594
FXUS63 KGRB 260754
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
254 AM CDT Sun May 26 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Rain is expected at times from late this morning through
  tonight. Non-severe thunderstorms are possible at times this
  afternoon and tonight. Around an inch of rain is expected
  across the area, with locally higher amounts up to 2 inches
  possible. Some minor flooding of streets and small stream is
  possible.

- A few rivers over central and eastern Wisconsin could surpass
  minor flood stage early this week.

- Areas of frost are possible Wednesday night into early Thursday
  morning across northern Wisconsin.

&&

.DISCUSSION...
Issued at 253 AM CDT Sun May 26 2024

Short Term...Today...Tonight...and Monday

Mostly cloudy skies and dry conditions were noted across much of
the region early this morning, save for some light showers or
sprinkles across far north-central Wisconsin associated with some
weak low level isentropic lift along a semi-stationary boundary
across the north. The main area of precipitation is expected to
continue developing across Iowa and track northeast through
Wisconsin later this morning and into the afternoon as a
negatively tilted mid level trough and attendant shortwave track
over Wisconsin. This trough will slowly move away from the region
tonight, as the moderate rain continues then slowly ends from west
to east later tonight into Monday morning. Some light rain showers
could continue through the overnight hours across northeast and
east-central Wisconsin as the trough remains close to that area.
Instability with the rain showers will be fairly modest, with
MUCAPEs of only a few hundred J/kg, but should be sufficient for a
few rumbles of thunder at times.

Highs today will be a few degrees below normal given the abundant
cloud cover and precipitation expected, with highs only in the
60s. The warmest temperatures should be realized before the steady
rain begins, with temperatures likely falling during the
afternoon as the moderate rain tracks through the area. Lows
tonight are expected to range from the upper 40s across the north,
to the middle 50s across east- central Wisconsin.

Another mid level shortwave will track through the upper
Mississippi Valley on Memorial day, which will bring the chance
for showers back to the region, especially during the afternoon.
MUCAPEs will again build to a few hundred J/kg during the
afternoon, which will bring the threat for some thunderstorms to
central and east-central Wisconsin. The modest instability should
prevent these storms from becoming severe. Highs on Memorial Day
should be a bit warmer as there are not expected to be rain
showers ongoing for the entire day. Highs are expected to range
from the upper 60s across the north, to around 70 across central
and east-central Wisconsin.

Long Term...Monday Night Through Saturday

The passage of two more shortwave troughs will keep the unsettled
weather going over northeast WI through Tuesday. By mid-week, the
mean flow becomes more amplified with upper troughs on each coast
and an upper ridge over the central CONUS. This ridge gets shoved
into the Great Lakes by the end of the week as the western trough
pushes toward the northern Plains. Some question by late week as
to whether this ridge can hold dry conditions intact or have
precipitation move into the region. Temperatures look to be below
normal through at least Wednesday, then warm to slightly above
normal at the end of the week.

Monday night through Tuesday...
One weak shortwave trough is forecast to swing through the western
Great Lakes Monday evening with a second, stronger shortwave
trough to follow on Tuesday. No real surface features or jet help,
thus precipitation to be dependent on mid-level forcing with each
trough. Instability wanes Monday evening and returns in limited
fashion Tuesday afternoon. Hardly any shear is advertised by the
models, thus any storms would be of the garden variety. Min
temperatures Monday night to range from the middle to upper 40s
north-central WI, upper 40s to lower 50s elsewhere. Max
temperatures Tuesday to be in the lower to middle 60s north-
central, middle to upper 60s for the rest of northeast WI.

Tuesday night and Wednesday...
Any lingering light shower activity will end Tuesday evening as
the second shortwave trough departs and high pressure begins to
build into the region during the overnight hours. Skies will
gradually become mostly clear with a diminishing wind. This would
send min temperatures down into the upper 30s to lower 40s north,
middle to upper 40s south. The high pressure will be centered over
the western Great Lakes on Wednesday, bringing plenty of sunshine
and a light north wind to the forecast area. Temperatures to still
be a bit below normal with readings in the lower to middle 60s
near Lake MI, middle 60s to around 70 degrees inland.

Wednesday night and Thursday...
Clear skies, light winds and a cool air mass aloft will bring a
cool night to the region Wednesday night. Cool enough such that
areas of frost cannot be ruled out over northern WI. By Thursday,
the high pressure will edge just to our east while the upper ridge
pushes toward the western Great Lakes. Anticipate another mostly
sunny day on Thursday and as winds veer more to the south-
southeast, expect a gradual boost in temperatures. Look for
readings in the middle 60s near Lake MI, mainly lower 70s inland.

Thursday night and Friday...
After a quiet Thursday night, precipitation trends become an issue
as the new 00z models are now showing the amplified flow to break
down faster than previously advertised. This would allow the
western upper trough to move across the central CONUS and could
bring a chance of showers to central WI as early as Friday
afternoon. A prevailing southerly wind will continue to pull
warmer air northward, thus max temperatures Friday to be near 70
degrees lakeside, lower to middle 70s north and middle to upper
70s south.

Friday night and Saturday...
If the amplified flow breakdown is correct, then rain chances
would reach all of northeast WI headed into the weekend as the
upper trough moves into the Upper MS Valley. Therefore, will need
to carry a chance of showers for both Friday night and Saturday
with a slight chance of thunderstorms. More clouds around on
Saturday should knock a couple of degrees off of max temperatures
with readings still near 70 degrees near Lake MI, but lower to
middle 70s inland.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1046 PM CDT Sat May 25 2024

VFR flight conditions will continue through Sunday morning, with
mainly mid/high clouds. A few sprinkles are possible in far north
central WI early in the TAF period, but will not mention in the
TAFs.

A low pressure system and associated strong short-wave trough
will move into the region on Sunday. Showers will overspread the
region from SW to NE in the late morning and early afternoon,
and become heavy at times in the afternoon and evening. As the
showers increase, flight conditions will drop to MVFR, then
IFR/LIFR. Most of the instability is expected to stay south of
the forecast area, so will not mention thunderstorms in the TAFs
for now. It is possible that a brief period of thunderstorms could
impact MTW in the late afternoon/early evening, but will let the
next shift assess this for the 12z TAFs.

Light S-SE winds will occur tonight into early Sunday, then
increase a bit from the SE-E during the day, then NE in the late
afternoon and evening. Small pockets of LLWS will be possible in
C/EC WI on Sunday.

&&

.HYDROLOGY...
Issued at 253 AM CDT Sun May 26 2024

Although heavy rain is not expected from this system, steady light
to moderate rain should persist for several hours once the rain
begins. There is fairly high confidence in rainfall amounts today
and tonight reaching an inch, plus or minus 0.25 inches, as this
represents the 50th percentile for all models. The probability of
exceeding 2 inches during this event is low but not zero,
representing the 90th percentile, and mainly focused across
central Wisconsin. Rainfall amounts around an inch would likely
have a low impact, while amounts closer to 2 inches would produce
street flooding in urban areas and bring small streams to bankfull
stage.

&&

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

&&

$$

DISCUSSION.....Kurimski/Kallas
AVIATION.......Kieckbusch
HYDROLOGY......Kurimski