020
FXUS63 KGRB 182338
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
638 PM CDT Tue Jun 18 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Gusty south winds could create large waves and strong currents
  at Lake Michigan beaches tonight. A Beach Hazard Statement has
  been issued.

- The gusty south winds will also create conditions hazardous to
  small craft on the lake and bay tonight.

- Thunderstorms are likely in central and north-central Wisconsin
  tonight, with a chance of storms in eastern Wisconsin late
  tonight.

- Periodic chances for storms will continue throughout the week.
  Some storms may produce strong winds and torrential rainfall.

&&

.DISCUSSION...
Issued at 219 PM CDT Tue Jun 18 2024

Short Term...Tonight and Wednesday

Main forecast concerns center on timing of convection into the
forecast area tonight, risk of severe and potential for heavy
rain.

The 19Z MSAS surface analysis showed an area of low pressure over
southwest MN with a warm front extended northeast/then east across
the northern sections of the Great Lakes. A cold front stretched
from the low pressure south-southwest through the central Plains.
Visible satellite imagery indicated plenty of instability cumulus
clouds across the forecast area. Radar mosaic had started to pick
up a few showers over western WI.

The cold front is forecast to slowly move into western WI by late
tonight and be the main focal point for showers and thunderstorms.
Despite some weakening of instability through the night, shear is
expected to increase into the 35-45 knot range after midnight over
northern and central WI. Therefore, some storms may become strong
toward central WI later tonight, but severe potential appears
minimal due to weaker instability. PW values pushing 2 inches
could bring locally heavy rain to central WI with localized 1-2
inch amounts possible. A warm and muggy night can be expected for
all with min temperatures to range from the lower to middle 60s
north-central WI, to the lower 70s eastern WI (inland from Lake
MI).

This cold front is expected to sag southeast across northeast WI
on Wednesday before grinding to a halt over southern WI late in
the afternoon as it begins to encounter the large upper-level high
pressure situated over the eastern CONUS. While there will be a
good chance of showers and storms (especially over central WI late
tonight into early Wednesday morning), model trends are for the
precipitation to become more scattered in nature through the day
due to the loss of upper support and weakening low-level jet.
Behind the cold front, dew points will slowly fall back into the
50s over northern/parts of central WI, so not as humid as the past
couple of days. Due to more clouds and continued rain chances, max
temperatures will be cooler with readings to range from the lower
to middle 70s north-central WI, to the lower 80s eastern WI
(slightly cooler near Lake MI).

Long Term...Wednesday Night Through Tuesday

An unsettled pattern will persist through the extended with
several rounds of rain and storms possible through the end of the
week. Main concern will be localized flooding due to successive
rounds of showers and anomalous moisture. The WPC currently
highlights portions of northeast Wisconsin under a marginal/slight
risk for receiving excessive rainfall through day five (Sunday).
A more robust system will then arrive to start the weekend,
bringing another wave of potentially strong storms Saturday into
Sunday.

Wednesday night through Saturday... Expect a brief dry spell on
Thursday as a cold front departs to the southeast, leaving an area
of large-scale subsidence and high pressure in its wake. Rain
chances will return to the forecast area Thursday afternoon as
return flow brings a surge of Gulf moisture up into the western
Great Lakes. Active weather will then make a comeback Friday and
Saturday as a potent shortwave skims the US/Canada border and
spins up a surface low over the Dakotas. Warm air advection precip
will likely be ongoing Friday and Saturday as a warm front lifts
north before becoming more widespread behind a cold front Saturday
afternoon and evening. Some strong storms may be possible
Saturday afternoon given favorable placement in the warm sector
paired with the return of dewpoints in the mid 70s. As a result,
instability upwards of 2,000 J/kg may be enough to sustain
stronger storms across portions of central and east-central
Wisconsin. Additionally, some synoptic support will be provided by
the right rear quad of an upper-level jet generating a convergent
zone on the nose of an increasing LLJ (40 to 50 knots). While it
is still too early to discern severe weather risk, initial
impression is that convective ingredients look favorable for
supporting thunderstorm activity Saturday afternoon and evening.
Medium-range models should paint a better picture in the next
couple of days, so will continue to monitor.

Rest of the extended... Scattered showers will be possible across
the forecast area through Sunday due to remnant cyclonic flow from
the departing system. A pattern change is then expected to start
the week as mid-level flow flattens out before a ridging regime
settles in across the eastern CONUS. This will bring about a more
prolonged period of dry and seasonable weather for much of the
Midwest, with temperatures averaging in the low to mid 80s.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 636 PM CDT Tue Jun 18 2024

A cold front is forecast to slowly move into western WI later
tonight and bring a chance of showers and thunderstorms into
central WI. Cigs will lower into the MVFR range once the
precipitation reaches the area and it is possible for a brief
period of IFR conditions under any storms. LLWS will occur at all
the TAF sites ahead of this front tonight.

Mainly MVFR conditions persist through Wednesday morning as the
cold front crosses the region and becomes stationary over
southern WI late in the day. There will still be a chance of
showers and thunderstorms with the front in the vicinity, mainly
through mid morning and again mid to late afternoon, but the north
should begin to dry out in the afternoon. In fact, VFR cigs should
return to the RHI/AUW/CWA TAF sites by the afternoon while cigs
improve to VFR at GRB/ATW/MTW in the afternoon.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement through Wednesday morning for WIZ022-040-
050.
&&

$$

DISCUSSION.....Kallas/Goodin
AVIATION.......JLA