156
FXUS63 KGRB 120914
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
414 AM CDT Sun May 12 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- A cold front will bring the potential for strong to isolated
  severe thunderstorms this afternoon and evening. The strongest
  storms could produce damaging winds and large hail. The most
  favorable time for the strongest storms is from 3 pm to 9 pm.

- Conditions hazardous to small craft are expected on Lake
  Michigan this afternoon and evening due to gusty south winds
  and high waves. Boaters on the Bay are urged to exercise
  caution.

- Areas of frost are possible over far northern Wisconsin north of
  an Arbor Vitae to Niagara line on Monday and Tuesday nights.

- After a dry period from Monday through Wednesday, the chance of
  rain will return from Thursday through Saturday.

&&

.DISCUSSION...
Issued at 414 AM CDT Sun May 12 2024

Short Term...Today...Tonight...and Monday

A 35 to 40 kt low-level jet was producing strong WAA over the U.P.
and far northern WI early this morning, leading to increasing
mid-level cloud cover and some sprinkles or light showers.
Temperatures were fairly mild, in the middle 40s to lower 50s at
most locations.

A warm front will lift northeast through the forecast area this
morning, with WAA sprinkles/showers exiting far northeast WI by
mid-morning. After a brief dry period in the late morning,
thunderstorms are expected to develop along a cold front as it
drops into northern WI during the afternoon. Meso-models suggest
that the threat of stronger storms will commence around 20z-21z
over the far north, with the greatest concentration of storms and
best chance of severe occurring in far NE WI, where convergence
and shear will be strongest. Instability will be somewhat limited
due to dew points in the upper 40s and lower 50s, but SBCAPE
should exceed 1000 j/kg in some areas. Deep layer shear of 35 to
40 kts will be sufficient for storm organization, with some of the
CAMs suggesting small bowing structures. Damaging winds are the
main concern, as evidenced by impressive inverted-v soundings, and
DCAPE values around 900 j/kg. Marginally severe hail will also be
possible, with favorable wet-bulb zero heights of 7500-8000 feet.
Ahead of the cold front, southwest winds gusting to 25 to 30 mph
will pump warmer air into the region, with highs rising into the
upper 70s to middle 80s over most of the CWA. The exception will
be near Lake Michigan, where south winds will keep much of Door
County and other coastal locations in the 60s to lower 70s.

The threat of strong and isolated severe storms will gradually
shift south during the early to mid-evening, with a decrease in
intensity occurring as instability wanes after sunset. Showers and
lingering storms should exit the southeast part of the forecast
area shortly after midnight, with clearing expected farther north.
Low temperatures will range from the upper 30s to lower 40s
northwest to the lower to middle 50s south.

The cold front will stall out south of the forecast area on
Monday, but the RRQ of a jet streak and some short-wave energy
will bring a chance of showers back into parts of central and
east central WI during the late morning and afternoon. High
temperatures should be in the middle 60s to lower 70s inland,
but E-NE winds off Lake Michigan will keep highs in the 55 to
60 range along the lakeshore.

Long Term...Monday Night Through Saturday

Relatively low impact weather is expected this work week.  Focus in
this part of the forecast revolves around timing rain chances from
Thursday into next weekend.

Precipitation chances...Rain chances will return late Wednesday
night into Thursday following a dry period from Monday night through
Wednesday.  Weak warm advection will be occurring ahead of shortwave
energy moving across the northern Plains and northern Mississippi
Valley, but instability looks meager at best so the chance for
thunderstorms appears low.

With shortwave energy slowly meandering east, precipitation will
remain in the forecast from Thursday night into Saturday.  Both the
Canadian and ECMWF indicate the potential for heavier precip on
Friday, but given the low amplitude nature of the energy, confidence
is relatively low with details like this.

Temperatures...Will start out below normal on Tuesday than steadily
warm into next weekend.  Forecast low temps are cold enough for
frost development over far northern WI on Monday and Tuesday nights.
With potential for rainfall on Thursday through Saturday, temps
could certainly be cooler than forecast.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1053 PM CDT Sat May 11 2024

VFR flying conditions are expected through the TAF period.
Showers and thunderstorms developing late Sunday afternoon into
early Sunday evening (22z-03z) could result in brief MVFR
visibility and gusty winds, but current indications suggest
coverage of thunderstorms will be scattered so have used VCTS to
show the potential. Included a TEMPO for RHI where confidence in
organized thunderstorms is highest.

In the meantime, a smattering of mid clouds and perhaps a
sprinkle will impact north-central Wisconsin the rest of the night
with this shifting to far northeast WI by daybreak before exiting.
Some uncertainty how far south the showers will extend out of Upper
Michigan, so continue to use VCSH, but only at RHI.

Light and variable winds will become out of the south overnight
and remain light. Southwest winds will increase Sunday morning,
gusting to around 25 mph by late morning. A period of LLWS is
expected across parts of central and north-central WI late tonight
as a low-level jet increases to 35 to 40 kts. Have included LLWS
for RHI, AUW and CWA where it is most likely, but held off at
GRB/ATW/MTW as it looks more marginal.

&&

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

&&

$$

DISCUSSION.....Kieckbusch/MPC
AVIATION.......JLA