260
FXUS63 KGRB 201145
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
645 AM CDT Mon May 20 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Showers and thunderstorms are likely today. Isolated strong or
  severe storms with gusty winds and hail are possible south of
  Highway 10 this afternoon.

- Confidence is increasing in potential for impactful severe
  weather and localized flooding across much of the forecast area
  Tuesday afternoon and evening. Damaging winds, large hail, and a
  few tornadoes will be possible with stronger storms.

&&

.DISCUSSION...
Issued at 414 AM CDT Mon May 20 2024

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

An upper trough moving from Iowa to northern Lake Michigan will
produce showers and elevated thunderstorms today. The associated
surface low should track south of the forecast area, and winds
will have a component from over the cool waters of Lake Michigan.
This should keep surface based convection mainly to our south. A
few models have the warm front getting into the far southern part
of the forecast area, and if so there could be some strong or
marginally severe storms with hail and strong winds in the
afternoon. Highs today will be close to normal.

The showers should exit the lakeshore area in the early evening,
with dry weather overnight. Wet ground and dewpoints in the low
50s may make for ground fog after midnight.

A strong, negatively tilted 500mb trough will approach the area
Tuesday. At the surface, an intensifying surface low will move
from northern Kansas to northwest Wisconsin. The surface warm
front is expected to stay to our south until early evening. This
will likely prevent surface based convection from initiating, but
there will be showers and elevated convection north of the front.
Precipitable water values are at all time records for this time of
the year, so very heavy rain and local flooding is possible,
especially in the afternoon and evening. There is also some
potential for large hail with the stronger storms in the
afternoon. The greatest threat of severe storms looks to be
Tuesday evening as the warm front moves north into our area.

Long Term...Tuesday Night Through Sunday

Focus for the long-term remains on severe weather and heavy
rainfall concerns Tuesday evening with the passage of a robust
system. Confidence is increasing in potential for impactful severe
weather and localized flooding across much of the forecast area
as more models come into agreement about the anomalous nature of
the driving surface low. The SPC currently highlights central
Wisconsin in a Day 2 enhanced risk for severe weather.

Tuesday evening storm chances... Dynamics with this system
continue to look favorable for a severe weather setup Tuesday
afternoon and evening as a negatively tilted shortwave swings to
our northwest. During this time, the attendant surface low is
expected to deepen to ~983 to 990 mb (once every ten year or more
climatological return interval) as it ejects up from the central
Plains. Timing of this system paired with the track of the warm
front will be critical in optimizing instability. An earlier
arrival during peak heating Tuesday afternoon may be enough to
maximize CAPE values along/north of the warm sector, allowing for
severe storm development as far north as the Highway 29 corridor.
This solution would potentially spell out a broken cap scenario
across portions of east-central Wisconsin Tuesday night where a
mean southeast wind at the surface advects a stable lake boundary
onshore. Otherwise, the assembly of other convective ingredients
may be enough to overcome limited surface-based instability. An
open Gulf will enable the transport of surface dewpoints in the
mid to upper 60s into central and east-central Wisconsin,
resulting in a broad swath of 1.5 to 1.8" PWATs. Additionally,
deep layer shear profiles (0 to 6 km shear of 50+ knots) and low-
level helicity (200 to 400 0-1 km SRH, 400 to 500 0-3 km SRH) may
support some discrete, rotating cells Tuesday night. Strong
rotating storms may be capable of producing tornadoes, which bears
emphasis. Sufficient lapse rates (~8 deg C/km) and a 50 to 60
knot low-level jet would also indicate large hail and damaging
winds as potential hazards. Suspect that the damaging wind threat
will decrease to the east given a lake-induced inversion and,
hence, lower likelihood of stronger winds aloft mixing down to the
surface. Heavy rainfall and localized flooding will continue to
be a concern, although highest QPF will likely remain off to our
west where the deformation zone is. We remain under a marginal to
slight risk for receiving excessive rainfall.

Rest of the extended... The remainder of the extended sees the
current pattern quieting down slightly as mid-level flow begins to
flatten out toward the end of the week. Next chances for precip
look to arrive Friday into Saturday, although models are
struggling with timing and amounts this far out.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 645 AM CDT Mon May 20 2024

An upper level disturbance will bring showers and MVFR conditions
in most areas today. Thunderstorms are possible, mainly south of
a AUW to GRB line. The activity will end from west to east during
the late afternoon and early evening hours.

IFR conditions are expected in most area overnight, due to wet ground
and moist low level air. Surface winds will be light.

A strong storm system moving from Iowa to northwest Wisconsin will
bring showers and thunderstorms to the area Tuesday and Tuesday
night. The activity will arrive from southwest to northeast. IFR
conditions are expected once the precipitation arrives. There is a
possibility of severe storms during the evening hours south of a
AUW to GRB line.

&&

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

&&

$$

DISCUSSION.....RDM/Goodin
AVIATION.......RDM