497
FXUS63 KGRB 200914
AFDGRB

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

Forecast discussion for routine morning forecast 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 06Z TAF Issuance
Issued at 1053 PM CDT Sun May 19 2024

VFR conditions will continue late this evening and most of
tonight with mainly mid and high clouds spreading across the area.
A few sprinkles or light showers are are possible, but will have
no impacts to flying conditions.

An area of showers will arrive from southwest to northeast after
08z ahead of an approaching low pressure system and frontal
boundary, continuing at times into Monday morning. Weak
instability could allow for a few embedded thunderstorms. A break
is expected for much of the late morning and afternoon, with
additional showers and storms developing later in the day as the
warm front lifts across the area, especially across eastern WI,
with more spotty activity further west. Still some uncertainty
just how long the break will be and where the second round of
activity will be focused. VFR conditions look to last through at
least 11z then MVFR and possibly some IFR conditions are expected
at times on Monday, associated with the heavier showers and
stronger storms. The strongest storms could produce gusty/erratic
winds, hail and heavy rain. Tried to limit the thunder/ceilings/
visibilities conditions as best as possible to 2-3 hour windows
where the threat looks to be greatest, but those may need to be
shifted/changed as models hone in on exact timing of each round of
showers/storms. Another round of showers and storms looks to
arrive late Monday night (after 06z).

Fog may develop on Lake Michigan Monday into Monday night as
dewpoints climb ahead of the low, but should stay mainly over the
lake, so will not include at MTW. In addition, some fog will be
possible across most land areas Monday night as winds die off and
recent rain will provide moist low levels, but winds just off the
surface and cloud cover may limit this threat.

&&

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

&&

$$

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