075
FXUS63 KGRB 131134
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
634 AM CDT Tue Aug 13 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Isolated showers and storms (15% to 30% chance) will be
  possible this afternoon and early this evening. Severe
  thunderstorms are not expected.

- Showers and some thunderstorms are forecast at times on (60 to
  80% chance) Thursday into Friday. While the severe potential
  remains low, favorable conditions will exist for heavy rainfall
  and localized urban and low-land flooding.

- Conditions hazardous to small craft possible on Lake Michigan
  Thursday and Thursday night.

&&

.DISCUSSION...
Issued at 347 AM CDT Tue Aug 13 2024

Short Term...Today...Tonight...and Wednesday

Shower/Storm Chances & Cloud Trends:

A dry start to the day is on tap today with plenty of sunshine,
with only a few patches of fog/clouds across parts of the area.
Then as temps climb, look for a cumulus field to develop in the
mid to late morning. Instability will build to 500-1000 J/kg by
early afternoon. A weak, nearly stationary boundary, along with
lake/bay breeze boundaries and steepening lapse rates will combine
with the instability to produce isolated to possibly scattered
sprinkles/showers and a few storms mainly this afternoon and early
evening. Little to no upper forcing, along with some dry air, a
weak cap above 10,000 feet and surface high pressure to overcome,
so think coverage will be limited. Weak shear under 20 kts will
lead to slow moving showers/storms that could drop some locally
heavy rain if/where the activity develops (PWATs around an inch).
The stronger showers/storms could produce some brief gusty winds
as well (with inverted-V soundings promoting evaporative cooling).
The outflow could also produce additional boundaries for other
storms to develop along. Any lingering shower/storm will end in
the early evening as instability wanes and temps cool.

Mostly clear skies are expected tonight, with clouds increasing
from the west Wednesday afternoon. The weak boundary/lake breeze
could touch off a shower near the MI border, but signs are not as
strong as today, so will keep a dry forecast.

Fog/Smoke/Temperatures:

Light winds and mostly clear skies will lead to some patchy fog
early this morning and again late tonight into early Wednesday.
Any fog will burn off near/shortly after sunrise.

A thin layer of smoke will remain across parts of the western
Great Lakes at times through mid-week. No impacts are expected
at/near the surface as the thicker smoke looks to stay west of
the area.

Temps look to be a little above normal today into Wednesday. Highs
each day will be from around 80 degrees to the mid 80s, with
slightly cooler readings near the shores of Lake Michigan. Lows
will be a little chilly (in the low 50s) in the cold spots across
north- central WI, with mainly upper 50s and lower 60s elsewhere.

Long Term...Wednesday Night Through Monday

Focus in this part of the forecast mainly revolves around the heavy
rainfall potential during the Thursday and Thursday night timeframe.
Thereafter, the upper level flow will transition to northwest which
will keep the higher end instability off to the south and west.

Wednesday night through Friday...Warm advection will be increasing
across the state ahead of shortwave energy moving east across the
northern Plains and northern Mississippi Valley.  Saturation will
primarily be above 10kft during the evening, but increasing ascent
further down the column and within increasing warm advection will
lead to deep saturation moving in overnight with rain chances
arriving after midnight.

Low pressure will then move across the region on Thursday and
Thursday night.  Precipitable water values will be increasing to
around 1.8-1.9 inches across the region.  Forecast soundings
indicate thunderstorm potential will be elevated in nature given the
cool boundary layer under ample cloud cover and precipitation.  Up
to 1000 j/kg will be possible over central to east-central Wisconsin
by late Thursday afternoon into Thursday evening that could result
in isolated strong storms, but the severe risk appears low.  A
higher risk appears to be heavy rainfall given some elevated
instability and slowing storm motion to under 10 kts by late in the
evening.

Rainfall amounts of 1-2 inches appears possible over primarily
central to east-central WI by the end of Thursday night.  Some light
showers will likely continue on Friday beneath the upper trough.
Given the recent dry weather and longer duration rainfall, the risk
of flash flooding appears low, and mainly confined to urban areas.

Rest of the forecast...The upper trough will be spinning over the
northern Great Lakes on Saturday, which could prolong the shower
activity over far northeast WI.  Behind this system, northwest flow
takes over and the weather looks relatively quiet until the middle
of next week.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 633 AM CDT Tue Aug 13 2024

Patchy ground fog will lift/burn off early this morning, giving
way to mostly sunny skies. Look for daytime cumulus to develop
later this morning and continue into the afternoon. Some of these
could produce some sprinkles, showers and possibly a storm.
Coverage looks too spotty to include in any one TAF at this time,
but with the high cloud bases expected (5,000-10,000 feet), only
the strongest activity would be able to reduce conditions to
MVFR. Otherwise, VFR conditions are expected. Patchy ground fog
will be possible again late tonight into early Wednesday.

&&

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

&&

$$

DISCUSSION.....Bersch/MPC
AVIATION.......Bersch