985
FXUS63 KGRB 290815
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
315 AM CDT Sat Jun 29 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

 - Scattered thunderstorms are expected this afternoon. An
   isolated strong storm is possible that could produce gusty
   winds and small hail.

 - Conditions hazardous to small craft are possible tonight into
   early Sunday morning.

 - Some rivers and streams will remain above bankfull or near
   minor flood stage into the middle of next week.

 - The next period to watch for heavy rain and stronger storms
   will be late Monday through Tuesday.

&&

.DISCUSSION...
Issued at 314 AM CDT Sat Jun 29 2024

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

An area of showers and isolated thunderstorms continues to track
through northeast Wisconsin early this morning coincident with a
mid level shortwave tracking through the western Great Lakes and
some modest MUCAPEs of 200 to 400 J/kg. This activity should move
out of the area by sunrise, with mainly quiet conditions expected
through the late morning.

Another mid level shortwave, shown on water vapor over North
Dakota, will track through the northern Great Lakes late this
morning and into the afternoon. A secondary cold front will
coincide with this secondary shortwave as it tracks through the
area. The shortwave is stronger this morning than what was
indicated by previous model runs, which will delay the arrival of
the secondary cold front as the stronger shortwave will track
through a bit later than previously forecast. This will allow
highs to be a few degrees higher than the previous forecast, with
the biggest difference in temperatures across north-central
Wisconsin. The later arrival of the front will mean some SBCAPE
will build ahead of the cold front this afternoon across east-
central and northeast Wisconsin as instabilities rise to 500 to
1000 J/kg. Effective shear is progged to be around 50 knots;
therefore, some strong to near severe storms are possible later
this afternoon into the early evening with hail and gusty winds
the main threats. CAMs models indicate this activity will be
fairly isolated, with a narrow window for any storms to develop,
so confidence in these storms having enough time to develop into
stronger storms is low. Highs today are expected to range from
the middle 70s across the north, to the lower 80s across east-
central and northeast Wisconsin.

High pressure will build in behind the departing cold front this
evening, building across the western Great Lakes on Sunday. This
will provide a period of dry weather with cooler temperatures
expected across the region. Lows tonight are expected to fall into
the lower 40s across the north, with lower 50s across east-
central Wisconsin. Highs Sunday are expected to range from around
70 across north-central Wisconsin, to the lower 70s across east-
central and northeast Wisconsin.

Long Term...Sunday Night Through Friday

Focus of this forecast continues to revolve around the thunderstorm
and heavy rainfall potential from Monday night through Tuesday night.

Monday night through Tuesday night...As a large high pressure system
shifts to the eastern Great Lakes on Monday night, return flow will
rapidly increase including a 35 to 45 kt low level jet that will
lead to surge of elevated moisture into the region from the west.
The strong moisture advection and convergence will lead to a large
area of rain spreading across the region from west to east.
Instability is lacking and less than 100 j/kg, so severe weather is
not expected despite decent wind fields.  But probabilities indicate
1/2 to 1 inch of rainfall will be possible in some areas.

After the first push of precip across the area on late Monday,
indications continue to suggest a relative lull in the precip on
Tuesday morning.  Then precip chances ramp up again late on Tuesday
afternoon into Tuesday evening.  The GFS has some concerning heavy
rainfall parameters including precipitable water values (pwats)
approaching their all-time high and relatively slow backward
propagating corfidi vectors (12 kts).  The ensemble means are more
conservative with precipitable water values but still approaching
the daily high around 2 inches. Given instability continues to look
marginal (600-1000 j/kg) for severe weather, think potential for
heavy rainfall has potential for greater impacts, particularly with
a moderate rainfall already expected on Monday night.

Another round of rain looks possible late in the week.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1019 PM CDT Fri Jun 28 2024

The steadier showers should be well east of the region by TAF
issuance, but scattered showers and isolated thunderstorms will
continue to develop ahead of a cold front through the overnight
period. Expect flight conditions to be in the IFR/LIFR categories
until the cold front moves through early Saturday morning.
Partial clearing and rising ceilings should lead to VFR
conditions regionwide by mid to late morning.

A short-wave trough and secondary cold front will bring a chance
of showers Saturday afternoon. There could a few thunderstorms as
well, but confidence in the occurrence or location of storms is
not high enough to include a mention in the TAFs at this time.
Low ceilings will return to north central and far northeast WI
during the late afternoon and evening hours.

LLWS will be ending in far eastern WI early in the TAF period.
Gusty west surface winds will develop in the wake of the cold
front passage, especially Saturday afternoon and evening.

&&

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

&&

$$

DISCUSSION.....Kurimski/MPC
AVIATION.......Kieckbusch