789
FXUS63 KGRB 051525
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1025 AM CDT Wed Jun 5 2024

New information added to update section

.UPDATE...
Issued at 1025 AM CDT Wed Jun 5 2024

A cold front is moving east across the Lake Michigan
shoreline at 10 am. The arcing band of rain along the front
continues from Manitowoc to Iron Mountain and Crandon. This precip
remains on track to exit Door County early this afternoon.

Dense fog along the lakeshore is improving south of Door County as
winds turn offshore. As the front crosses Door county in the next
1-2 hours, visibilities will improve rather quickly so anticipate
ending the Dense Fog Advisory at its expiration time.

Lastly, there is a rather abrupt back edge to the cloud mass
behind the front with strong solar insolation occurring over
eastern Minnesota and western Wisconsin. This will aid in the
development of instability and thunderstorm development this
afternoon.

Dynamics look rather robust late this afternoon into this evening
when thunderstorms remain on track to impact the region. Timing
hasn`t changed much and focused on the strongest storms moving
into north-central WI around 5 pm and the Fox Valley around 7 pm.
Primary concern remains on the damaging wind potential given the
strong wind fields aloft. The threat of strong/severe storms
should diminish quickly in the 8-9 pm hour.

&&

.KEY MESSAGES...

- Gusty southwest to west winds up to 35 mph are expected late
  this morning and afternoon. Brief higher gusts in excess of 40
  mph will be possible in any strong showers and storms in the
  afternoon and evening.

- Conditions hazardous to small craft will be possible at times
  from late today through Friday.

- Less humid conditions arrive from west to east today, with much
  cooler and less humid conditions on Thursday.

&&

.DISCUSSION...
Issued at 413 AM CDT Wed Jun 5 2024

Short Term...Today...Tonight...and Thursday

Main weather concerns will be assessing best chances for showers/
storms, if any strong/severe storms will occur, how gusty winds
get, and temp trends.

Storm Chances/Trends:

The showers and storms have remained well behaved during the
overnight hours as instability waned and lapse rates weakened.
Only had some brief gusty winds to 30 mph and a few heavy
downpours reported after midnight. This activity will continue to
shift eastward as a cold front pushes east through the state,
ending over far northeast WI by around noon/1PM. Could get a
slight uptick in activity over eastern WI as a sharp/quick moving
upper shortwave rotates across northeast WI. This could touch off
some storms and heavier rain.

Attention then turns to the afternoon as additional scattered
showers and storms are expected to spread into the region as
another shortwave will drop into the state, along with the LFQ of
an upper jet and cold front. While overall instability will be
modest, with SBCAPEs of 400-700 J/kg, impressively steep low and
mid level lapse rates approaching 10C/km and inverted-V signature
on forecast soundings will aid is transporting strong winds to the
surface. Bulk shear still forecast to climb up to 40 kts, with
effective shear 5-15 kts lower. Some gusts could approach severe
criteria after after 3PM, with instability and lapse rates
weakening in the late evening. Some hail will also be possible,
but it should mainly be under severe criteria in nature due to
limited CAPE in the hail growth zone. But with the steep lapse
rates and wet bulb heights dropping under 10K feet, some 1/2 to
possibly 3/4" hail will be possible in the strongest storms. SPC
has introduced a Marginal Risk for a severe storm for this
wind/hail risk. Brief heavy rain will be possible with any storm,
but the flooding risk will remain very low.

On Thursday, additional pop-up showers are likely at times in the
late morning and afternoon as the upper low drops into the Great
Lakes and cool/cyclonic flow dominates the region. May be able to
get a rumble of thunder, but instability will be weaker than
today (under 200 J/kg), so will not include any thunder mention at
this time. The stronger showers could produce some gusty winds
and small hail.

Winds/Temps/Dewpoints:

Winds will become west this morning and increase through the
morning, with gusts to 25-35 mph by early afternoon. Gusts near
Lake Michigan look to be reduced due to reduced mixing, but may
approach small craft criteria near the shore. West-northwest
winds will remain gusty on Thursday, gusting to 35 mph. Cooler air
aloft and decent mixing will provide a better chance for small
craft conditions on Thursday.

Highs today look to climb mainly into the 70s, with an 80 possible
where the clouds are slower to move in. It will be cooler tonight,
especially over eastern WI, with lows mainly in the 50s. With the
continued NW flow and a push of CAA, Thursday will be much cooler
than early in the week, with highs in the upper 50s (north) and
60s (elsewhere). After a sticky Tuesday and a muggy night for
much of the area, dewpoints and humidity levels will decrease
through the day behind the front. Much less humid air arrives on
Thursday as dewpoints drop into the 40s.

Long Term...Thursday Night Through Tuesday

The long term forecast will be heavily influenced by an upper-level
closed low progged to set up camp over the eastern Great
Lakes/southeast Ontario region through late this weekend. As a
result, on and off chances for light showers and isolated
thunderstorms are anticipated. However, due to the finer details
changing from run to run, the main forecast challenge will be
determining when and where the precip will occur.

Any lingering diurnally driven showers and isolated thunderstorms
associated with the closed low moving over the western Great Lakes
will quickly come to an end Thursday evening. Dry weather is
anticipated for Friday as drier air will be present in the mid-
levels. A shortwave looks to drop over the Upper Mississippi Valley
sometime Saturday before phasing with the main closed low over the
eastern Great Lakes/southeast Ontario area on Sunday. This would
bring the next chance for showers and storms, but with weak forcing
and lack of deep-layer moisture, it is too difficult to discern when
the precip will actually occur. But, given the nature of this
pattern, the best chance for showers and storms would be during the
afternoon and evening.

The weekend shortwave will push the main closed low to the east and
allow for brief ridging to build over the area for Monday. The next
signal for precip appears to be Tuesday with the departure of the
ridge.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 659 AM CDT Wed Jun 5 2024

A mix of LIFR/IFR/MVFR conditions will continue this morning as
an area of showers exits from west to east. A rumble of thunder is
also possible, but chances are too low to include. Some dense
fog has also developed on/near Lake Michigan which impacted MTW
for a brief time, but as winds turn to the west, any fog near/on
Lake Michigan will get pushed east of the area. Conditions will
improve to VFR later this morning and early afternoon with west-
northwest winds increasing to 15-28 kts.

Scattered showers and storms are expected to spread west to east
across much of the area after 2-3pm this afternoon, then end in
the late evening. These thunderstorms will have high bases, with
dry air below. This could produce local strong downdrafts, with
winds in excess of 40 mph, below the cloud bases, and possible
strong surface winds, along with small hail. A period of LLWS is
expected as winds at 2000 ft increase to 35-40 kts this evening
and overnight.

&&

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

&&

$$

UPDATE.........MPC
DISCUSSION.....Bersch/Kruk
AVIATION.......Bersch