736
FXUS63 KGRB 011727
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1227 PM CDT Mon Jul 1 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Periods of heavy rain will be possible from after midnight
   tonight through Tuesday night. There is a 40 to 70 percent
   chance of rainfall greater than 1 inch over central to east-
   central Wisconsin during this timeframe. In general, 1 to 2
   inches of rain will be possible across the entire area, with
   locally higher amounts up to 3 inches. This could lead to
   minor river and stream flooding, along with urban flooding.

 - There is a high risk of rip currents on Lake Michigan beaches
   on Tuesday and Tuesday night due to high wave action. Beach
   goers are advised to remain out of the water when waves are
   high.

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

&&

.DISCUSSION...
Issued at 316 AM CDT Mon Jul 1 2024

Short Term...Today...Tonight...and Tuesday

The main focus of the short term forecast will be the arrival of
rain late tonight, starting off a rainy Tuesday.

Before the rain arrives, a quiet Monday is expected for the
region, with clear sunny skies through the early afternoon. As
warm air advection begins aloft on the back side of the departing
high pressure, clouds will begin to move in through the evening
hours.

Rain will arrive after midnight tonight as a low pressure system
passes to our northwest. Widespread rainfall is expected for a
good portion of Tuesday, with two periods of potentially heavier
rainfall as a pair of shortwaves crosses the region. The first of
these will be sometime Tuesday morning, potentially bringing some
scattered thunder but severe is unlikely as instability will be
quite limited. Heavy rainfall will be possible however, as PWAT
values push to near their climatological maxima of 2+ inches. As a
result, this initial round could bring a widespread inch or more
to the region. Behind the shortwave, precipitation could continue
into the afternoon, but will be lighter and less focused. A few
areas could see breaks in rain during this time as well, before
the next round of heavier convectively driven rainfall arrives
Tuesday evening (see extended discussion). Between the two rounds
of heavier rainfall, a widespread 1-2 inches of rainfall with
local amounts up to 2-3 is possible, which may create some
flooding concerns for lowland areas, river rises, and/or urban
flooding if the higher rainfall swaths cover the same areas.

Long Term...Tuesday Night Through Sunday


Main focus throughout this forecast period revolves around the
potential for heavy rainfall on Tuesday night, and the potential for
showers and thunderstorms on the Fourth of July and the subsequent
weekend.

Tuesday night...Shortwave energy will push a cold front across the
region from late evening through the overnight.  Dynamics are not
particularly impressive along the front, but there is upper support
via the right rear quad of an upper jet to enhance ascent. However,
precipitable water (pwats) values are near the daily max in
climatology (2.05 inches) and forecast soundings indicate skinny
cape up to 500-600 j/kg with high freezing levels. Despite
sufficient wind fields for severe weather, these parameters continue
to support a heavy rain threat across the area.  Probabilities
highlight the Fox Valley and lakeshore areas at having the greatest
potential for an additional inch of rain (10-20% chance) and think
isolated amounts of at least an inch are possible.  With
preconditioning of the soil from rains earlier in the day, think
impacts will revolve around minor river/stream and urban flooding.
Most of the rain will exit by late Tuesday night or early Wednesday
morning.

Independence Day through next weekend...A relatively strong
shortwave trough and associated low pressure system will track
across the Northern Plains into Minnesota on Thursday and over
Wisconsin on Friday. Little has changed with the timing of the
system, and areas over central WI have the highest probabilities of
rain by late Thursday evening. Timing could be close to firework
celebrations and festivals, and trends will be closely monitored.

This system will pass across the region on Friday, bringing a high
chance (70-80%) of showers and storms.  While the rain looks to
depart the region by late Friday night, flattened troughing aloft
will likely create light precip at times next weekend but
predictability is low with regards to the timing and severity.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1226 PM CDT Mon Jul 1 2024

VFR flying conditions are expected for the remainder of the
afternoon and evening as a high pressure system slowly shifts off
to the east. High based cirrus clouds will start to spread over
central WI this afternoon and into eastern WI this evening ahead of
an approaching low pressure system. Currently have rain arriving to
the central and northern WI TAF sites (AUW, CWA, and RHI) during the
06-07Z hour and the eastern WI TAF sites (AUW, GRB, and MTW)) during
the 08-09Z hour. Cigs will likely lower into the MVFR range as rain
begins to fall and will continue to lower toward IFR as rainfall
intensifies, mainly across central WI Tuesday morning. May also see
periods of borderline MVFR/IFR vsbys with the heaviest showers
Tuesday morning. Left any mention of thunder out of the 18Z TAFs as
instability is forecast to be quite weak and confidence is low, but
won`t rule out a few rumbles of thunder just yet. Lastly, did
include LLWS at all TAF sites, except MTW, early Tuesday morning
with 10-15kt southerly winds at the surface, and 35-40kt
southwesterly winds at around 2km above ground.

&&

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

$$

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