185
FXUS63 KGRB 111732
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1232 PM CDT Sat May 11 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- A cold front will bring the potential for strong to isolated
  severe thunderstorms on Sunday afternoon and evening. The
  strongest storms could produce large hail and damaging winds.
  The most favorable time for the strongest storms is from 3 pm to
  9 pm.

- Conditions hazardous to small craft will be possible on the Lake
  on Sunday afternoon and evening due to gusty south winds and
  high waves. Boaters on the Bay are urged to exercise caution.

&&

.DISCUSSION...
Issued at 401 AM CDT Sat May 11 2024

Short Term...Today...Tonight...and Sunday

A small upper level low was producing some showers over northern
Door County early this morning, and cyclonic NW flow supported
widespread cloud cover over northern and eastern WI. The
southwest portion of the forecast area was mostly clear, which
provided a nice view of an impressive display of Northern Lights.
Temperatures were in the 40s.

Clouds will persist across northern and eastern WI this morning,
and over mainly eastern WI in the afternoon. There will be a small
chance of afternoon showers in eastern WI, due to convergence
along lake breeze boundaries, the passage of a short-wave trough,
and SBCAPE up to 150 j/kg. high temperatures will be in the 60s,
except 55 to 60 near Lake Michigan.

A weak ridge of high pressure will reside over the forecast area
this evening, then shift east overnight. As this occurs, a
35 to 40 kt southwesterly LLJ will take aim on NC WI and a surface
warm front will lift toward the southwest part of the forecast
area (central WI). WAA should result in scattered shower
development over much of the forecast area overnight. Lows will
be in the 40s.

On Sunday, the warm front will lift northeast during the morning,
causing showers to eventually shift east. Thunderstorms are
expected to develop along a cold front as it drops into northern
WI during the afternoon. There are big model discrepancies
concerning the degree of instability that develops along and ahead
of the cold front during peak heating. Some models show very deep
mixing causing dew points to drop well into the 40s, while the
GFS continues to inflate dew points into the middle to upper 50s.
Average mixing ratios suggest a middle ground between these two
extremes, with dew points in the lower 50s. This should support
SBCAPE values of 800-1200 j/kg. Deep layer shear values have
increased from previous runs, with 0-6 km bulk shear of 35 to 40
kts along and just ahead of the front. Instability/shear
parameters indicate potential for strong to severe storms.
Inverted-V soundings and DCAPE values around 900 j/kg suggest
potential for damaging winds, and favorable wet-bulb zero heights
around 8000 feet are supportive of large hail. High temperatures
will be in the 70s and lower 80s, except 60s near Lake Michigan,
due to south to southeast winds off the lake.

Long Term...Sunday Night Through Friday

Focus in this part of the forecast mainly revolves around
thunderstorm chances on Sunday evening.  Otherwise, generally quiet
weather is expected until the end of the upcoming week.

Sunday night...As a cold front drops south, the chance of
thunderstorms will continue over central to northeast Wisconsin
through the evening.  Surface based instability to around 1000 j/kg
along with deep layer shear of 25-30 kts will be present at the
start of the evening, though models indicate instability will be
rapidly diminishing through the evening.  Therefore think that the
risk for strong to isolated severe storms will continue for the
first few hours into the evening before the threat wanes with
nocturnal stabilization and southward push of the front.  DCAPES
upwards of 900 j/kg with inverted V profiles suggest strong winds
will be the primary threat from any storms that develop or continue
into the evening.

Precip chances for the rest of the period...After the cold front
settles to the southern Great Lakes and high pressure builds into
northern Wisconsin from the north, the next chance of rain will
return during the Thursday to Friday time frame.  Instability looks
relatively low during this period and so will the threat of severe
weather.

Temperatures...After the warm day on Sunday, temperatures will fall
back early next week and closer to normal.  Tuesday looks to be the
coolest day in the upper 50s to lower 60s.  The rest of the period
will be slightly above normal and in the 60s to low 70s.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1232 PM CDT Sat May 11 2024

VFR flying conditions are expected through the TAF period. A
scattered to overcast deck of clouds will continue this
afternoon, slowly decreasing as sunset approaches. Then, a mid-
deck is expected to spread across most spots this evening into
Sunday morning. A few showers and sprinkles will continue this
afternoon across eastern WI, with additional activity possible
later this evening into Sunday morning, but should not impact
aviation. Will use the VCSH to cover the shower/sprinkle chance.

Northwest winds will gust to around 20 kts through the afternoon,
then subside by early evening. Winds will become light and
variable for a time this evening then turn to the south later
tonight and remain light. Southwest winds will increase Sunday
morning, gusting to around 25 mph by late morning.

A period of LLWS is expected across parts of central and north-
central WI late tonight as a low-level jet increases to 35 to 40
kts. Have included LLWS for RHI, AUW and CWA where it is most
likely, but hold off at GRB/ATW/MTW as it looks more marginal.

&&

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

&&

$$

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