431
FXUS63 KGRB 121153
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
653 AM CDT Wed Jun 12 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- There is a Marginal Risk of severe thunderstorms across north
  central and central Wisconsin late this afternoon and evening.
  Overall confidence in the severe weather threat is low, but a
  few of the strongest storms may produce gusty winds and hail.

- Well above normal temperatures are forecast from Sunday into the
  middle of next week. Heat indices may surpass 90 degrees at
  times.

- There are periodic chances for thunderstorms Saturday night
  through the middle of next week. It is uncertain if and when any
  of the storms would become severe. Locally heavy rainfall will
  be possible at times.

&&

.DISCUSSION...
Issued at 403 AM CDT Wed Jun 12 2024

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

Precipitation trends, severe weather potential this evening,
warmer temperatures and gusty winds on Thursday will be the main
forecast concerns.

Surface analysis showed an ill-defined warm front situated from
west central WI southeast-ward into far southern WI early this
morning. WV imagery and upper air analysis indicated a seasonably
strong jet streak extending from northern MT into northern MN.
Subtle short-waves embedded in the fast zonal flow were moving
through the Dakotas.

Precipitation Trends and Severe Thunderstorm Potential: The
surface warm front is expected to lift northeast across the
forecast area this morning, with broad WAA developing across the
region. A weak short-wave and RRQ of an 80-90 knot upper level
jet streak will move through the region in the late morning and
early afternoon, and should generate isolated to scattered showers
and a few weak thunderstorms. Additional scattered thunderstorms
are expected to develop as a 35-45 knot low-level jet increases
ahead of an approaching cold front late this afternoon and
evening. By this time, instability will be increasing across the
western part of the forecast area, with MUCAPE reaching 1000 j/kg
or a little higher. Deep layer shear will be 35 to 50 knots, which
is supportive of storm organization. Negative factors for severe
storms include possible earlier convection, a lack of significant
upper level forcing, a later arrival time after peak heating, and
marginal instability that wanes into the evening hours. Right now,
think that SPC has the right idea with a Marginal Risk over the
western half of the area, but overall confidence in how and
where this will play out is low, as models show a multitude of
possibilities. The overall threat of storms will continue to
spread southeast across the forecast area this evening, then shift
out of east central WI during the overnight period as the low-
level jet veers west. The cold front will move through overnight
into Thursday morning and instability will develop over our SE CWA
in the late morning/midday. However, with west flow at 925/850
mb, low-level convergence will be shallow/weak along the boundary,
leading to isolated storms at best. A short-wave trough and
secondary cold front will impact northern WI in the afternoon,
bringing scattered showers.

Temperatures: Broad WAA will boost H8 temperatures to +17 to +18 C
by the end of the day, which would typically support highs into
the middle to upper 80s. However, increasing clouds and isolated
to scattered showers in the late morning/early afternoon will hold
readings back a bit. Went with highs in the upper 70s to middle
80s, but cooler lakeside. A mild night is in store as a weak cold
front slowly moves through the region. Lows should be in the upper
50s and 60s. Despite CAA on Thursday, deep mixing should allow
temperatures to reach into the middle 70s northwest to the lower
to middle 80s southeast.

Gusty Winds on Thursday: Deep mixing through 6-8k feet should
tap into strong winds aloft, resulting in west winds gusting to
around 30 mph in most areas. Gusts should not be as strong over
Lake Michigan, due to more stable conditions over the relatively
cool waters. Thus, expecting gusts to remain below the 25 knot
threshold for a Small Craft Advisory.

Long Term...Thursday Night Through Tuesday

A 500mb trough will swing east of the area Thursday night which
will result in a comfortable northwest flow pattern through
Saturday morning. High temperatures on Friday should be close to
normal with highs in the 70s. It will be cool again Friday night
across the north with lows in the 40s in many spots. On Saturday,
the 500mb flow flattens with the next system moving into portions
of the area Saturday afternoon. The ECMWF was the slowest model
with the rain not arriving until Saturday evening while the other
models moved rain into portions of the area by late Saturday
afternoon. The highest chances of rain will be across central
Wisconsin. The prospects for thunder Saturday afternoon seem small
as the best instability remains well to the south and west.

On Saturday night, the models are indicating a 40 knot 850mb jet
cranking up during the evening. Most unstable CAPE values were
highest across southwest Wisconsin with values of at least 500 to
1,500 J/KG. Elevated thunderstorms are expected to be on the
increase during the mid evening and overnight in response to the
low level jet. Some stronger storms are possible across central
Wisconsin as 0-6km shear values were from 35 to 40 knots. Probably
more of a concern will the potential for locally heavy rainfall,
especially if there is some training along the warm front. Sunday
continues to be an interesting day as models continue to show
lingering showers and storms through the morning, and possibly
into the afternoon. With the ongoing convection and cloud cover,
there is concern about the forecast high temperatures. We could
end up being several degrees cooler if the clouds don`t break up
in the afternoon. Also, the ECMWF has a more pronounced 500mb
trough moving across the area that was not seen in the other
models. This feature was probably a response to some convective
feedback.

For early next week, the ECMWF has the 500mb ridge building across
the east coast with the area being in the ring of fire which means
there will be periodic chances of thunderstorms through the middle
of next week. The details and timing of the these storms this far
out is murky due to the model differences. Any of these complexes
of storms could bring the potential for stronger storms and locally
heavy rainfall. Unseasonable warm to maybe hot conditions are
likely early next week with the heat index values at times
climbing into the 90s.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 631 AM CDT Wed Jun 12 2024

Patchy fog was already mixing out at issuance time, so will not
include any mention with the new TAFs.

A couple areas of precipitation should impact the region today
into tonight. An initial batch of showers and storms over MN
will shift into the forecast area late this morning and impact the
western TAF sites into mid-afternoon. Additional showers and
storms are expected to develop ahead of a cold front later this
afternoon into tonight. Have attempted to frame out the best
4-hour period for thunderstorms during the evening and early
overnight hours at each TAF site. Once the storms have ended,
there may be some potential for a brief period of MVFR ceilings
over mainly NC WI late tonight.

LLWS is expected to form later this afternoon into tonight, as a
35-50 knot low-level jet develops.

&&

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

&&

$$

DISCUSSION.....Kieckbusch/Eckberg
AVIATION.......Kieckbusch