering showers
should dissipate with the loss of daytime heating mid to late
evening, with mostly clear skies the rest of the night. Enough
gradient wind ahead of low developing in the central Plains to
restrict any fog. Decent range to lows, with low 40s north-central
and low 50s central WI.

On to Friday where some potential for strong or severe storms
is still in the mix, but it is not clear cut. Showers and
thunderstorms are expected to develop late tonight ahead of the
low close to nearby warm front, higher instability on nose of
low-level jet. This line of storms then shifts into WI on Friday
morning, but though they may hold together some, they will be
outpacing greater instability that doesn`t budge much from the
upper mississippi river valley. Maybe a brief downpour and some
gusty winds, but not expecting this activity to be severe so long
as it arrives late morning/early afternoon. In wake of this the
question is how much instability will build up mid to late
afternoon ahead of the primary cold front sweeping in from the
west and warm front lifting into southern WI. SE winds to north of
the warm front will limit the instability with the flow off Lake
Michigan and could limit northward progression of the warm front.

Overall, think greatest risk of strong to severe storms ends up
over southwest/south central WI perhaps into central WI. Effective
shear sufficient for organized storms at least over 30 kts more
likely 40 kts farther west into the cwa you go. So, if we realize
sufficient insolation and realization of instability forecast
(MLCAPES over 1000J/kg), severe storms could occur. SPC has the
southern half of the forecast area within a marginal risk for
severe with large hail and damaging winds the primary hazards
(5-14 probability). The primary tornado threat is expected to stay
further south in the warm sector near the warm front, but the
probability for our southern areas is non-zero as the warm front
will be close by. PWATS ramp up to 1.5 inches or 175 percent of
normal so heavy rain is possible (WPC has area in marginal risk
for excessive rainfall) but strong winds will result in quick
storm motions so main issue in terms of heavy rain/flooding will
be whether there is training of storms across repeated areas.
Highs on Friday are expected to range from the middle 60s across
the north, to the lower 70s across east-central and portions of
central Wisconsin.

Long Term...Friday Night Through Thursday

A cold front will move across eastern Wisconsin Friday evening.
Bufkit soundings would suggest the greatest risk of severe weather
would be over central Wisconsin where CAPE values are over 1,000
J/KG and shear values around 40 knots. The severe weather
parameters drop off as you head northeast to Green Bay where these
values were only 20 to 30 knots with CAPE values of 1,000 J/KG or
less. The Storm Prediction Center Day 2 Outlook has a Marginal
Risk south of a Wausau to Antigo to Sturgeon Bay line. The
amount of coverage and intensity in strong or severe storms will
depend on how much instability there is by late afternoon. The
strongest storms should exit northeast Wisconsin around 04z with
some lingering showers across north-central and far northeast
Wisconsin into early Friday morning.

Saturday will be the day to spend outdoors with highs in the upper
60s to middle 70s under partly cloudy. Attention then turns to
Sunday afternoon into the early morning of Memorial Day where a
soaking rain is expected as low pressure moves across the region.
There are still some uncertainty in the track of the storm that
could impact the amount of thunder is seen across northeast
Wisconsin. As the low moves away on Memorial Day, instability
showers are expected to continue at times into the afternoon and
early evening. A northwest flow will continue on Tuesday, bringing
additional chances of showers and perhaps a few storms. Dry
conditions are expected on Wednesday with only a small chance of
rain on Thursday.

During the period, temperatures should average at or slightly
below normal.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 702 PM CDT Thu May 23 2024

VFR conditions are expected this evening into early Friday, with
any lingering showers across eastern/northern WI ending early this
evening. Conditions will deteriorate from west to east on Friday
as a cold front sweeps across the state. A weakening line/area of
showers and isolated storms will be crossing the area mainly
between 15-21z, likely impacting AUW/CWA, but lower chances the
further north/east you go. A stronger line/area is forecast to
arrive in central and north- central WI between 21-23z then exit
eastern WI by around 03z Sat.

Winds will become light after sunset. Could approach LLWS
criteria in central WI (AUW/CWA) late tonight, but will hold off
including as winds aloft look to stay under 25-30 kts. Another
chance at LLWS Friday afternoon as winds at 2000 ft pick up out of
the southeast, but if we remain gusty at the surface, LLWS will
be limited/reduced.

&&

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

&&

$$

DISCUSSION.....JLA/Eckberg
AVIATION.......Bersch