547
FXUS63 KGRB 041731
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1231 PM CDT Sat May 4 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms are expected later
  this morning into this evening, with additional showers and
  thunderstorms Tuesday through Wednesday.

- Low clouds and fog, locally dense, possible across parts of
  central and north-central WI late tonight into early Sunday.

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

&&

.DISCUSSION...
Issued at 409 AM CDT Sat May 4 2024

Short Term...Today...Tonight...and Sunday

Main forecast concerns will be pinning down the timing of rain
today into this evening, storm chances, fog/frost potential and
cloud/temp trends.

Today...a dry start to the day is expected with mid and high
clouds spreading across the region. Clouds will thicken and lower
from west to east, as a shortwave, low pressure and cold front
approach. An area of showers associated with these features (and
possible weakening MCV?) will arrive as a LLJ aids in moisture
transport back into the area and we get into the RRQ of an upper
jet. Still some slight timing differences, but most of central and
north-central WI should see the rain by 18-19z, with the band
working east and weakening across the area through the afternoon.
Additional showers and a few storms look to develop across eastern
WI in the afternoon ahead of the front. Models have trended
higher with rain amounts, with some CAMs showing over 0.75" in
parts of central and northern WI. While the PWATs do climb to
between 1.00-1.25", think the high amounts are a little ambitious
as the band will be moving rather quickly. Ensembles/NBM, along
with WPC, more in the 0.10-0.40" range which seems more reasonable
for most spots. Instability will be tough to come by due to
clouds, but a narrow ribbon of instability (SBCAPE 200-500 J/kg)is
still forecast to develop/advect into eastern WI this afternoon.
This will keep the chance for a few non-severe storms going
through around sunset. If storms can form during peak heating,
lapse rates do steepen for a time, so a strong storm would be
possible with some small hail. Not too much wind to bring down
from aloft, but a weak inverted-V profile could aid in a brief
gusty winds.

Highs will be coolest where the clouds/rain arrive first, with
upper 50s and lower 60s expected across parts of central and north
central WI. Further east, highs in the upper 60s to low 70s are
expected. Cooler readings are expected near Lake MI as onshore
winds are expected.

Models are again hinting at some clouds/fog across Lake Michigan
as dewpoints climb ahead of the front. Little to no fog was
observed yesterday (with only a little over the far southern part
of the lake), and with similar moisture and water temps, would
think fog chances are low. But low clouds are more likely and will
add some extra clouds over eastern WI, but will hold off on any
fog mention for now. Will need to monitor the area of low
clouds/fog over MI and see how it behaves as it works westward.

Tonight...the rain showers and isolated storms will exit east of
the area by midnight. Recent rains, decreasing winds, and at least
partial clearing skies should allow for fog development, mainly
across central and north-central WI. While the the HREF is
currently showing a 20-40% chance for visibilities under a mile,
it is similar to what it showed yesterday when fairly widespread
dense fog developed just to our west. Have added fog for much of
central and north-central WI after midnight. Lows look to drop
into the upper 30s and 40s for most spots. But if clouds clear a
little quicker across parts of central WI, we could see temps
quickly drop into the mid 30s. Frost/freeze headlines have begun
from Marathon Co. southward, so a frost advisory may be needed if
temps trend a little colder.

Sunday...dry conditions will prevail as high pressure builds into
the western Great Lakes. Lingering low clouds and some fog is
expected in the morning, especially across central and north-
central WI. Some daytime cu are expected in the later morning and
afternoon due to lingering low level moisture. Winds look to be
just light enough for a lake breeze to develop across far eastern
WI in the afternoon. 850mb temps drop back to around 3-6C, which
will support highs mainly in the 60s to near 70. RHs look to drop
into the 30s in the sandy soil regions, but with the recent wet
weather, continued green-p and lack of gusty winds, the fire
weather threat will be minimal.

Long Term...Sunday Night Through Friday

Dry start to the extended period Sunday night through most of
Monday with a ridge of high pressure over the region. Deep mixing
to around 4000ft Monday afternoon could lead to slightly elevated
fire weather conditions with RHs falling to around 30 percent and
southeast winds gusting to 20-25 mph. Fire weather concerns will
be greatest across the sandy soil regions of northern and central
WI where green up has not fully occurred yet.

The remainder of next week will be dominated by an initially well
organized cyclone moving into the northern Plains Tuesday. Rain
may begin to push into the area as early as Monday night as a
warm air advection regime sets up bringing in a surge of deeper
moisture with PWAT values around 1.2-1.3". Embedded thunderstorms
may develop Tuesday afternoon and evening as instability
increases, however, morning rain and cloud cover along with
easterly winds off the lake and bay could limit the intensity of
any storms that do develop. Confidence in any storms becoming
severe is low at this time, however, there is still ample time for
the forecast to change. In terms of rainfall amounts the long
term grand ensemble (GEFS, EPS, and GEPS) shows a 30-60% for over
0.5" of QPF Monday night to Tuesday night.

Periods of showers and storms are expected Wednesday through
Friday as the aforementioned cyclone slowly shifts east and
weakens. The disorganized nature of this weakening system makes
specific forecast details difficult to pin down. For now expect
lots of clouds and periods of rain along with a few storms through
the second half of next week. With the recent and forecast
rainfall many area rivers and streams will likely continue near
or at bank full through next week.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1226 PM CDT Sat May 4 2024

A band of showers and thunderstorms was moving across central into
northeast Wisconsin early this afternoon. Some of the stronger
storms could produce wind gusts up to 30 knots and pea size hail.
Meanwhile, dense fog was moving northward along the Lake Michigan
coastline and will impact KMTW and all the airport sites in Door
County due to low visibilities or low CIGS. Another cluster of
stronger storms are possible after 21z across east-central
Wisconsin, mainly south of Appleton. Some of these storms could
produce wind gusts to around 40 knots and hail up to one inch
in diameter. MVFR CIGS are expected for a period of time behind
the cold front followed by improving CIGS during the evening.
Some patchy fog is possible late tonight across central and
north-central Wisconsin where skies clear the longer and winds
become fairly light towards sunrise on Sunday.

&&

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

&&

$$

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