283
FXUS63 KGRB 112019
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
319 PM CDT Sat May 11 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

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

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

- Well above normal temps expected on Sunday, then dropping back
  closer to normal by Tuesday.

&&

.DISCUSSION...
Issued at 318 PM CDT Sat May 11 2024

Short Term...Tonight and Sunday

Main forecast concerns are dealing with diminishing showers this
afternoon, cloud shower/trends tonight and the storm/severe
threat and how warm it will get on Sunday.

For tonight...look for the clouds and any lingering light
showers/sprinkles across eastern WI to fade as cyclonic flow
loosens its grip on eastern WI, a weak shortwave exit and any
weak convergence along the bay/lake breeze weakens as the 150-200
J/kg SBCAPE decreases late this afternoon and evening. Some pea
sized hail will be possible in the stronger activity. Mid-clouds
will increase from west to east during the evening and overnight
hours as a LLJ and WAA push in, which looks to limit our view of
the Northern Lights, but some pockets of clearing are expected.
Some showers and sprinkles will be possible overnight as the
WAA/LLJ combine with a weak shortwave, with the best chances over
northern WI. Some very weak instability (MUCAPE under 300 J/kg)
arrives overnight across northern WI, so a few rumbles of thunder
are possible. Lows look to drop mainly into the 40s.

For Sunday...morning showers and possibly a stray storm will be
possible in the morning hours as the LLJ/WAA weaken and the warm
front pushes north. Focus will then turn to the cold front working
south into the area in the very late morning and afternoon.
Expect strong heating to occur ahead of the front with showers and
storms developing by mid-late morning over far northern WI then
spreading southeast through the afternoon hours. Still some
discrepancies with moisture/instability, as differences in how
much mixing will occur and how much moisture will advect in. GFS
appears to be too high with its dewpoints with low 60s in parts of
the forecast area. With the expected airmass and mixing through
the day, it will be tough to get mid-50 dewpoints and have leaned
toward the model consensus of upper 40s and low 50s. This would
allow SBCAPEs to climb to between 800-1300 J/kg, with up to 1000
DCAPE in the afternoon. Impressive lapse rates up to 10 C/km in
the low-levels and up to 8 C/km in the mid-levels, along with wet-
bulb zero heights under 10,000 ft, will aid in hail development.
Shear will be on the low side, but 0-6km bulk shear should be
around 30-35 kts. Winds aloft not that impressive, but sounding
showing the inverted-V, which will support bringing the stronger
winds down to the surface. So many severe weather parameters will
be there for at least pulse-like severe storms with wind gusts to
50-60 mph and large hail the main threats. SPC has added Marginal
Risk was added for parts of northeast WI to account for the severe
weather threat.

925mb climb to between 17-22C, supporting highs upper 70s to mid
80s. The earlier arrival of the front may hold down temps over the
far north. Cooler readings are also expected near the immediate
shore of Lake Michigan.

Long Term...Sunday Night Through Saturday

 Main period of focus for the extended forecast is Sunday night
as thunderstorms will remain possible along a southward sagging
cold front. Otherwise, mostly dry and quite conditions expected
until the end of the upcoming week.

Sunday night...Thunderstorms will likely be ongoing into Sunday
night along a cold front that is forecast to drop south across
the region. Models point to an environment capable of sustaining
a few isolated strong to severe thunderstorms through the evening
with MUCAPE around 750-1000 J/kg and surface to 6km bulk shear
around 30- 35kt. Instability will rapidly diminish after sunset
with the lose of incoming solar radiation. Expect the threat for
strong to severe storms to greatly diminish by around 9-10pm
Sunday. In terms of impacts DCAPE values of 900-1000 J/kg with
inverted V profiles suggest strong thunderstorm outflow wind
gusts will be the main hazard. Wet bulb zero heights around
8000 ft also point to the possibility of large hail with the
strongest updrafts.

Rest of extended precip chances...Confluent flow aloft may
support an area of showers from the Fox Valley to the lakeshore
Monday afternoon. A rumble or two of thunder may be hear south an
Oshkosh to Manitowoc line, but don`t expect much in the way of
impacts from these showers. High pressure then takes control of
the region keeping the middle of the week dry. Chance for showers
and storms return toward the end of the week as a short-wave
passing north of the region drags a cold front across Wisconsin.
There is still considerable differences in timing with this
system as the Canadian has the precipitation arriving Thursday
morning, while the GFS and ECMWF hold the precipitation back
until Thursday night. Regardless instability does not look too
impressive with this system currently which keeps the threat for
severe weather low at this time.

Temperaturs...High temperatures Monday will be a few degrees
cooler than Sunday but still in the mid 60s to mi 70s across the
region. Tuesday looks like the coolest day of the upcoming week
with highs forecast to reach the low to upper 60s, which is near
to or just slightly cooler than normal for mid May. Temperatures
then trend slightly warmer through the end of the week with highs
each day forecast to reach the mid 60s to mid 70s. Highs for
lakeshore areas will be about 5 to 10 degrees cooler each day.
With clearing skies and light winds will have to watch for low
temperatures dropping below freezing early Tuesday and Wednesday
mornings, mainly across far northern WI.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1232 PM CDT Sat May 11 2024

VFR flying conditions are expected through the TAF period. A
scattered to overcast deck of clouds will continue this
afternoon, slowly decreasing as sunset approaches. Then, a mid-
deck is expected to spread across most spots this evening into
Sunday morning. A few showers and sprinkles will continue this
afternoon across eastern WI, with additional activity possible
later this evening into Sunday morning, but should not impact
aviation. Will use the VCSH to cover the shower/sprinkle chance.

Northwest winds will gust to around 20 kts through the afternoon,
then subside by early evening. Winds will become light and
variable for a time this evening then turn to the south later
tonight 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 hold off at GRB/ATW/MTW as it looks more marginal.

&&

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

&&

$$

DISCUSSION.....Bersch/GK
AVIATION.......Bersch