245
FXUS63 KGRB 190849
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
349 AM CDT Thu Sep 19 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Well above normal high temperatures in the 80s continue today.

- Locally dense fog across mainly eastern Wisconsin early this
  morning. The fog should burn off by 9 am.

- There is a marginal risk of severe storms in north-central and
  central Wisconsin late this afternoon and evening. Damaging
  winds are the main risk.

- The recent dry spell over the last month has led to worsening
  drought conditions across much of the area. Rainfall totals of
  a quarter to half inch are expected, especially north and west
  of the Fox Valley from tonight into Friday afternoon. Localized
  rainfall amounts around an inch are possible across north-
  central Wisconsin, but confidence is not high in seeing these
  higher amounts.

- 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 349 AM CDT Thu Sep 19 2024

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

The stretch of summer-like temps continue late this week. Main
forecast concerns will be the chances for strong to severe storms
this afternoon/evening, fog potential/trends, and how warm we
will get today/Friday.

Thunderstorm & Heavy Rain Potential:

We will start off dry today with some high clouds around as weak
high pressure pushes east of the region. Then attention turns to
the vertically stacked low pressure moving from far southeast
Alberta early today to central Manitoba Friday morning and the
associated negatively tilted shortwave trough and cold front.
These features are producing some showers and storms across MN/IA
early this morning. Expect this activity to percolate through the
morning, arriving in central and north-central WI later this
morning and/or early afternoon. This activity will be outpacing
the instability and is expected to weaken as it reaches the area.
The main event will get going this afternoon across eastern MN
and far western WI as 2000-3000 SBCAPE is forecast to develop
ahead of the front. The greatest threat for severe weather will be
within the first few hours of initiation west of the area, then
as the area congeals into one or more clusters, it will begin to
encounter more dry/stable air, as nocturnal cooling begins and a
low-level inversion developing, so a weakening trend is expected.
Still some uncertainty on just how fast this occurs as we could
get MUCAPE of 1000-1500 J/kg, along with decent bulk shear of ~40
kts, into parts of central and north central WI late afternoon
toward sunset. If this occurs, isolated severe storms would be
possible before the boundary layer begins to cool off. Gusty winds
and small hail will be the main threats. The cluster of
showers/storms will spread east across the area in the late
evening and into Friday morning.

Some much needed rainfall is expected across the area, but not
all spots will see the heavier (0.50+") amounts as the activity
will be pretty progressive. Spots that do see repeated storm
activity could see 1+ inch totals as PWATs climb to ~1.5"
(150-200% of normal). While brief heavy downpours are expected
(1-2" per hour rates) in any storm, the flooding threat will be
very low due to the movement of the storms and the recent dry
spell.

Fog Potential/Trends:

Webcams and surface obs showing dense fog once again across far
eastern WI near Lake Michigan and Green Bay, as a healthy low-
level inversion, light onshore flow off the warm waters, and
mostly clear skies remain in place. It is a little difficult to
assess how widespread the fog is due to the high clouds obscuring
the view to the surface. Expect the fog to continue across Oconto,
Marinette, Manitowoc, and Kewaunee counties early this morning.
The fog will be dense fog at times and is expected to lift/mix out
by 8-9 AM. May need another SPS to cover the dense fog. Patchy
ground fog is possible further inland early this morning. Some fog
will be possible late tonight into early Friday across central
and north- central WI as the rain/clouds exit.

Temperatures:

Well above normal temps will continue today and Friday. Impressive
925mb temps of 20-23C and 850mb temps of 15-17C remain across the
area today, which support highs in the low to mid 80s for most of
the area. Could see a couple spots in the upper 80s in/west of the
Fox Valley. With dewpoints in the 50s to low 60s, heat index
values will remain close to the actual air temps. It will be a
relatively warm night, with lows only dropping into the upper 50s
to mid 60s. 850/925mb temps cool off slightly on Friday as weak
CAA arrives, but highs still look to remain in the upper 70s to
the lower half of the 80s. Some higher dewpoints in the mid 60s
are expected to linger into the late morning or early afternoon
over the east, making it feel a little humid, but heat index
values will remain in the low to mid 80s. Records still look to be
safe, but we will be close to a few of the lower ones.

Long Term...Friday Night Through Wednesday

The long stretch of 80 degree weather in September is about to
come to an end this weekend as 500mb ridging across the western
Great Lakes flattens as a system passes north of the Canadian
border. However, temperatures early next week will still be at
or possibly a few degrees above normal.

For Friday night, weak high pressure will move across the area.
On Saturday, the last day with high temperatures in the 80s for
this stretch are expected as southerly winds increase ahead of
a cold front. It will be warm, but still at least several degrees
away from the record high for the date. On Saturday night,
a cold front will move across the area, bringing a chance of
showers and thunderstorms. Some significant difference in the
bufkit soundings, but the GFS sounding at Green Bay was suggest a
better chance of storms with elevated CAPE and stronger shear
values. Some showers could linger across the area Sunday morning.
A surface low could bring some much needed Sunday night into
Tuesday. Rainfall over the last month to month and a half has
been very dry across much of the area. Green Bay has only received
0.36 inches of rain this month. The GFS as a has 500mb trough
swinging across the area on Wednesday, bringing a continued chance
of rain.

The latest GFS mos guidance for early next week has warmed by at
least a few degrees between Monday and Wednesday, which is not
matching up well with the ECMWF MOS guidance for this time period.
Temperatures could still end up at or a few degrees above normal
early next week.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1047 PM CDT Wed Sep 18 2024

A weak upper level disturbance will produce high clouds overnight.
Moist air near the lake and bay will likely produce areas of dense
ground fog at the airports immediately west of the bay and Lake
Michigan like MTW, OCQ, SUE and MNM could have dense ground fog
late tonight 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 2100 UTC west of a EGV to ISW line. VFR conditions
are expected outside of any convective activity.

Scattered showers and thunderstorms will move across the area
Thursday night, with IFR conditions likely west of a IMT to STE
line after midnight and MVFR to the east.

&&

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

&&

$$

DISCUSSION.....Bersch/Eckberg
AVIATION.......RDM