766
FXUS63 KGRB 031748
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1248 PM CDT Fri May 3 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms are possible
  Saturday into Saturday evening with more showers and
  thunderstorms Monday night through Wednesday.

- Slightly elevated fire weather conditions, mainly across
  northern WI, this afternoon and again Sunday and Monday
  afternoons due to RHs dropping into the 20-35 percent range.

- A few rivers will remain at or reach bankfull stage into the
  weekend.

&&

.DISCUSSION...
Issued at 352 AM CDT Fri May 3 2024

Short Term...Today...Tonight...and Saturday

Main forecast issues will be dealing with lingering low clouds/fog
early today, relative humidities in the 20/30s this afternoon and
any fire weather issues, and shower/thunder chances Saturday
afternoon.

Today...rain showers associated with a shortwave trough and weak
low pressure/front will exit east of the area early this morning,
with only some patchy fog/drizzle/sprinkles left around sunrise
across mainly northern/eastern WI. As drier air advects in from
the west, look for any light precip to end/exit, with the fog and
low clouds mixing out through the morning hours. Plenty of sunshine
is expected in the late morning and afternoon as high pressure
builds across the western Great Lakes. Mixing to at least
4000-5000 ft will support warmer temperatures, with highs climbing
into the upper 60s to middle 70s, and west winds gusting to 20 to
30 mph (strongest in northern WI) and low relative humidity
(20-35 percent) in the sandy soil areas. If we can mix closer to
7000 ft, as the RAP suggests, wind gusts would be a little
higher and RH`s a little lower. The recent rainfall, along with
the continued green-up, will limit the overall fire weather
concern. However, locations across northern WI may see slightly
elevated fire weather conditions as green-up has been the slowest,
the deepest mixing is expected and winds will be the highest this
afternoon.

Tonight...dry conditions continue as high pressure slides east of
the region. Mostly clear skies are expected during the evening,
with mid and high clouds spreading east across the area overnight.
Lows look to mainly be in the 40s with a light south/southeast
wind.

Saturday...a dry start to the day is expected, but skies look to
become cloudy through the morning. An area of rain showers will
spread west to east across the area during the morning, as another
shortwave trough and frontal boundary approach from the west.
This first area looks to be weakening as it traverses the area,
but additional showers look to develop later in the morning and
afternoon ahead of the front. Rain amounts look to remain under a
quarter inch for most spots. As for thunder chances, assuming the
front pushes through in the afternoon, should get at least 500
J/kg of CAPE across eastern WI by early-mid afternoon. This would
be enough to support a few storms, but severe weather is not
expected. Will carry a slight chance for thunder in the afternoon
across parts of central and eastern WI. Light south/southeast
winds will increase in the morning ahead of the front, then shift
to the west/northwest behind the front in the afternoon. Winds
could get a little gusty, up to around 35 mph ahead of the front,
but will be difficult to break into the inversion to mix down the
stronger winds. Will hold gusts closer to 20-25 mph for now. As
CAA arrives behind the front, better mixing will promote gusts to
~25 mph.

Long Term...Saturday Night Through Thursday

 Main focus of the extended period revolves around a well
organized cyclone that is forecast to bring showers and
thunderstorms to the region early to middle of next week.

Scattered showers and an isolated thunderstorm or two may linger
Saturday night into early Sunday morning across far eastern WI as
an upper-level trough and surface cold front depart from the
region. A ridge of high pressure is forecast to build over area
Sunday returning dry and calm conditions to the region through at
least Monday afternoon. The ridge will also ushering in a drier
air mass which may lower RHs to around 30 percent Sunday and
Monday afternoons. These lower RHs and warmer temperatures in the
low 60s to around 70 degrees may bring slightly elevated fire
weather conditions to far northern WI, where vegetation has yet to
green up, Sunday and Monday afternoons.

While there are still slight differences in timing and intensity,
ensemble models are in decent agreement that a near vertically
stacked low pressure system will eject out of the Great Basin
toward the norther Plains late Monday into Tuesday. An initial
wave of warm air advection out in front of the low along with
diffluent flow aloft may bring a round scatter showers and storms
Monday night into Tuesday. Perhaps the best chance for strong to
potential severe storms comes Tuesday afternoon and evening as
much of the CWA may reside in the warm sector allowing for an
increase in instability, however, the question remains if there
will be a forcing mechanism present for thunderstorm initiation.

Periods of rain and thunderstorms will remain possible through
Wednesday and Thursday as the cyclone brings several pieces of
jet energy across the region. With several area rivers and
streams already at or near bankfull the active pattern for next
week may create some minor flooding concerns particularly in low
laying and flood prone areas.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1235 PM CDT Fri May 3 2024

Expect VFR conditions to prevail through the TAF period, with
patchy mid/high clouds expected into early Saturday, then
thickening and lowering clouds in NC/C WI as showers arrive toward
midday. Considered the possibility of patchy fog tonight, but the
air mass has become well mixed this afternoon (warm temps and low
dew points leading to large dew point depressions) and boundary
layer winds will be increasing to 20 to 25 kts late tonight.
Models are hinting at some low clouds coming off Lake Michigan
(near MTW) Saturday morning, but confidence is too low to mention
a ceiling at this time.

Gusty west winds will drop off quickly late in the day, become
light SE overnight, then moderate SE-S later Saturday morning.

&&

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

&&

$$

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