351
FXUS63 KGRB 021752
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1252 PM CDT Thu May 2 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Round of showers and a few storms are forecast today and
  tonight. Best chance for thunderstorms will be this afternoon
  and evening with brief heavy rain, small hail and gusty winds
  expected to be the main impacts with any storms that do develop.

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

- Additional showers are possible Saturday into Saturday night
  with more showers and thunderstorms Monday night through
  Wednesday.

&&

.DISCUSSION...
Issued at 410 AM CDT Thu May 2 2024

Short Term...Today...Tonight...and Friday

Timing the precip, assessing thunder/heavy rain potential, along
with gusty wind and fog (land/lake) chances highlight the main
forecast concerns in short term, as a low pressure and upper
trough/shortwave push across the western Great Lakes.

Rain/Thunder Trends: Initial push of FGEN/WAA will spread
thicker/lower clouds across the area this morning, along with a
few light showers and sprinkles over the south/west. Some of the
radar returns are virga early this morning as there is plenty of
dry air to overcome, but the stronger echoes are reaching the
ground. As the boundary layer moistens and stronger forcing
arrives later this morning and afternoon, a large area of showers
and embedded storms (currently over IA) will overspread most
locations. Activity looks to become more scattered later in the
day into tonight as strongest WAA push and LLJ exits/weakens.
Could get a little more organized activity overnight, especially
over eastern WI, as the front/shortwave swings east across the
state. Models consistent with bringing the surface instability as
far north as the Hwy 10 corridor this afternoon/evening, near the
surface warm front, with elevated CAPE up to 400-700 J/kg
spreading across much of the area this evening. Severe weather is
not expected as instability will be limited and lapse rates are
not steep, but a strong storm with brief gusty winds and possibly
small hail can`t be ruled out near/south of the front as deep
layer shear looks to be 35-50 kts. Friday is looking dry as
shortwave lifts to the north and the front pushes to our east.

Heavy Rain & Flooding Threat / Rivers: Overall trend the last few
days has been to back off on QPF totals with the system today,
with chances of an inch or more now between 10-25 percent in
central WI, ender 10 percent elsewhere. This will help
limit/reduce the flood threat across the region. Plus, we have had
recovery time in between each round of rain the past week. That
said, models continue to advertise a possibly band of heavier
rain (over an inch), but the location is not pinned down. With
PWATs climbing to between 1.0-1.2 inches, some locally heavy
downpours will be possible, possibly creating some minor low-land
flooding. While we already have a handful of rivers at bankfull,
not expecting any significant flooding over the next couple of
days...with mainly slow rises in many of the rivers into/near
bankfull (except the Yellow where a faster rise is expected),
with a few sites in central/northeast WI possibly approaching
flood stage depending on if/where any of the heavier rainfall
falls into the basins that are already at/near bankfull.

Temps / Winds: It will be cooler for most spots today as eastern
flow prevails. NBM actually came in warmer over the south and
cooler over the north, likely due to the cooler start north and
warmer start south. Will trend in this direction, which brings
highs in the 50s and low 60s. The east flow will keep temps
cooler near Lake Michigan. Lows in the 40s to low 50s are expected
tonight. Despite being behind the front on Friday, we lose the
flow over the lake and get more sunshine, so it looks to be 10-15
degrees warmer. Highs look to range from the mid 60s to low 70s,
with cooler readings near the immediate shores of Lake Michigan.

It will be less windy today as winds do a full 180, with light and
variable winds becoming east this morning. As winds aloft pick up
to 30-40 kts at 2000 ft, some of this will mix down later this
morning and afternoon. But with limited mixing and inversion in
place, surface gusts should hold between 20-30 mph, highest across
central WI where a few higher gusts are possible as winds aloft
are the highest. Winds will drop off for a period this evening as
the low moves overhead, then winds will shift and increase out of
the west behind the surface low/front, with west winds gusting up
to 25 mph overnight into early Friday. CAA not overly impressive,
so gusts may be reduced. Winds aloft not impressive (up to 25
kts) on Friday, so even with some weak CAA (early in the day) and
mixing up to 2000 ft, surface winds should remain under 25 mph
for most locations.

Fog Potential: Some patchy fog developed during the early morning
hours across far northern WI where winds were the lightest and
clear skies prevailed for part of the night, but was limited due
to the increase in clouds during the early morning hours. Any fog
this morning will quickly burn off/lift after sunrise. Some models
are hinting at some lake fog tonight into early Friday as
dewpoints climb into the 40s and winds shift to the east/southeast
then south. Lake Michigan water temps sitting in the low to mid
40s, so would need at least mid 40 dewpoints and ideally upper 40s
dewpoints to get fog to develop on the lake. Forecast dewpoints
upstream in MI look to be in the upper 40s to mid 50s, so fog will
definitely be possible, possibly pushing into the bay and
adjacent land. Any fog that does develop will get pushed away from
the shore Friday morning as the flow turns west/northwest.

Long Term...Friday Night Through Wednesday

The active spring pattern will continue this weekend and into
next week with a few rounds of showers and storms possible. Rising
water levels on area rivers and streams will remain a minor
concern through the extend period, particularly in northeast WI
and along the Wisconsin River where several locations are already
at or near bankfull.

Dry start to the weekend as a weak high pressure system and dry
air mass move across the region behind a departing cold front
Friday night. There is a chance for scattered showers late
Saturday morning through Saturday evening as a digging upper-
level trough and PV anomaly are forecast to move across the
Minnesota Arrowhead. Not expecting any thunderstorms during this
period as instability is expected to be nearly zero. Deep layer
moisture is also lacking compared to the Thursday/Thursday night
system with PWAT values expected to be around 0.5-1". In terms of
QPF the GEFS ensemble is the most aggressive with a 20-40% chance
for over 0.1" while the EPS and GEPS are more convective with
around a 5-25% for greater than 0.1" of QPF by Saturday night.

A ridge of high pressure is progged to take control of the region
Sunday and most of Monday returning dry and mostly sunny
conditions to the region. Ensemble guidance has consistently been
showing the next weather maker pushing out of the Rockies toward
the Northern Plains late Monday into Tuesday. Warm advection rain
looks possible late Monday as mean southerly flow brings deeper
Gulf moisture to the region. With the low center currently
forecast to reside over the Dakotas Tuesday that would place
central and northeast WI in the warm sector and thus allowing for
increasing low-level moisture and instability. Forecast soundings
are also showing 20-40kt of surface to 6km bulk shear Tuesday
afternoon. While there is still plenty of time for the forecast
to change with those convective elements in play, Tuesday
afternoon and evening will be a time period to monitor for a few
potentially strong thunderstorms.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1243 PM CDT Thu May 2 2024

Widespread rain is impacting the region at issuance time, with
flight conditions deteriorating to MVFR/IFR in the steadier/heavier
showers. This looks to continue at the western TAF sites, but
we`ll likely see a break in the showers for a while in eastern WI
during the mid to late afternoon.

A weak low pressure system and associated warm front and cold
frontal passage will produce numerous showers and a few
thunderstorms this evening, with shower activity ending with the
passage of the cold front overnight. Widespread MVFR/IFR
conditions are anticipated. Clearing skies will occur from west to
east late tonight into early Friday, and gusty west winds will
pick up Friday morning.

Pockets of LLWS are expected this afternoon and evening.

&&

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

&&

$$

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