Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 270903
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
403 AM CDT Tue Jul 27 2021

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 402 AM CDT Tue Jul 27 2021

The first linear MCS that produced widespread damaging winds over
northern and eastern WI has weakened and shifted across Lake
Michigan, while another west to east oriented line of storms was
dropping south across the forecast area. This line was being
sustained by WAA/moisture convergence and a weak short-wave
trough, and MUCAPE of 1500-2000 j/kg over north central/central
WI. Expect the strrongest cells to move through central WI early
this morning, with convection weakening in the east as it
encounters an air mass that was overturned by earlier storms. All
of this convection will result in the frontal boundary getting
shoved to our south this morning, which should bring a lull in the
precipitation this afternoon into tonight, with only isolated
storms anticipated. Highs today should be in the upper 70s and
lower 80s north, and middle to upper 80s south, with an easterly
wind component keeping the lakeshore counties in the 70s. Lows
tonight should be in the upper 50s to middle 60s.

On Wednesday, a surface/850 mb warm front will lift back toward
the region, as low pressure moves into MN. Strong WAA will cause
scattered/numerous thunderstorms to develop during the afternoon.
With increasing instability and very strong deep layer
shear/helicity, severe thunderstorms (including supercells) look
like a good bet. High temperatures will be in the lower 80s north,
and middle to upper 80s south, with slightly cooler readings
along the lakeshore. Heat indices may reach the middle 90s in
parts of central WI.

.LONG TERM...Wednesday Night Through Monday
Issued at 402 AM CDT Tue Jul 27 2021

Severe weather indices are pointing toward a potentially
significant severe weather and heavy rainfall event Wednesday
night. A warm front will lift into central WI, accompanied by
MUCAPE of 2000-4000 j/kg, deep layer shear of 55-70 knots, 0-3km
helicities of 400-600, low LCL heights and abundant low-level
CAPE. Supercells, including potential for a high-end severe event
and a few tornadoes, are a concern for especially the southwest
half of the forecast area, where SPC has highlighted an enhanced
risk of severe storms. With PWATS around 2 inches and 850 mb Tds
of 14-17 C, torrential rainfall is also a significant concern.
Most of the convection should shift south of the region by
Thursday morning.

The next chance of thunderstorms will be late Friday night into
Saturday, when a cold front and associated short-wave trough
move through. Instability, moisture and shear are much more modest
than the midweek set-up, but could still see a few strong to
severe storms.

Canadian high pressure should bring drier and somewhat cooler
weather for the end of the weekend and early part of next week.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1241 AM CDT Tue Jul 27 2021

A line of thunderstorms with strong to damaging winds will
continue to progress through far northeast and east central WI
early in the TAF period. At this time, it appears that the worst
of the storms will stay north of OSH, but a gust front will pass
through with a wind shift to the north shortly after 06z, and
there is a chance of storms between 07z-09z. More thunderstorms
are developing in an area of WAA/moisture convergence and
increasing elevated instability over northwest WI, and these
storms will likely track southeast through parts of central WI
early this morning. It appears that weather will quiet down later
this morning/afternoon, but will take a closer look at these trends
with the 12z TAFs.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....Kieckbusch
LONG TERM......Kieckbusch
AVIATION.......Kieckbusch


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