642
FXUS63 KGRB 121152
AFDGRB

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

Updated aviation portion for 12Z TAF 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 12Z TAF Issuance
Issued at 640 AM CDT Sun May 12 2024

VFR flying conditions are expected through mid-afternoon.
Showers and thunderstorms will develop along a cold front over
northern WI late this afternoon, then drop south during the
evening. The storms could be strong to severe, with strong gusty
winds and hail. MVFR vsbys may accompany the heavier showers. Have
attempted to frame out the most likely three-hour period for
storms at each TAF site. The showers and storms will shift south
of the region shortly after midnight, with clearing occurring to
the north.

Models are suggesting some smoke or haze from Canadian wildfires
may arrive in the wake of the cold front tonight, but not sure if
this will have an impact on surface conditions, so will leave it
out of the TAFs for now.

Southwest winds will increase later this morning, with gusts to
around 25 kts. A brief period of LLWS is expected across NC WI
early this morning, with another round of LLWS possible in C/EC
WI tonight.

&&

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

&&

$$

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