172
FXUS63 KGRB 211120
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
620 AM CDT Fri Jun 21 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Thunderstorms with locally heavy rain are likely this afternoon
  and tonight. The rain could produce small stream and urban
  flooding, especially north of highway 8.

- Strong to severe thunderstorms are possible Saturday with
  damaging winds the main threat. Heavy rains may cause some
  rivers to rise to near flood stage.

- More showers and thunderstorms are expected Monday night into
  Tuesday, mainly for northern and northeastern WI.

&&

.DISCUSSION...
Issued at 425 AM CDT Fri Jun 21 2024

Short Term...Today...Tonight...and Saturday

Jet energy moving through the southwest upper flow will induce
rising motion over a warm front that was over northern Illinois
early this morning.

There are two or three discernible jets that will move across the
region this afternoon and tonight. The first one will produce
overrunning showers and thunderstorms that will arrive in central
and northcentral Wisconsin by afternoon and in the east by evening.
the activity will continue tonight. An inch or two of rain is
possible, especially north of highway 8. Some Urban and small
stream flooding is possible the severe threat looks to be rather
low, as the storms are not surface based and the air aloft is
warm.

Cool outflow from the storms this afternoon and tonight will
likely keep the warm front south of the forecast area through much
of Saturday, though the ECMWF and Canadian bring it into the Fox
Valley during the afternoon. Another jet streak will move across
the area Saturday, and produce additional showers and storms. In
addition to additional heavy rain, there could be some severe
storms along and just north of the warm front. Damaging winds
look to be the main threat, though a tornado cannot be ruled out.

Long Term...Saturday Night Through Thursday

Persistent high pressure over the southern US keeps an active sub-
 tropical jet running across the northern CONUS. There will be
several chances for rain and thunderstorms through the extended
period as multiple short-waves cross the region. At this point
the flooding concern through the long term is rather broad brush,
but will become more focused to areas that see the greatest
rainfall amounts Friday and Saturday.

Saturday night through Sunday...Areas of heavy rain and scattered
strong to possibly severe thunderstorms are likely to be ongoing
into Saturday night as a cold front sweeps across the region and
a low pressure system starts to depart to the east. Strong wind
gusts are expected to be the main hazard with any thunderstorms
by this time period. The axis of heaviest rain by this time will
likely have shifted to east-central WI. By the time the rain is
all said and done much of the region will likely be looking at
1.5-2.5 inches of rain, with locally higher amounts where the
strongest convection ends up.

A short-wave following behind the main departing system is
progged to dig south over northern WI Sunday returning a chance
for showers and a few thunderstorms. With colder air moving in
aloft low-level lapse rates are forecast to steepen Sunday which
along with decent elevated instability and strong effect shear
could sustain a few stronger thunderstorms, mainly for northeast
WI where ascent is expected to be maximized.

Monday through Tuesday...Ensemble guidances is in decent
agreement with a brief mover over the area Monday keeping much of
the day dry. Temperatures and humidity will be on the rise Monday
as return flow develops during the afternoon. The next period
worth watching for possibly strong thunderstorm is Monday night
into Tuesday morning across north and northeast WI as a low-level
jet develops ahead of an approaching short-wave. Forecast sounds
do show strong elevated instability and deep-layer during this
period, however, a fairly strong cap is expected to develop
during the overnight hour limiting the window for any convection.
If any storms do develop strong winds, hail and heavy rain would
be the main hazards.

Rest of the Extended...With a ridge building over the northern
Rockies during the middle part of next week the region will
reside in northwest flow regime bring cooler temperatures back to
the region. The northwest flow may also bring a drier air mass to
the region which would limit the precipitation chances middle to
end of next week.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 620 AM CDT Fri Jun 21 2024

Flying conditions will deteriorate today as showers and thunderstorms
move in from the west. The activity is north of a warm front that
will be moving slowly north from northern Illinois. Periods of
showers and thunderstorms will continue tonight and Saturday, with
significant rain possible.

Ceilings will fall to IFR from west to east during the late afternoon
and early evening hours today, with IFR conditions in most areas
tonight and Saturday.

&&

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

&&

$$

DISCUSSION.....RDM/GK
AVIATION.......RDM