581
FXUS63 KGRB 170923
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
423 AM CDT Mon Jun 17 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- There is a Marginal to Slight risk of severe storms across the
  region today. The best chance of severe storms will be across
  parts of central and north central WI this morning, and over
  northern WI later this afternoon and evening.

- There is a Slight Risk of excessive rainfall today, mainly west
  and north of the Fox Valley. A complex of storms with heavy
  rainfall will move through central, north central and far
  northeast WI this morning, and additional thunderstorms with
  heavy rainfall may occur over northern WI later this afternoon
  and evening. There is a continued risk of excessive rainfall
  from Wednesday through Friday.

- Well-above average temperatures are expected through Tuesday.
  Heat indices may reach the lower 90s in the Fox Valley today,
  and middle to upper 90s across most of the forecast area on
  Tuesday.

- Periodic thunderstorm chances are possible through much of the
  week. Some strong storms and locally heavy rainfall may be
  possible at times.

&&

.DISCUSSION...
Issued at 422 AM CDT Mon Jun 17 2024

Short Term...Today...Tonight...and Tuesday

Conditions were quiet across GRB CWA early this morning, with
an unstable (MUCAPE 1500-2000 j/kg) but capped (CINH of -150 to
300 j/kg) air mass in place. However, upstream convection has
rapidly expanded and become more organized over central and
southern MN and NW IA, due to strong WAA/moisture convergence on
the nose of 40 kt low-level jet, possible MCV and in the presence
of a cold front. This activity is expected to move into GRB CWA
this morning, and will pose a heavy rainfall and at least
marginal severe risk.

Upstream convection will move into the forecast area this morning,
most likely between 13z-14z. Most models suggest the MCS will move
across the northwest two-thirds of the CWA; closer to the
position of the incoming cold front, and the current forecast
follows this scenario. Will watch trends closely early this
morning and adjust farther south, if necessary. Given with fairly
strong CAPE, steep mid-level lapse rates of 7.5 to 8 C/km and
deep layer of 25 to 35 kts over the NW part of the forecast area,
suspect at least a marginal risk of severe storms will exist as
the line of storms arrives. In addition, we will likely pick up a
quick half inch to an inch of rain, which could lead to minor
urban flooding. This convection should move out of far NE WI
around midday. The cold front is expected to be bisecting the CWA
from central into NE WI this afternoon, with increasing southerly
flow resulting in overrunning. This should result in a continued
threat of thunderstorms, especially over N WI during the late
afternoon and evening. The severe threat is uncertain due to the
stabilizing effects of the morning convection, but locally strong
storms and pockets of heavy rainfall will be possible. The front
will lift back north as a warm front and move out of the forecast
area overnight, leading to a period of dry weather that will
persist through Tuesday. Partly cloudy skies and south winds
gusting to 30 mph will reinforce the hot air mass over the region,
with heat indices soaring into the middle to upper 90s Tuesday
afternoon.

Highs today should range from the lower to middle 70s far
northwest to the middle to upper 80s southeast. Lows tonight
should be in the middle 60s north to the lower 70s south. Highs on
Tuesday will be mainly in the upper 80s and lower 90s.

Long Term...Tuesday Night Through Sunday

500mb ridge across the eastern United States will flatten by the
end of the work week, resulting in an active westerly flow
pattern into next weekend.

For Tuesday night, thunderstorms are expected mainly along the
weak cold front approaching the area from the west. Some
stronger/severe storms are possible across north-central during
the evening with CAPE values around 2,500 J/KG. The storms are
expected to gradually weaken during the late evening and
overnight. On Wednesday, the GFS model is quicker with the surface
frontal passage compared to the NAM/ECMWF/Canadian models. NAM
Bufkit soundings on Wednesday indicated around 1,000 J/KG of CAPE
and 0-6km shear values around 30 knots which would support strong
storms and possibly a few isolated severe storms into Wednesday
evening. The greatest risk of severe weather would be from central
into northeast Wisconsin. On Wednesday night, the surface front
sinks southward into southern Wisconsin, resulting in a dry period
across the far north. The front is expected meander across the
region into next weekend with a few rounds of showers and storms
on the north side of the boundary.

A more potent system is expected to send the front northward as a
warm front Friday night into Saturday. Showers and storms are
expected along the warm front with a few strong or severe storms
possible. A cold front is expected to move across the area
Saturday night. No significant changes were made to the Sunday
period. High temperatures during the period will run at or above
normal.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1112 PM CDT Sun Jun 16 2024

VFR conditions will continue late this evening. Models showing
some lower ceilings (MVFR and possibly IFR for a time) developing
across central and northern WI overnight into Monday morning and
possibly again Monday afternoon/evening. VFR conditions look to
prevail for most/all of the TAF period at GRB/ATW/MTW. We continue
to monitor upstream for shower/storm develop as a cold front
approaches. Storms continue to have a hard time firing as a cap is
in place holding things in check, along with weaker lapse rates.
With instability decreasing, chances for any widespread
shower/storm activity is fading. While a spotty storm may still
occur, will remove thunder as chances are under 20%.

Additional rounds of showers and storms are expected on Monday as
instability will be in place. One round looks to be in the morning
with whatever develops across MN/IA and tracks northeast into the
region. Another is possible later in the day. There are still plenty
of unanswered questions as to when and where the storms will
actually form as details regarding where the boundaries will end
up remains illusive. Have added some PROB30s to get the ball
rolling on targeting the best chances for showers/storms.

Surface winds will remain under 10 kts for most of the night, but
winds aloft could allow for a few gusts to 20 kts, mainly over
eastern WI. LLWS is expected, mainly through 10-12z, as winds at
2000 ft will remain near 35 kts. Winds could be a little erratic
on Monday due to overnight boundaries/storms and questions on
where the cold front will stall, but should eventually become
south/southeast. An additional period of LLWS is expected Monday
evening/night.

&&

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

&&

$$

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