463
FXUS63 KGRB 182247
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
547 PM CDT Wed Sep 18 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Near-record high temperatures are expected on Thursday.

- Locally dense fog is possible across mainly eastern Wisconsin
  late tonight into Thursday morning.

- There is a marginal risk of severe storms in north-central and
  central Wisconsin from late Thursday afternoon into Thursday
  evening. Most locations are forecast to see a quarter to a half
  inch of rain from Thursday afternoon through Friday afternoon.

- A pattern shift is expected this weekend, with highs dropping closer
  to normal Sunday through the middle of next week. Confidence is
  low on the timing/location and rainfall amounts for early next
  week.

&&

.DISCUSSION...
Issued at 243 PM CDT Wed Sep 18 2024

Short Term...Tonight and Thursday

The latest RAP analysis and satellite radar imagery show a
vertically stacked low pressure system spinning over eastern
Montana early this afternoon. An arcing band of clouds is moving
east and well ahead of the system into the northern Mississippi
Valley. Cirrus along the leading edge of this cloud band is
dissipating as it spreads east across Wisconsin. Otherwise, it is
another warm September day with temps in the upper 70s to middle
80s across the region. As this upper low slowly moves east, fog
potential early Thursday morning and thunderstorm potential late
Thursday afternoon are the focus of this forecast.

Fog potential: While cirrus will continue to invade the region
from the west, conditions will be favorable for fog development
over northeast WI late tonight into Thursday morning. Models
continue to show light onshore flow off the warm waters of the Bay
and Lake combined with mostly clear skies and temps falling well
below their cross-over temps. The highest probabilities will be
just west of the Lake Michigan and Green Bay shoreline over inland
Oconto, Marinette, Manitowoc, and Kewaunee counties. Locally dense
fog will be possible starting around or shortly after midnight
through about 8-9 am Thursday morning.

Thunderstorm potential: A very dry airmass remains present over
eastern WI this afternoon where precipitable water values are
estimated at about 60% of normal. This dry airmass will shift east
as the moisture axis ahead of low pressure over the northern
plains slides into the state on Thursday afternoon. By 4 pm
Thursday, precipitable water values are expected to climb to 175%
of normal over north-central WI.

The increasing moisture will lead to a more unstable airmass
invading the region on Thursday. Thunderstorms are expected to
develop over eastern Minnesota at peak heating in the afternoon.
Some guidance develops additional thunderstorm activity further
east into central WI after 5 pm where instability will reach over
1000 j/kg. If storms can move into central WI before nocturnal
cooling sets in, a few strong storms appear possible due to deep
layer shear over 30 kts and dcapes over 1000 j/kg.

Temperatures: A little warmer on Thursday with highs in the low to
middle 80s. Lows tonight will fall into the upper 40s to middle
50s.

Long Term...Thursday Night Through Wednesday

Tha main forecast concern is severe thunderstorm potential for the
western part of the forecast area Thursday evening, and
precipitation chances and amounts for the entire forecast area
Thursday night into Friday. Cooler temperatures are also
anticipated later this weekend and early next week.

The blocking upper level ridge will break down, with a negatively-
tilted short-wave moving through the western Great Lakes later
Thursday into Friday. An associated cold front will push through
the region, with PWATs increasing to near 1.5 inches along and
ahead of the boundary. The main severe threat will occur west of
the forecast area Thursday afternoon, but lingering daytime
instability and deep layer shear of 35-40 kts may allow a few
strong to marginally severe storms to impact NC/C WI during the
evening. Convection should weaken as it shifts farther east during
the overnight/Friday morning period. Probabilities are fairly
high for most of the region to receive a tenth to a quarter inch
of rain with this system, though there is a 30 percent chance of
a half inch or more in NC/C WI. Most of the rain will be east of
the forecast area by midday Friday. Generally dry conditions are
anticipated Friday night into Saturday, but another frontal
boundary could bring a chance of showers or storms Saturday night
into Sunday.

As was mentioned previously, the details of the forecast become
muddled later in the weekend into next week, with low confidence
in timing/location/precipitation amounts. There is higher
confidence in temperature trends, with the much above normal
temperatures coming to an end later this weekend/early next week.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 547 PM CDT Wed Sep 18 2024

A weak upper level disturbance will produce increasing high
clouds tonight. Ground fog is possible late tonight, at the
airports immediately west of the bay and Lake Michigan. Places
like MTW, OCQ, SUE and MNM could have dense ground fog from around
08z through 13z Thursday.

Middle and high clouds will increase Thursday, as a weak cold front
approaches from the west. Scattered showers or thunderstorms are
possible after 1800 UTC west of a IMT to STE line. VFR conditions
are expected outside of any convective activity.

&&

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

&&

$$

DISCUSSION.....MPC/Kieckbusch
AVIATION.......RDM