029
FXUS63 KGRB 301718
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1218 PM CDT Sun Jun 30 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

 - The cold spots over north-central Wisconsin could see low
   temperatures in the 30s tonight. Widespread frost is not
   expected, but protecting cold sensitive plants are encouraged.

 - Periods of heavy rain will be possible from late Monday night
   through Tuesday night. There is a 40 to 70 percent chance of
   rainfall greater than 1 inch over central to east-central
   Wisconsin.

 - The next chance of widespread rain will arrive late on July 4
   into July 5.

&&

.DISCUSSION...
Issued at 229 AM CDT Sun Jun 30 2024

Short Term...Today...Tonight...and Monday

Cold air advection into the region will setup a fairly robust area
of high pressure over the western Great Lakes for the latter half
of the weekend through Monday. As result, expect clear skies and
cooler temperatures for both today and Monday. High temperatures
will run below normal as well, with highs in the upper 60s to
middle 70s today, and in the lower to middle 70s on Monday. The
only concern weather- wise for this early week period will be
temperatures tonight, as a few of the colder spots may drop into
the middle to upper 30s tonight. While this is unlikely to support
any widespread frost, cold sensitive plants may need some
additional protection.

Long Term...Monday Night Through Saturday

Focus of this forecast continues to revolve around the
thunderstorm and heavy rainfall potential from Monday night
through Tuesday night. Secondary focus is weather for the
Independence Day holiday.

Monday night through Tuesday night...As a large high pressure system
shifts from the eastern Great Lakes to the northeast coast 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 200 j/kg, so
severe weather is not expected despite decent wind fields.  But
probabilities appear to have increased (30-50%) over central to east-
central Wisconsin for 1+ inches of rainfall by midday Tuesday.
Isolated amounts to 1.5 inches appear possible over central WI.

After the first push of precip exits across the area on Tuesday
morning, indications continue to suggest a relative lull in the
precip on later Tuesday morning or Tuesday afternoon.  This is a
little slower than guidance was showing yesterday. Then precip
chances ramp up again on Tuesday evening along the cold front.  The
ensemble means continue to show precipitable water values (pwats)
approaching their daily high and in excess of 2 inches.   Like the
previous shift mentioned, chances of heavy rain have shifted south
somewhat over the past 24 hours closer to the higher instability.
Chances of an additional 0.50 inches of rain lie around 20 to 30%
over central Wisconsin.

To summarize, probabilistic data continues to show potential for 1-2
inches of rain over central to east-central Wisconsin from late
Monday night through Tuesday night.  Given the marginal instability,
the severe weather risk remains lower than heavy rainfall.

Thursday thru Friday (July 4-5)...Low pressure will be moving into
the northern Mississippi Valley on Thursday and into the western
Great Lakes on Friday. The latest data suggest timing of rain will
hold off until late on Thursday night, and heavy rain will be
possible again on Friday.  Considering the timeframe, details could
certainly change.  Temperatures are forecast to be near normal on
Independence Day.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1217 PM CDT Sun Jun 30 2024

VFR flying conditions are expected at all terminals through the
TAF period as a high pressure system settles in over the region.
A few isolated areas of fair weather cumulus clouds may develop
this afternoon, but should have minimal aviation impacts. Only
potential concern would be fog development overnight as winds
become calm, but was not confidence enough to include fog in the
TAFs at this time. Otherwise, winds will veer around and increase
Monday morning ahead of the next low pressure system. Flying
conditions will likely deteriorate Monday afternoon/evening as the
aforementioned low is forecast to bring periods of heavy rain to
the region through Tuesday.

&&

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

&&

$$

DISCUSSION.....Uhlmann/MPC
AVIATION.......GK