414
FXUS63 KGRB 201200
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
700 AM CDT Sat Jul 20 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Showers and thunderstorms are likely across northern WI this
  afternoon. Severe storms are not expected, but locally heavy
  rainfall may occur.

- A chance of showers and thunderstorms will be possible at times
  from Sunday afternoon through the middle of next week. The
  severe thunderstorm potential is low for Sunday and Monday, but
  is too early to determine for Tuesday and Wednesday. Locally
  heavy rain is possible.

&&

.DISCUSSION...
Issued at 356 AM CDT Sat Jul 20 2024

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

High pressure brought quiet conditions to the forecast area
early this morning, with partly cloudy skies and patchy fog in a
few locations. A cold front was dropping south through southern
Ontario and northern Lake Superior. This front will become the
focus for shower and thunderstorm development as it moves through
GRB CWA this afternoon and tonight.

The back door cold front will sag south through northern WI this
afternoon. A weak upper trough over the region won`t provide any
forcing, as the main short-wave will remain well to our west.
However, increasing moisture (dew points in the lower to middle
60s) and instability (CAPE around 750-1000 j/kg) along the front
should lead to numerous showers and scattered thunderstorms across
north central and far northeast WI. With PWATs increasing to 1.25
to 1.75 inches and weak flow (5 to 10 knots) aloft leading to
slow movement of any storms, there is potential for locally heavy
rainfall. Severe weather is not anticipated, as deep layer shear
is expected to be weak (5 to 15 knots). Highs will be mostly in
the upper 70s to middle 80s.

The cold front will move through the rest of the forecast area
tonight, but with the diurnal decrease in instability and a lack
of any other forcing mechanism, suspect that precipitation coverage
will be isolated to scattered at best. Low temperatures will range
from the lower to middle 50s far north to the lower to middle 60s
in C/EC WI.

On Sunday, the weak front should be situated south and west of
the forecast area, but models show scattered showers and storms
redeveloping in the southwest part of the forecast area during
the afternoon. The best chance should be southwest of a line from
Merrill to Oshkosh, where CAPE values of 750-1000 j/kg will
reside. Weak shear and light steering winds aloft will again
support a localized heavy rainfall threat with any storms. Farther
northeast, plenty of sunshine and a more comfortable air mass will
make for a nice day. Highs will be in the 75 to 80 range.

Long Term...Sunday Night Through Friday

Northeast Wisconsin will remain on the west side of a broad upper
trough through midweek, and then transition to weak ridging by
the end of the week.

A surface boundary residing over the area on Monday, combined with
daytime heating, will bring a chance for scattered showers and
isolated thunderstorms. MLCAPES are as high as 1000 to 1500 J/kg,
but weak shear and lack of upper forcing results in a low severe
thunderstorm potential. Pwats of 1.0 to 1.5 inches could bring
locally heavy rain.

There are higher chances (40-70%) for showers and thunderstorms
on Tuesday and Wednesday as a mid-level shortwave approaches from
the northwest, along with the sfc front still over the area.
CAPE values are similar, but 0-6km shear increasing to 15-30 kts.
Pwats are in the 1.0 to 1.5 inch range. May need to keep a closer
watch for a few isolated stronger storms, but this will also
depend on the position of the front and timing of the shortwave.
Models are not in great agreement on this at the moment.

Models continue to diverge late in the week, with the GFS quick to
move the front out and allow upper ridging to build. The ECMWF and
Canadian are slower with this. Sometime late Wednesday-Thursday
any remaining precipitation should clear out with dry days to
follow to end the week.

Temperatures will be near to slightly below normal, with daily
highs in the mid 70s to low 80s, and daily lows in the 50s to low
60s.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 643 AM CDT Sat Jul 20 2024

VFR conditions will prevail through this evening, with the
possible exception of MVFR/IFR vsbys with any thunderstorms
across northern WI this afternoon/early evening.

SCT-BKN stratocumulus clouds )bases around 6k ft AGL) will impact
parts of the region early this morning, but should be replaced by
daytime cumulus development (bases 4-5K ft AGL)regionwide later
this morning morning. A cold front will bring showers and storms
to northern WI this afternoon and early evening, and confidence
was high enough to include a TEMPO group for thunderstorms at RHI
for the mid to late afternoon. Precipitation trends get more
muddled this evening, as it`s uncertainty how much convection
will persist along the cold front after daytime instability wanes.
Since there is little or no forcing aside from the cold front,
convection should decrease for the late evening and overnight
hours, with only isolated to scattered showers and a stray storm
peristing near the advancing front. Later tonight, there is
potential for low clouds and fog to develop over north central and
central WI, with IFR/LIFR conditions possible.

.OSH...Current 6-7K ft AGL ceilings should last for a few more
hours, but SCT-BKN daytime cumulus development is expected to
occur by 16z-17z, with bases in the 4-5K ft AGL range. There is a
small chance of a shower late tonight as the cold front moves
through, but confidence was too low to mention in the OSH TAF.
Winds will remain light and less than 10 kts, with W-SW winds
eventually turning NW-N after the overnight frontal passage.
There is a chance of MVFR fog late tonight.

&&

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

&&

$$

DISCUSSION.....Kieckbusch/KLJ
AVIATION.......Kieckbusch