817
FXUS63 KGRB 061632
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1132 AM CDT Mon May 6 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Relative humidity values are expected to drop into the middle
  20s and 30s this afternoon. Recent rain, lack of gusty winds
  and continued green-up will limit fire weather concerns.

- Next round of showers and storms arrives Tuesday morning. Best
  chance for thunderstorms will be Tuesday afternoon and evening.
  There is a marginal risk for severe storms south and west of an
  Antigo to Kewaunee line.

- Showers will again be possible Wednesday afternoon through
  Thursday with a few storms possible Wednesday afternoon and
  evening, mainly in central WI.

&&

.DISCUSSION...
Issued at 419 AM CDT Mon May 6 2024

Short Term...Today...Tonight...and Tuesday

Quiet early May weather continues today, then active weather
arrives on Tuesday. Main forecast concerns will be low relative
humidities today and rain amounts / severe weather risk on
Tuesday.

Rain Amounts / Thunder / Severe Threat / Clouds:

Any patchy ground fog over north/northeast WI early this morning
will burn off quickly after sunrise, with sunny skies prevailing
to start the day as high pressure slides into the eastern Great
Lakes. Forecast soundings showing less moisture in the boundary
layer today, but could just be enough to get some fair weather
cumulus clouds to develop, especially north/west of the Fox
Valley. High clouds will increase from the west in the afternoon
and evening.

Attention turns to the deep low pressure that will sit and spin
across western SD/ND today into Tuesday. Large band of rain will
spread northeast across the area on Tuesday, arriving after 12z in
central WI, as an occluded front approaches, and strong FGEN and
PWATs of 1.0-1.25" sweep across the state. The band will be moving
pretty quickly, with most spots only seeing the rain for 3-5
hours before it shuts off. This band will be encountering drier
air as it works in, so some weakening is expected as well. Models
vary from 0.10" to over an inch of rain with this first wave, with
HREF mean between 0.30-0.70" which seems reasonable, but leaning
toward the lower amounts for most locations. Locally higher
amounts would be associated with any storms in the afternoon.

Speaking of the storm threat, a few embedded non-severe storms
will be possible with the first band of rain, but MUCAPEs look to
lag the best forcing. Chances for thunder look to be under 15% in
the morning so will not include. The threat for strong to
possibly severe storms is increasing later in the day, especially
across central and east-central WI. Still some questions regarding
just how much clearing/heating will occur, which will determine
how unstable we can get. But models continue to show MLCAPE
increasing to 500-1000 J/kg, possibly as much as 1500 J/kg with
shear up to 30-35 kts (at least early in the afternoon, then
decreases late in the day). In addition, mid-level lapse rates
approach 7.5-8.5 C/km and a shortwave will all combine to produce
another round of showers and storms. If the instability is
realized, low-topped supercells would be expected, aided by 0-3km
CAPE climbing to 300 J/kg. Damaging winds, hail (aided by wet-
bulb zero heights under 8000 ft) and an isolated tornado would be
possible. SPC has introduced a marginal risk of severe weather
from Antigo to Kewaunee southward, which seems reasonable with
expected ingredients but still some uncertainty. If it becomes
clearer that the better instability will be realized and work far
enough north into our area, the severe threat will increase.

Temps / Relative Humidity / Winds:

925/850mb temps support highs in the upper 60s to low 70s across
most of the area today, with cooler readings near Lake Michigan as
an onshore flow will prevail. Southeast winds will increase later
this morning and afternoon with gusts of 15 to 20 mph expected at
most spots. Relative humidities this afternoon will drop into 20s
to mid 30s across most locations away from Lake Michigan, lowest
in the sandy soil areas. Recent rain and lack of gusty winds will
limit the overall fire weather concern today. However, slightly
elevated fire weather conditions are possible this afternoon as
soils dry out, especially over northern WI where the green-up has
been the slowest.

