206
FXUS63 KGRB 151953
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
253 PM CDT Thu Aug 15 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Periods of showers and thunderstorms through Friday night will
  produce an additional 0.5 to 2 inches of rain. Rain will be
  heavy at times, and may result in localized flooding in urban,
  low-lying and poor drainage areas.

- A few strong storms are possible later this evening over central
  Wisconsin. Torrential rainfall and brief gusty winds are the
  main hazards. A few strong to severe storms may also develop
  over central and east central WI Friday afternoon.

- Conditions hazardous to small craft will continue on the Bay and
  Lake through Friday morning. Dangerous swimming conditions due
  to high waves and strong currents are expected on Lake Michigan
  beaches.

- Quiet weather conditions and near-normal temperatures in the mid 70s
  to low 80s are expected Sunday into much of next week.

&&

.DISCUSSION...
Issued at 250 PM CDT Thu Aug 15 2024

Short Term...Tonight and Friday

Precipitation trends, and heavy rainfall and severe thunderstorm
potential, will be the main forecast concerns.

A vertically stacked low pressure system was situated over far NE
ND/NW MN, and was drifting ESE toward the forecast area this
afternoon. A short-wave trough wrapping around the low was
generating a large band of moderate rain across the NE half of WI.
This band was still filling in over far NE WI, while the back
edge was moving through central WI. The rain band will shift out
of far NE WI early this evening. Another short-wave over eastern
ND and far NE Nebraska will move into central WI this evening, and
lift northeast through the rest of the forecast area overnight.
By the time this wave arrives, an unstable air mass (MUCAPE of
600-1200 j/kg) will be in place, so thunderstorms will become more
likely. Not expecting any severe storms due to the nighttime
arrival, high wet-bulb zero heights (poor hail potential) and weak
shear (15-25 kts), but torrential rainfall will be likely due to
continued high PWATs (1.3-1.7 inches). Convection will wane with
the departure of the wave late tonight into Friday morning. Yet
another short-wave will sweep across C/EC WI Friday afternoon, and
trigger another round of strong storms with heavy rainfall. Could
see an isolated severe storm or two with damaging winds, as
temperatures warming to near 80 and dew points in the upper 60s
yield SBCAPE of 1-2K j/kg. Although shear values have trended
downward slightly, models still show 20-25 kts over C/EC WI, and
CAMs show some evidence of better convective organization over the
southern part of the CWA. Farther north, scattered to numerous
showers and a few thunderstorms should also develop beneath the
upper low. These will be slow moving, with potential for locally
very heavy rainfall.

Southeast winds will gradually taper off late this afternoon and
evening, but current marine and beach hazard headlines still look
to be on target.

Lows will drop into the middle to upper 60s tonight. Highs on
Friday will be in the 70s to around 80 range.

Long Term...Friday Night Through Thursday

Slow moving stacked low pressure system only slowly exits the
western Great Lakes by late this weekend. Rain chances will not
end until Sunday. As upper level ridge persists over the southwest
and southern Plains, northwest flow will become established south
central Canada to the lower Mississippi River valley. This will
yield surface high pressure and mainly dry weather over the
western Great Lakes Sunday through midweek with small chances for
rain returning late in the week. This pattern will keep temps near
normal with highs most days in the mid to upper 70s and low
temperatures in the 50s.

Showers/storms: Mid-level low over northern WI and western Upper
Michigan on Friday evening settles slowly to lower Michigan
Saturday afternoon. Shortwave rotating along southern edge of the
low will result in a period of showers and storms Friday afternoon
and these will carry into Friday evening. Based on being on north
edge of stronger winds H85-H5/5kft-20kft and effective shear, the
MLCAPEs over 1000J/kg forecast could be sufficient enough for a
stronger storm into early Friday evening over east-central to
northeast WI. Gusty winds the primary hazard. locally heavy rain
remains a hazard as well though with PWATs near 1.75 inches and
warm cloud depths over 11kft. CAMs point to convection that will
have some motion west to east due to the shortwave, but any
training convection would pose an issue and could cause issues for
areas that see the most rain today and Friday as well as poor
drainage, low-lying and urban areas. Kept chancy pops into the
overnight hours over northeast to east-central WI as the low will
be too close to assume showers will taper even in wake of the
late afternoon/early evening shortwave. Finally, given the deep
moisture and weakly cyclonic/neutral low- level flow still in
place on Saturday along with daytime heating, instability, no
need to alter chance pops over much of the area through the day.
Pops finally taper off Saturday evening, allowing dry conditions
to return on Sunday.

Winds: As the low tracks to the east, southerly winds shift more
north-northwest on Saturday. Pressure gradient between exiting low
and approaching high pressure system will remain tight across Lake
Michigan. Strongest winds/gusts will be later Saturday night through
Sunday. Possible, but not a sure thing, that a Small Craft Advisory
will be needed over southern zones on Lake Michigan (south of
Sturgeon Bay). With the offshore wind direction, highest waves
will be directed toward the western shoreline of lower Michigan
and northern Indiana, so there will not be the need for any Beach
Hazard Statements for our forecast area. Peak wind gusts on land
may reach 25 mph on Sunday, highest over east-central WI closer to
Lake Michigan.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1242 PM CDT Thu Aug 15 2024

A large area of rain associated with a short-wave trough will
continue to shift northeast through the forecast area this
afternoon, then exit far northeast WI this evening. Flight
conditions will deteriorate to IFR during the steadiest/heaviest
period of rainfall. There will be a lull in the precipitation
after the initial band moves through, but IFR/LIFR ceilings and
patchy fog will remain. More showers and scattered thunderstorms
should redevelop as another short-wave moves through later this
evening and overnight. Have attempted to frame out the best time
for thunderstorm development at the TAF sites. After the overnight
convection moves out, there should be a general lull in the rain
in most areas until Friday afternoon. Remaining IFR/LIFR ceilings
and fog should improve to MVFR during the mid to late morning.

Southeast winds will gust up to 25 kts this afternoon. Winds will
decrease and slowly shift to the south and southwest tonight into
Friday morning.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement through Friday morning for WIZ022-040-
050.
&&

$$

DISCUSSION.....Kieckbusch/JLA
AVIATION.......Kieckbusch