981
FXUS63 KGRB 141202
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
702 AM CDT Wed Aug 14 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Patchy fog, locally dense, may result in hazardous travel
  conditions for this morning`s commute.

- Showers and thunderstorms will impact the area from late tonight
  through Friday night. Periods of moderate to heavy rainfall are
  expected, which could lead to localized flooding in urban and
  low-lying areas.

- Conditions hazardous to small craft will be possible on the Bay
  and Lake on Thursday and Thursday night. Dangerous swimming
  conditions are also expected on Lake Michigan beaches.

&&

.DISCUSSION...
Issued at 359 AM CDT Wed Aug 14 2024

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

Today...light winds and clear skies will allow for patchy fog to
expand before sunrise, with locally dense fog in some river valleys
and other low-lying areas. This may cause locally hazardous travel
concerns for the early morning commute. The low clouds and fog
will burn off/mix out shortly after sunrise.

Otherwise, a quiet mid-August day is on tap for the region as weak
high pressure slides across the Great Lakes. Some high clouds
will drift in from the west through the day, along with daytime
cumulus later this morning and afternoon, mainly away from the
lake. Expect dry conditions for most/all of the area, with the one
exception being near the MI border, or possibly near the Bay, as
a spotty shower will be possible this afternoon along the lake/bay
breezes. Less instability than yesterday, generally under ~500
J/kg, plus a weak cap just above 10,000 feet, mixing into dry air,
little to no upper support and weak shear, will make it difficult
for any shower/storm to form. If a shower were able to form, it
would be pretty short-lived and very isolated with a brief heavy
downpour and wind gust (inverted-V soundings) being the only minor
concerns. Thinking the better convergence will be in the U.P.,
and with the chance in any one spot under 10%, will stick with a
dry forecast.

It will be a warm day with highs climbing into the low to mid 80s
for most of the area, with slightly cooler readings near the shores
of Lake Michigan. Humidity levels will be pretty comfortable with
dewpoints mainly in the 50s to around 60.

Tonight/Thursday...dry conditions are expected through around
midnight, then look for rain to begin to spread into parts of
central and north-central WI overnight as the initial push of WAA/
isentropic lift and shortwave energy arrives. A secondary
shortwave and slug of deeper moisture (PWATs of 1.7-2.0") arrives
Thursday morning, helping spread this area of rain across the
entire area. Most of the rain looks to be light to moderate, but
embedded areas of heavy rain are expected at times as soundings
showing very deep moisture, up to ~40,000 feet and warm cloud
depths, making for efficient rain production. Chances for thunder
will be low with the initial WAA precip (but some rumbles are
certainly possible), as only some weak elevated instability will
be present. Better chances for thunder will be later in the day as
more unstable air advects in from the southwest, but the activity
looks to be more isolated/scattered in nature. Severe weather
threat will be low, but as CAPE builds to ~1000 J/kg, some strong
storms are possible with gusty winds and hail. Latest rainfall
amounts have come down, with most spots staying under an inch
during the day on Thursday. Some locally higher totals are
possible associated with any storms in the afternoon, but the
storms are expected to have enough movement to limit the heavy
rain threat in any one spot. So while some locally heavy rain will
be possible, the lower totals, plus several days of mainly dry
conditions, should keep any flooding threat low through sunset.
Southeast winds will become a little gusty, especially across
eastern WI with gusts to around 30 mph expected.

Lows will drop into the 50s over northern WI and remain in the
60s further south. Highs will be cooler than the past few days due
to the clouds/rain, with most spots holding in the upper 60s to
mid 70s.

Long Term...Thursday Night Through Tuesday

This portion of the forecast will start out wet as low pressure
crosses the region from Thursday night through Friday night.  Then
after the low moves out, relatively seasonable late summer weather
returns through the middle of next week.

Rainfall trends:  Low pressure will slowly move across northern
Wisconsin on Thursday night and Friday before exiting over northern
Lake Michigan on Friday night.  Ahead of the low on Thursday night,
an axis of instability (perhaps up to 1000 j/kg of elevated cape)
will lift northeast across central to northeast WI.  Precipitable
water values will remain high at 1.8-1.9 inches during the evening,
so think locally heavy rainfall will remain possible in this area.
Forecast soundings indicated cloud layer winds of 20-30 kts, which
is significantly higher than models were showing yesterday.  As a
result, think the forward storm motion will keep the flash flooding
threat rather low.  A few strong storms cannot be ruled out though,
which could produce small hail.

The chance of showers and storms will continue at times on Friday as
as the low slowly meanders east across the state.  Will likely see
an increase of coverage of the precip as instability builds with the
heat of the day.  Deep layer shear of 20-25 kts looks rather
marginal for severe weather, but a moist profile and pwats around
1.5 inches will aid in rainfall efficiency of the showers.

Light showers will likely stick around on Friday night into Saturday
morning before ending.

Rest of the extended... Northwest flow takes over after the system
departs later on Saturday.  No significant systems are forecast to
impact the region from Sunday through next Wednesday.  Temperatures
should remain near normal.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 651 AM CDT Wed Aug 14 2024

Patchy ground fog will burn off shortly after sunrise. Otherwise,
look for VFR conditions today as high clouds drift across the
region, along with a few daytime cumulus. Clouds will slowly
lower and thicken overnight, with showers arriving from west to
east late tonight into Thursday morning. Conditions do not look
to drop to MVFR until around 12z in central and north-central WI
when the heavier showers are expected. Widespread showers, along
with embedded storms, are expected much of Thursday, bringing poor
flying conditions.

&&

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

&&

$$

DISCUSSION.....Bersch/MPC
AVIATION.......Bersch