190
FXUS63 KGRB 010334
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1034 PM CDT Tue Apr 30 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

 - A low pressure system will bring a period of showers and
   thunderstorms from late this evening into the overnight hours
   tonight. No severe weather is expected, but a few storms could
   bring brief gusty winds and small hail. Another round of
   showers and storms are likely from Thursday into Friday.

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

 - 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 223 PM CDT Tue Apr 30 2024

Short Term...Tonight and Wednesday

The latest RAP analysis and satellite/radar imagery show widespread
low stratus across north-central to far northeast Wisconsin within
a shallow, cool airmass. The back edge of this cloud cover has
been slowly shifting to the northeast as return flow on the
western flank of a ridge axis increases over the northern
Mississippi Valley. Looking upstream, an elongated area of low
pressure exists to the lee of the Rockies ahead of a potent
shortwave trough. Showers and thunderstorms have developed on the
leading edge of the moisture axis in a region of elevated moisture
convergence from eastern South Dakota to Iowa. As low pressure
tracks northeast across the region, precip and thunderstorm trends
along with gusty winds on the backside of the system are the main
forecasts concerns through Wednesday afternoon.

Showers and thunderstorm potential: Low pressure will be
intensifying as it moves from southern Minnesota early this
evening to western Lake Superior by Wednesday morning. Strong QG
forcing will be passing across the northwest half of Wisconsin as
the low passes to go along with steep mid-level lapse rates
upwards of 7 C/km.

This forcing combined with a 40-50 kt low level jet will lead to
a swath of showers and embedded thunderstorms lifting from
southwest to northeast across the region from late evening through
the overnight hours. The HREF indicates that elevated instability
will weaken though the night, but can reach 500-750 j/kg as
storms enter central WI late this evening. As a result, a few
strong storms will be possible, primarily over central WI, but no
severe weather is expected. The storms will become more isolated
over north- central and far northeast WI late tonight as
instability wanes. There is a 30-40% chance of 0.50" or more of
rainfall over parts of central to north-central WI tonight.

Gusty winds Wednesday: As the low lifts across Lake Superior,
boundary layer winds will increase in a cold advection regime on
Wednesday morning. HRRR/RAP/GFS all point to substantial low level
moisture beneath the inversion and within a broad thermal
troughing regime. As a result, some drizzle may stick around into
Wednesday morning over far northern WI. The cloud cover and light
precip will likely hinder wind gust potential across the region.
Forecast soundings and NBM probabilities indicate the greatest
potential for wind gusts above 35 mph will occur over east-central
WI (40-60%) in the 1-2 pm hour, but those probabilities fall to
under 20% for wind gusts greater than 40 mph. Therefore, while it
will be windy, gusts do not appear strong enough for a wind
advisory at this time.

The gusty winds of 30-40 mph could lead to isolated power line
fires, but recent rainfall and potential for underperforming winds
due to cloud cover should limit the potential substantially. Think
the potential is greatest over central WI to far northeast WI
where green up has yet to occur in the sandy soil/forested areas.

Long Term...Wednesday Night Through Tuesday

An active pattern containing several systems and multiple rounds
of precip is set to persist through the extended. Main focus will
be on an end of week system that will bring rain chances to the
western Great Lakes starting Thursday afternoon. As a result, the
majority of the forecast area is under a slight risk for
excessive rainfall Thursday through Friday. Expect river/stream
levels to be on the rise through the end of the week, with some
locations reaching bankfull by the end of the week.

Thursday/Friday precip chances... Rain chances will begin to ramp
up toward the latter half of the week as a robust trough spins up
a lee cyclone that will eject into the upper Midwest by Friday
morning. Warm air advection precip will likely arrive Thursday
afternoon out ahead of a warm front before becoming more
widespread Thursday evening along a trailing cold front as it
moves east across the forecast area. Models are still struggling
to hone in on QPF amounts, although ensemble guidance currently
shows a strong signal for at least half an inch of storm total QPF
through Friday. Given strong poleward moisture transport and an
open Gulf, some models (GFS and ECMWF) would suggest closer to
0.75 to 1" of event QPF. Most convective elements look to remain
to our south and east, although an elevated thunderstorm with
some gusty winds cannot be ruled out Thursday evening through
Friday morning with MUCAPE values in the 300 to 400 J/kg range
(NAM). Best chances for thunder currently look to be in east-
central Wisconsin, although any storms will have to overcome a
robust surface inversion due to easterly flow advecting a stable
lake airmass onshore. Drier conditions will then arrive behind the
cold front on Friday as winds increase and veer to westerly. Gusts
between 20 and 30 mph will be possible during this time, mainly in
central Wisconsin.

Saturday/Sunday precip chances... As Friday`s surface low
continues to decay and exits to the north, another upper-level
disturbance is progged to develop in its wake, bringing
additional precip chances through the weekend. However, there is
still model disagreement about the timing of this feature this far
out in the forecast period, and thus how fast precip chances will
arrive. Overall QPF amounts look to be lower than the previous
system as most of the prime Gulf moisture struggles to make it up
into the forecast area. Convective elements currently don`t look
too impressive, although this may change as additional models come
in 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.....MPC/Goodin
AVIATION.......RDM