406
FXUS63 KGRB 142346
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
646 PM CDT Wed Aug 14 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Periods of showers and embedded thunderstorms will bring 1 to 2
  inches of rain to much of the region late tonight through Friday
  night. Rainfall will be heavy rainfall at times, resulting in
  locally higher amounts, and localized flooding in urban and low-lying
  areas.

- Conditions hazardous to small craft will occur on the Bay and
  Lake from Thursday into 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 301 PM CDT Wed Aug 14 2024

Short Term...Tonight and Thursday

Precipitation trends and heavy rainfall potential will be the
main forecast concerns.

Low pressure organizing in the Northern Plains will shift east
into central MN and far NW WI on Thursday. As the system nears,
a swath of moderate rainfall is expected surge northeast across
the forecast area later tonight into Thursday. Expecting somewhat
of a decrease in shower activity over the southwest part of the
forecast area in the afternoon. Widespread rainfall amounts of a
half inch to an inch, with locally higher amounts, are likely
through Thursday.

Moisture Availability: An influx of Gulf of Mexico moisture will
cause PWATs to surge to 1.5 to 2 inches (150-200 percent of
normal) late tonight into Thursday. 850 mb dew points are also
quite impressive; rising to 12-16 C. Forecast soundings indicate
saturated warm cloud depths to 3.5 to 4 km, leading to efficient
rainfall production.

Forcing: A lead short-wave will shift northeast across the
forecast area later this evening and overnight, as a 35-45 kt
low-level jet cranks up. A stronger, negatively-tilted short wave
will move through the forecast area on Thursday.

Instability: Left thunderstorms out of the forecast for tonight,
as elevated instability is slow to arrive. The leading edge of
instability aloft will arrive in the southwest part of the
forecast area Thursday morning, and in the northeast half in the
afternoon. Surface-based instability is expected to remain
southwest of the forecast area until Thursday evening, which in
combination with overcast skies and lingering showers, should
limit any severe potential during the afternoon.

Winds: Southeast winds will crank up late tonight into Thursday as
the low pressure system approaches. Widespread gusts to 25 to 30
mph are expected on Thursday. Gusts to 25 to 30 kts and building
waves on Lake Michigan will result in hazardous conditions for
small craft (starting around 15z/Thu), and dangerous swimming
conditions on the beaches of Door, Kewaunee and Manitowoc
counties.

Temperatures: Lows will drop into the upper 50s to middle 60s
tonight. Highs on Thursday will only be in the middle 60s to
middle 70s.

Long Term...Thursday Night Through Wednesday

Highest impact in the long-term is Thursday night - Friday night.

Models coming into fairly good agreement with having the heart of
the upper low centered roughly over northern Wisconsin around 12z
Friday. Expecting a lull in precip at this time (PoPs generally down
to 10% to 20% range) as dry air/falcon punch being indicated by
models. This precip lull, however, will be short-lived. By
afternoon/evening, this low is over the Great Lakes, and models are
showing an additional slug of moisture and energy via a 1.5 PV
anomaly. PWATs with this additional band of energy are around 1.5 to
1.6 inches, and the 0-6km layer has saturation of around 85 to 95%.
This feature has been evident for the past 24 to 36 hours of model
runs, so there is consistency. Ultimately, have boosted QPF in
northern WI to the 0.1 to 0.25 inch range. Would not be surprised if
this needs to be boosted again, as precip looks to be at least
mildly efficient.

For severe weather potential... Deep layer shear of 20-30 kts looks
rather marginal for severe weather. The vertical wind profile looks
fairly unidirectional by Friday PM, so very plausible for a stronger
shower/storm to potentially get some of these winds to the surface
and yield a brief 30 to 35 kt wind gust as a primary hazard. SPC has
our CWA in `general` thunder, and that is probably fine. Could
perhaps make a case for `marginal`, but will see how Thursday plays
out. In all, do not see severe weather as a primary threat at this
time.

Rest of the extended... Northwest flow takes over after the system
departs later on Saturday.  No impactful systems are expected across
the region from Sunday through next Wednesday. Medium-high
confidence in near-normal temperatures into the middle of next week.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 646 PM CDT Wed Aug 14 2024

This evening and overnight will continue to see VFR flying
conditions. Mid and high clouds will stream in from the west, but
most of the precipitation and lower cigs will hold off until
after 12Z/Thu. As the next system approaches, expect lowering
cigs, light to moderate rain, and drops in visibility due to the
rain throughout Thursday, especially from late morning through the
afternoon. Mainly IFR with pockets of LIFR conditions are
expected. Some embedded thunderstorms are possible across the
state, but the likelihood at any one TAF site was too low to
include in the TAFs.

Meanwhile, SE winds will increase Thursday with gusts of 20 to 30
kts forecast from mid-morning through late afternoon.

&&

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

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

$$

DISCUSSION.....Kieckbusch/Kotenberg
AVIATION.......KLJ