106
FXUS63 KGRB 171151
AFDGRB

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

Updated aviation portion for 12Z TAF 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 12Z TAF Issuance
Issued at 644 AM CDT Mon Jun 17 2024

A cold front has pushed into the northwest part of the forecast
area, and MVFR ceilings have formed in the shallow cool air behind
it. A line of thunderstorms was approaching from northwest WI and
eastern MN, and is expected to impact the western TAF sites
between 14z and 17z. The tail end of these storms may brush
through GRB around midday, but locations farther south are
expected to remain mainly dry. Additional showers and storms
should impact mainly northern WI later this afternoon and evening
as the front lifts back north as a warm front. Persistent low
clouds over northern WI should gradually improve tonight as the
warm front lifts north.

LLWS is expected to develop overnight tonight.

&&

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

&&

$$

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