811
FXUS63 KGRB 010859
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
359 AM CDT Wed May 1 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Gusty west winds from 30 to 40 mph are expected over central and
  eastern Wisconsin today. While recent rains will limit the fire
  weather potential, the gusty winds will bring a threat of
  isolated power line fires.

- Another round of showers and storms is forecast Thursday
  through Friday morning. Best chance for thunderstorms will be
  Thursday afternoon and evening with small hail and gusty winds
  expected to be the main impacts with any storms that do develop.

- Due to recent and upcoming rainfall, rivers and streams will be
  on the rise late this week. Some rivers are forecast to reach
  bankfull stage by Friday or Saturday.

&&

.DISCUSSION...
Issued at 359 AM CDT Wed May 1 2024

Short Term...Today...Tonight...and Thursday

Main forecast concerns will be pinning down best chances for
rain/thunder, how windy it will be today and temp trends as the
active weather pattern continues across the northern CONUS to
start off the month of May.

Precip / Thunder / Cloud Trends:

Low pressure will track east across Upper Michigan today, with the
majority of the heavier precip ending by 12z across far
northern/northeast WI as the best forcing exits. Thunder has been
isolated through the early morning hours, only associated with the
strongest activity. Any threat for thunder will end by sunrise as
any weak elevated instability exits into MI. Lingering clouds,
light showers, sprinkles and areas of drizzle is expected this
morning where the low clouds remain, mainly focused across
northern WI. Clouds are likely to continue for most/all of the day
over the north, with at least a period of clearing expected
further south.

The next round of rain will arrive from the southwest on Thursday
as low pressure organizes over Iowa and a push of WAA arrives.
Best chances for rain will be in the later morning and afternoon
hours. Thunder chances will increase in the afternoon but remain
on the low side (under 20%) as the elevated instability will
struggle to push into northeast WI.

Winds / Fire Wx Concerns:

Limited mixing and a weak inversion has keep winds in check
through the overnight hours. As the low pressure pulls away this
morning, a push of CAA along with more unidirectional wind will
aid in the strong winds aloft to be transported down to the
surface. The strongest winds and best mixing look to occur across
central and eastern WI, where gusts to 40 mph are expected.
NBM/HREF probabilities continue to highlight these areas with a
80%+ chance of 30mph gusts, but chances for 40mph drop to under
40% and are spotty. An inversion will limit just how far we can
mix into, which look to also hold gusts under advisory criteria.
The expected cloud cover over north-central WI will further limit
gust potential there.

No major changes in the thinking regarding fire weather as the
gusty winds could lead to isolated power line fires today. But
recent rainfall and winds generally staying under 40 mph will
limit the threat. Potential still looks to be the greatest across
central WI to far northeast WI where green up has yet to occur in
the sandy soil/forested areas. Will continue to highlight in the
fire weather products, but hold off on adding to the HWO.

Temps / Frost Potential:

Have trimmed back highs slightly today due to the clouds and CAA,
but most spots still look to climb into upper 50s and 60s, with
north-central WI likely holding in the low to mid 50s. If skies
can stay clear overnight, lows in the low-mid 30s are likely in
the Northwoods with areas of frost. If more clouds stick around,
even mid clouds, temps should hold in the upper 30s and limit
frost potential. Frost/freeze headlines have begun for north-
central WI, so have added at least some patchy frost in the
forecast for the north tonight. If cloud trends back off on
coverage, a frost advisory may be needed. High temps look to hold
mainly in the 50s on Thursday, with a 60 possible south of Hwy 29
possible.

Long Term...Thursday Night Through Tuesday

An unsettled pattern is expected to end the week through this
weekend with several rounds of precipitation forecast to move
through the region. The area is under a marginal to slight risk
(5- 15%) for excessive rainfall Thursday morning through Friday
morning. The cumulative affect of the recent and forecast rainfall
does raise minor flooding concerns. A few area rivers and streams
are already at or near bankfull and several others are showing
increasing water levels and may reach bankfull by the end of this
week.

As mentioned above the CWA is under a marginal to slight risk (5-
15%) for excessive rainfall Thursday morning through Friday
morning as a well organized cyclone is progged to move northeast
over the upper Mississippi Valley. Periods of moderate to heavy
rainfall are expected Friday morning as a right rear jet quad
provides additional upper-level support to the aforementioned
cyclone. A few embedded thunderstorms are also possible Friday
morning, but the combination of meager 100-200 J/kg MUCAPE, 10-
20kt effective shear and southeast winds off Lake Michigan
creating a stable boundary layer should keep any storms that do
develop weak. Precipitation will come to an end Friday as a much
drier air mass moves over the region behind a cold front. By the
time rain comes to an end most areas are looking at a 40-60%
chance for greater than 0.75" of QPF with up to 1" possible for
areas that see the heaviest showers.

The dry conditions are not expected to last long as the northern
half of WI remains under the influence of the cyclonic flow from
the main surface cyclone that will be slowly moving northward
over Manitoba and Ontario this weekend. There is still
uncertainly with exactly when the second wave of precipitation
will cross the region with ensemble cluster analysis showing a
broad window between late Saturday morning and early Sunday
morning. Don`t expect much in the way of convective active during
this time as the most recent 00Z models are showing little to no
instability and modest 5-6 C/km mid- level lapse rates. Available
moisture with this second wave of precipitation is substantially
less than the Thursday/Friday system. Most areas are looking at a
20-30% for greater than 0.25" of QPF this weekend.

Another brief dry period is expected Sunday afternoon through
Monday morning as a weak ridge of high pressure moves across the
western Great Lakes. Ensemble models are in decent agreement with
the next system to track across the upper Midwest Monday
afternoon through Tuesday. It is too far out to dig much into the
details with this system, however, the current general synoptic
pattern with a low center moving over the Dakotas into western
Minnesota would support the possibility for thunderstorms.
Details on the finer mesoscale environment will be refined as the
higher resolution models come into range.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1033 PM CDT Tue Apr 30 2024

Showers and isolated thunderstorms will move across the region
overnight as an upper level disturbance and surface front approach
from the Plains states. Flight conditions will lower to MVFR/IFR
in the precipitation.

The rain will end by daybreak Wednesday. MVFR ceilings are likely
west of an ESC to STE line, with VFR ceilings to the east.
Southwest winds will gust to over 30 knots mostly south of a AUW
to SUE line.

Another weather system will bring additional showers and
thunderstorms to the region Thursday and Thursday evening.

&&

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

&&

$$

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