Lows tonight will stay in the 40s to around 50 for most of the
area. Winds aloft will really ramp up overnight (to nearly 40 kts
at 2000 ft), but an inversion should limit gusts at the surface.

Highs on Tuesday will be tricky with the wave of rain/clouds
holding temps down over the north, but some brief clearing behind
the band and south/southwest winds allowing for a push of warmer
air in the afternoon. Still some question just how far north this
push will be and how warm it will get. Looking like most spots
will likely stay in the 60s, but some 70s will be possible over
the south. As mixing increases Tuesday morning, southeast winds
will get gusty, up to ~35 mph.

Long Term...Tuesday Night Through Sunday

There is a chance for showers and a few storms nearly everyday
during the extended period. However, the potential for any severe
weather or a washout is low at this time.

Tuesday night through Thursday...Showers and a few storms may
linger through Tuesday night into Wednesday morning, mainly
across northern WI where the deeper moister remains thanks in
part to northerly winds off Lake Superior. Expect much of the
region to turn dry Wednesday morning as drier air filters over
the area behind an occluded front. The dry conditions may not
last long as the next round of showers and storms is progged to
arrive Wednesday afternoon as a weakening cyclone slowly moving
east out of the northern Plains merges with a surface low pushing
northeast out of the central Plains. As these systems merge
models are picking up on a band of heavier rainfall setting up
across central and southern WI. Best chance for stronger storms
would be across central WI where a swath of 100-200 J/kg MUCAPE
is expected to develop and mid-level lapse rates approach 7-
7.5 C/km. The exact location of the strongest storms will be tied
to how far north clouds clear out Wednesday morning. Chance for
storms developing over east-central WI looks rather low at this
time given easterly winds off Lake Michigan creating a more
stable boundary layer. Showers may linger across central and east-
central WI through Thursday, however, winds turning northeasterly
brings cooler drier air to the region greatly limiting any
convective potential. Additionally, with the winds shifting around
to the northeast Thursday it will be noticeable cooler with highs
in the mid 50s to near 60 degrees compared to the low 60s to mid
70s expected Wednesday.

Rest of the extended....Ensembles are trending toward a mostly
dry Friday as a brief ridge of high pressure develops over the
region. Depending on how much rain falls Thursday and how quickly
clouds clear out Thursday night there is the potential for fog or
frost to develop across northern WI with temperatures forecast to
fall to or near freezing Friday morning. Temperatures should
recover back into the 60s by Friday afternoon. Models have come
into a broad consensus with a quick moving short-wave passing
over the region next Saturday that may bring another round of
showers and a thunderstorms.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1119 AM CDT Mon May 6 2024

VFR conditions will prevail through 12z Tuesday. Wind shear
will develop between 04-07z Tuesday and continue through
Tuesday morning, although wind shear should diminish across
central Wisconsin around 16z. A round of rain will move
into central Wisconsin between 12z and 15z, and across north-
central and northeast Wisconsin between 15z and 18z. CIGS
will fall into the MVFR category and then into the IFR category
across the north late Tuesday morning through Tuesday afternoon.
Scattered thunderstorms are possible Tuesday afternoon that
could produce some gusty winds and small hail.

&&

.MARINE...
Issued at 419 AM CDT Mon May 6 2024

Southeast winds will increase Tuesday morning, with some gusts to
25 kts possible. Cooler waters will create more stable conditions
compared to land, so still some question just how much of the wind
aloft will mix down. The persist southeast winds will produce
building waves, especially south of Algoma on Lake Michigan,
where waves will climb to around 4 feet Tuesday morning. Have
held off on a Small Craft Advisory due to the uncertainty and
better chances staying just to our south. As dewpoints increase
Tuesday afternoon, dense fog will be possible, which could
linger into at least Tuesday evening.

Gusty and persistent northeast winds will bring a period of
hazardous conditions for small craft Wednesday night into
Thursday.

&&

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

&&

$$

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