746
FXUS63 KGRB 151756
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1256 PM CDT Thu Aug 15 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Periods of showers and thunderstorms will bring 0.5 to 2 inches
  of rain to much of the region from today through Saturday. The
  rainfall will be heavy at times, resulting in locally higher
  amounts, and localized flooding in urban and low-lying areas.

- A few strong storms are also possible this evening that could
  produce brief gusty winds and torrential downpours, mainly in
  central WI.

- Conditions hazardous to small craft will occur on the Bay and
  Lake through Friday morning. Dangerous swimming conditions due
  to high waves and strong currents are expected on Lake Michigan
  beaches.

- Quiet weather conditions and near-normal temperatures in the mid
  70s to low 80s are expected Sunday into next week.

&&

.DISCUSSION...
Issued at 419 AM CDT Thu Aug 15 2024

Short Term...Today...Tonight...and Friday

Rain & Storm Chances / Heavy Rain & Severe Wx Potential:

An unsettled period of weather is on tap for the region today into
Friday as vertically stacked low pressure slowly tracks from
eastern ND early this morning, to near Duluth Friday morning, to
northeast WI Friday afternoon. Much of the initial batch of
WAA/isentropic lift precip is getting eaten up by the dry air, but
continued WAA, an approaching LLJ, shortwave and increasing
moisture/lift will allow the shower/storm activity to our
south/west to continue to expand/push northeastward this morning
and exit this afternoon. Thunder chances will be low due to
little to no instability. Another shortwave and LFQ of a jet push
in later this afternoon/evening, with another wave of showers and
better chance of some storms expected, but it will be more
isolated/scattered in nature. A few lingering showers/storm are
possible late tonight into early Friday, but most spots look to
stay dry as we should be in between the better forcing/moisture
and in a pseudo dry slot. On Friday, look for shower/storm
coverage to increase through the morning, peaking in the afternoon
as we max out instability/moisture/forcing as the low pushes
overhead.

The widespread flood risk appears to be low due to the progressive
nature of most of the shower/storm activity, plus there will be
breaks in between each wave, limiting the threat to reach flash
flood guidance. But localized flooding will remain possible where
storms occur as soundings still showing deep saturated warm cloud
depths to 4 km, along with PWATs of 1.5-2.0" (150-200% of normal)
today and 1.4-1.8" on Friday, leading to efficient rainfall
production, and very heavy downpours. Most spots look to see up to
a half inch of rain with the first wave, then the showers/storms
later today into this evening could drop locally heavy rain (over
an inch), especially in central WI. Any training storms or
if/where storms pivot over the same location on Friday will lead
to localized heavier rain totals (over an inch) with some flooding
possible. especially if it coincides with heavy rain today.

The threat for organized severe weather remains low due to the
lack of sufficient shear and much surface heating/ instability.
Could be a small window very late this afternoon/early evening,
especially if we can get a little clearing, mainly across central
WI, where some decent elevated instability (up to ~1700 J/kg)
advects in. Some small hail and gusty winds (to around 40 mph)
would be the main hazards with the strongest storms.

Winds / Marine & Beach Hazards:

Pressure gradient will tighten up as low pressure approaches from
the west, allowing southeast winds to increase through the morning
hours. Gusts of 20-30 mph are expected by late morning and into
the afternoon, highest over eastern WI where some locally higher
gusts are possible. The persistent southeast winds will create
large waves and dangerous swimming conditions on the beaches of
Door, Kewaunee and Manitowoc counties. A Small Craft Advisory and
Beach Hazards Statement remain in effect. As the low moves
closer/overhead, winds will decrease later tonight into Friday.

Temperatures / Humidity:

It will be cooler today with highs in the upper 60s to mid 70s
for most locations. A little more humidity will be in the air as
dewpoints climb into the low to mid 60s. It will be a relatively
warm and muggy night across the region, with lows only dropping
into the 60s. Guidance came in a good 5 degrees warmer in many
spots on Friday, but with the expected cloud cover and precip
around, temps will be hampered a bit, especially over the north.
Will hold most highs in the 70s, but a couple spots could touch 80
if the rain hold off a little longer and especially if we can get
some peaks of sun in the south. Dewpoints will be in the mid and
upper 60s for most spots on Friday, with a couple spots hitting
70 degrees possible, making for a humid day.

Long Term...Friday Night Through Wednesday

Rainfall trends through Saturday continue to be the focus of this
forecast.  Thereafter, typical late summer weather is forecast as
the upper flow turns northwest which is conducive for quiet
conditions.

Rainfall trends:  The forward motion of the vertically stacked low
pressure has slowed over the past 24 hours, which doesn`t shift east
across northern Lower Michigan until Saturday.  Deep saturation
within the cyclonic circulation will bring scattered shower activity
to the region at times through Saturday afternoon.  The most
widespread activity should reside over eastern WI.

While instability will be very low for most of the period, perhaps
minor destabilization could take place with diurnal heating on
Saturday that could lead to a slight thunderstorm chance. Otherwise,
the chance of storms is very low.  There is potential for an
additional quarter to a half inch of rain across northeast WI.

Rest of the forecast:  Northwest flow aloft and high pressure at the
surface will bring seasonable late summer weather to the area from
Sunday through Tuesday.  There are some indications of a cold front
diving southeast through the region during the Wednesday to Thursday
time frame, but predictability is low this far out.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1242 PM CDT Thu Aug 15 2024

A large area of rain associated with a short-wave trough will
continue to shift northeast through the forecast area this
afternoon, then exit far northeast WI this evening. Flight
conditions will deteriorate to IFR during the steadiest/heaviest
period of rainfall. There will be a lull in the precipitation
after the initial band moves through, but IFR/LIFR ceilings and
patchy fog will remain. More showers and scattered thunderstorms
should redevelop as another short-wave moves through later this
evening and overnight. Have attempted to frame out the best time
for thunderstorm development at the TAF sites. After the overnight
convection moves out, there should be a general lull in the rain
in most areas until Friday afternoon. Remaining IFR/LIFR ceilings
and fog should improve to MVFR during the mid to late morning.

Southeast winds will gust up to 25 kts this afternoon. Winds will
decrease and slowly shift to the south and southwest tonight into
Friday morning.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement through Friday morning for WIZ022.

Beach Hazards Statement through Friday morning for WIZ040-050.
&&

$$

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