Severe Thunderstorm Warning 25, NWS Green Bay, WI

Area Forecast Discussion
National Weather Service Green Bay WI
707 PM CDT Thu Jul 27 2023

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Friday
Issued at 425 PM CDT Thu Jul 27 2023

The main focus from this forecast period revolves around the
potential for severe thunderstorms this evening and overnight. There
is also potential for these storms to continue into Friday morning
or redevelop during Friday afternoon.

This afternoon's radar and satellite imagery showed discrete
thunderstorms (some severe) developing along the MN/WI border
associated with a weak shortwave and surface cold front. Closer to
home, isolated showers developed in portions of central and far
northeast WI. While the isolated showers will have no impact, the
discrete thunderstorms are expected to track into the western
portion of the forecast area late this afternoon into this evening,
then track east/southeast across the remainder of the forecast area
through late tonight.

CAMs continue to agree on a storm mode solution with these discrete
cells combining to create a more structured line segment as it moves
across the forecast area. The primary threat will be damaging winds
due to the likely storm mode, but large hail will also be possible
with steep mid-level lapse rates, effective shear of 35 to 45 kts,
and CAPE of 2000 to 4000 J/kg in place. The greatest severe threat
will be in north-central WI, where SPC has an enhanced risk, but the
threat for damaging winds and large hail will be possible for the
remainder of the area as well. Locally heavy rainfall is also
possible within the storms with PWATs between 1.5 to 1.75 inches.
The severe threat should decrease late tonight as the atmosphere
gets worked over. However, there are signs as the cold front moves
into the area, it will slow down and become nearly stationary over
central and east-central WI. This could lead to additional
thunderstorms, possibly strong, through Friday morning. But, this
solution will depend on the evolution of the line of storms tonight.
If the line of storms exits by Friday morning, some models indicate
additional showers and thunderstorms developing during Friday
afternoon into Friday evening. Ran with a blend of models solution
for Friday keeping mainly chance PoPs in the forecast for locations
mainly along and south of Highway 29.

Before the cold front moves through, overnight temperatures will be
balmy with readings mainly in the 60s. Temperatures on Friday will
be cooler than Thursday with readings mainly in the 80s, however, it
will still feel quite humid for locations south of Highway 29 as dew
points will be in the middle 60s to low 70s.

.LONG TERM...Friday Night Through Thursday
Issued at 425 PM CDT Thu Jul 27 2023

Summary: Rain showers and thunderstorms Friday night taper by
early Saturday morning. Dry conditions setup for much of the
weekend, although an outside chance of some light rain is possible
Sunday. Northwest flow aloft prevails into the beginning and
middle of next week to create near-normal temperatures and minimal
precipitation chances.

A cold front extending from Quebec westward into the Upper Midwest
will be located over east-central and parts of northeast Wisconsin
Friday evening. Rain and thunderstorm chances continue mainly
along and south of the front, which keeps the best chances over
east-central WI and the Fox Valley as chances taper northward quickly
in the Northwoods. The front slides eastward Saturday morning to
keep only a Slight Chance of precip in far southern counties
towards 12Z Saturday.

As the associated low centered in the Canadian Maritimes finally
enters the North Atlantic and the front drops out of our area, all
precip chances cease for the forecast area. A dry day is expected
Saturday with a shortwave ridge entering the area from the
northwest. Persistent northwest flow is expected then into Sunday,
although a shortwave trough may move southeastward into north-
central Wisconsin and create some very low precip chances. Have
kept with PoPs below mentionable for now with this Sunday rain
shower potential, but may be something to watch and require future
increases in PoPs for those north-central Wisconsin counties.

Eastern Wisconsin will be on the far western edge of an upper-
level low pressure centered over northern Quebec Sunday night into
Monday as high pressure builds over the northern Plains. This
should help to keep temperatures near-normal to warm into early
next week. That high pressure may shift east by Tuesday night and
allow for the next potential for thunderstorms into Wednesday.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 705 PM CDT Thu Jul 27 2023

Thunderstorm activity over Northwest Wisconsin has slowly
organized into a linear mesoscale convective complex. Current
observational trends suggest the main thunderstorm complex will
continue to push eastward while turning to the south with time. An
outflow boundary with a surface wind shift to the northwest is
anticipated to develop to the south of the thunderstorm complex
particularly after the 02Z time frame. There remains question as
to the amount of active thunderstorm development that will take
place along the outflow as it sags south of Highway 29 and KGRB.
Regardless, will maintain a TEMPO for thunderstorms with strong
wind gusts of 40 to 45 knots possible along the outflow and wind
shift over the forecast area. Considerable uncertainty remains
with the southward progression of the outflow late tonight and
also for the daytime hours on Friday. Zonal westerly flow through
Friday with additional shortwave energy bring additional
thunderstorm chances. Time periods of potential activity are
between 11-15Z for KATW and KOSH and after 20Z for portions of
central Wisconsin.

.KOSH...
VFR conditions anticipated through approximately 03Z. An outflow
boundary with scattered to numerous thunderstorm activity is
expected to impact KOSH in the 03-06Z time window. Low confidence
exists with wind direction behind the wind shift but wind gusts to
40 knots will be possible. MVFR visibility restrictions will be
possible in a rain cooled airmass through daybreak with additional
chances for thunderstorms 11-15Z as upstream convection from
Minnesota passes through the region. Considerable uncertainty
exists with location of thunderstorm activity after 20Z Friday.
&&

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

SHORT TERM.....Kruk
LONG TERM......NLW
AVIATION.......Behnke

Hazardous Weather Outlook
National Weather Service Green Bay WI
453 PM CDT Thu Jul 27 2023

WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-282200-
Vilas-Oneida-Forest-Florence-Northern Marinette County-Lincoln-
Langlade-Menominee-Northern Oconto County-Door-Marathon-Shawano-
Wood-Portage-Waupaca-Outagamie-Brown-Kewaunee-Waushara-Winnebago-
Calumet-Manitowoc-Southern Marinette County-
Southern Oconto County-
453 PM CDT Thu Jul 27 2023

This Hazardous Weather Outlook is for north-central and northeast
Wisconsin.

.DAY ONE...Tonight

Very warm temperatures and humid conditions are expected through
this evening. Heat index values could approach 100 degrees through
this evening for locations in central and east-central Wisconsin.
If you have outdoor plans, be prepared for very warm and humid
conditions.

Severe thunderstorms are possible from late this afternoon
through tonight, capable of producing damaging winds to 70 mph,
large hail, and locally heavy rainfall. The greatest risk for
severe storms will be in north-central Wisconsin, but damaging
winds, large hail, and locally heavy rainfall is also possible
across the whole area. A severe thunderstorm watch is in effect
through 11 PM.

.DAYS TWO THROUGH SEVEN...Friday through Thursday

There is a chance of additional thunderstorms on Friday. Some of
these storms could become strong to locally severe, capable of
producing damaging winds and hail.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to report hail, wind gusts over 40 mph, and
wind damage to the National Weather Service in Green Bay.

LMZ521-522-541>543-282200-
The Bay of Green Bay south of a line from Cedar River MI to Rock
Island Passage-Lake Michigan from Washington Island to Sheboygan-
453 PM CDT Thu Jul 27 2023

This Hazardous Weather Outlook is for portions of northwest Lake
Michigan and the waters of Green Bay.

.DAY ONE...Tonight

Thunderstorms are expected tonight and would pose a hazard to
mariners.

.DAYS TWO THROUGH SEVEN...Friday through Thursday

Thunderstorms are possible at times from Friday into Friday
night, which could pose a hazard to mariners.

Uhlmann/Kruk

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