186
FXUS63 KGRB 041144
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
644 AM CDT Thu Jul 4 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Next round of widespread showers/storms arrives from the west
  this afternoon and evening, becoming more widespread overnight.
  Some storms across central Wisconsin may become strong to
  severe, with the main threats being flooding, gusty winds, and
  small hail.

- Elevated potential for flooding this evening into Friday. River
  and stream levels will likely be on the rise, with some urban
  flooding also possible depending on where swaths of heaviest
  rain occur.

- On and off chances for rain/storms continue through early next
  week as upper-level flow remains unsettled.

&&

.DISCUSSION...
Issued at 400 AM CDT Thu Jul 4 2024

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

Severe and heavy rain potential are main concerns through tonight.

Shortwave trough over the northern plains moves to MN by tonight
while weak sfc low and warm front reach western WI this evening.
As has started to become more evident, unsettled weather is going
to be present in portions of our area late today into this evening
that will impact 4th of July activities, including fireworks displays.
Ahead of this though, through at least early afternoon conditions
will stay mainly dry with temperatures pushing into the low to mid
80s. Unlike yesterday though, winds will be considerably lighter.

The dry weather will come to an end though. Storms are already
developing as close as southwest MN this morning in warm/moist
advection ahead of the low. Isolated convection could make it into
western WI late this morning, though greater chances hold off
until mid to late afternoon over IA/MN and southwest WI as mid-
level jet arrives and interacts with building instability in the
vicinity of the warm front and on nose of modest low-level jet.
This convection will try to roll east, at least partially, as
upper trough and H85 low tied to it remain progressive. As these
features arrive, expect showers and some storms to expand first
into central WI late afternoon into early evening and then
eventually to east-central WI by late evening. The activity over
central WI could be strong or even severe, at least in isolated
fashion. Greatest risk for severe would be between 3p-8p. Locally
heavy downpours will also be a factor with 0.5-1.0/hr rainfall
rates.

Leftovers of this activity will shift east/northeast the rest of
the evening. Mixed signals still on extent of the showers and
storms by time they reach east-central WI. If convection is more
robust, heavy rain and localized flooding could occur with local
amounts over an inch occurring pretty easily given higher PWATS
over 1.25 inches shifting across along with warm cloud depths
rising over 11kft. Most guidance dampens this convection though.
00z HRRR and Canadian are the most aggressive with east-central to
northeast WI showers and storms through late evening. Better
chances for widespread moderate to heavy rain will already be
setting up farther northwest, from central to north-central WI
within mid-upper diffluence and large scale Q-vector convergence
and stronger low-level convergence near the H85 low. Though weaker
whatever instability is left by this point (MUCAPES up to
400j/kg) will be feeding into this stronger lift to produce waves
of moderate to heavy rain. Decent agreement from deterministic
models and ensembles that greatest risk of 2 inches of rain
tonight will occur from central to north-central WI. Debated
whether to issue a flood watch in these areas, but did not as
these parts of the cwa have largely missed out on the heavy rain
that occurred recently on Tuesday night and thus the flash flood
guidance is higher compared to the still saturated areas of east-
central WI (where rainfall amounts tonight should remain under an
inch). Even so, if rainfall amounts end up toward the reasonable
high-end amount (90th percentile) of 2.5-3.0 inches or HREF max
values of 3-4 inches, there would be certainly be some issues with
flooding in low-lying and urban areas.

Into Friday, it looks like a washout with showers for much of the
time over much of the area as the upper trough slides right
across and sfc low tracks from central WI to northern Lake
Michigan, putting the majority of our area within moist, cool
cyclonic NE-N flow. Lowered temps from NBM as this pattern would
suggest it will be a struggle to reach low 70s. Being beneath the
upper and sfc lows poses some concern for rotating storms as
there will be ambient low-level vorticity present though current
HREF projections for 0-1km/0-3kmSRH are very muted and there
is no signal in terms of the NST. MLCAPES increase to 500-750j/kg
during the afternoon centered on northeast and east-central WI so
though there could be some rumbles of thunder, seems greatest risk
will be from locally heavy rain.

Long Term...Friday Night Through Wednesday

Main focus will revolve around heavy rain/flooding potential
through Friday evening as robust shortwave energy drags a surface
low and accompanying rain chances across the upper Midwest.
Confidence is increasing in periods of moderate to heavy rainfall
Friday afternoon and evening, which may kindle flash flood
concerns across portions of central to east-central Wisconsin.
Otherwise, the remainder of the extended sees unsettled upper-
level flow bringing several additional rounds of rain/storms to
the forecast area through mid-week.

Friday evening rain... As decaying low pressure treks through
the western Great Lakes Friday afternoon, precip will be ongoing
across much of eastern Wisconsin by Friday evening. Current
ensemble guidance shows a more aggressive rainfall scenario than
previously suggested, highlighting a 60 to 80 percent signal for
exceeding one inch of rainfall north and west of the Fox Valley.
However, there are still some questions with how progressive this
system will be, and hence how much rain we end up getting. Flash
flooding may be a concern across portions of east-central
Wisconsin into Friday evening where recent periods of heavy rain
have dropped flash flood guidance (6-hour guidance ~1 to 1.5
inches in the Fox Valley).

Rest of the extended... Saturday looks to be slightly drier,
although some wrap-around shower activity will be possible during
the afternoon and evening as the surface low departs to the east.
Coverage will likely be spotty due to the influence of high
pressure to the south and lack of any sustaining surface
features. Nonetheless, a swath of surface dewpoints in the low to
mid 60s and resultant CAPEs exceeding 1,000 J/kg may be enough to
generate some pulse storms during peak heating. On and off
rain/storm chances will then continue through mid-week as another
shortwave digs across the northern Plains. A first glance shows
yet more heavy rain concerns as a corridor of Gulf moisture and
high PWATs (~1.75 inches) make their way up into Wisconsin. Expect
temperatures this weekend through early next week to run right
around average for this time of year, plateauing in the upper 70s
to low 80s.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 643 AM CDT Thu Jul 4 2024

Mainly VFR conditions expected through this afternoon, with
deteriorating flight conditions to MVFR over north-central and
central WI this evening as showers and some thunderstorms move
into the area from the west. IFR conditions possible in the
heavier showers and storms. Main uncertainty is how far east
these showers and storms reach during the evening.

Eventually, all areas will see showers and some storms with
locally heavy rainfall overnight as low pressure begins to cross
Wisconsin. Flight conditions will drop to IFR or MVFR. LIFR
possible central to north-central WI.

Winds through the TAF period will be significantly lighter than
yesterday, with mostly 10 knots or less.

&&

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

&&

$$

DISCUSSION.....JLA/Goodin
AVIATION.......JLA