991
FXUS63 KGRB 071153
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
653 AM CDT Tue May 7 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Periods of showers and embedded thunderstorms are expected this
  afternoon and tonight. If sufficient breaks in the rain occur,
  conditions may become supportive for a few strong storms late
  this afternoon into early this evening. The best chance for
  strong storms will be south of a Marshfield to Waupaca to
  Oshkosh line.

- Conditions hazardous to small craft are expected on Lake
  Michigan south of Two Rivers through early this evening due to
  gusty southeast winds.

- Areas of frost are expected to develop over parts of central
  and north-central Wisconsin on Thursday night.

&&

.DISCUSSION...
Issued at 423 AM CDT Tue May 7 2024

Short Term...Today...Tonight...and Wednesday

The main forecast concerns will be precipitation trends and
potential for a few strong thunderstorms in the southwest part
of the forecast area later today.

A deep upper level low was situated over the western Dakotas,
with a potent negatively-tilted short-wave trough lifting
northeast toward the western Great Lakes. A weakening band of
showers and embedded thunderstorms was moving through SE MN
and eastern IA, ahead of the short-wave, and coincident with
a strong and persistent area of frontogenetic forcing and
deep moisture convergence. This band is poised to reach the far
southwest part of GRB CWA by 12z/Tue.

Today, strong forcing with the short-wave and frontogenesis
will push the band of moderate showers through the SW part of the
forecast area between 12z-15z, and through most of NE WI between
15z-18z. Little or no instability is expected with the main rain
band, so only expect isolated embedded thunder at best. However,
with PWATs increasing to 1.0-1.25 inches, much of the area could
see a quick half to three-quarter inch of rain. The most
difficult question for today is how much clearing and developing
instability will occur in central WI before an occluded front
approaches the area late in the day. Most models show a small area
of marginal instability (SBCAPE of 250-750 j/kg) developing in
parts of central WI and the western Fox Valley; roughly along and
south of a line from Marshfield to Waupaca to Oshkosh. Scattered
storms could develop in this region as the occluded front arrives
during the late afternoon/early evening. Given that deep layer
shear will be decreasing rapidly during this time frame,
confidence in severe weather development is low. However, a few
strong storms with gusty winds, small hail and heavy downpours
will be possible.

The weakening short-wave and occluded front will lift into
northern WI during the evening, taking lingering showers and
isolated storms with it. Small precipitation chances will remain
across the north overnight into Wednesday. Sunshine, warmer
temperatures and developing weak instability may result in a
slight chance of thunderstorms in our southwest counties Wednesday
afternoon.

Temperatures will be a tough call today, and will be heavily
moderated by showers and cloud cover. Most of the CWA should see
highs in the middle 50s to middle 60s, with the warmest temps over
the far north (heating before the rain band arrives) and far
south (recovery in the afternoon after the first band moves
through). Lows tonight should be in the middle 40s to lower 50s.
Highs on Wednesday should range from the upper 50s/lower 60s far
north to the lower to middle 70s south.

Long Term...Wednesday Night Through Monday

Relatively low impact weather is expected from Wednesday night into
early next week as the upper flow veers to the northwest and
temperatures fall below normal.  The highest impacts will likely
occur via rain chances, low temps cold enough for frost, and gusty
winds at times.

Precipitation chances:  Elongated upper troughing will extend from
the central Plains to Quebec from the Wednesday night through
Thursday night time frame.  Surface low pressure will be moving east
across the mid-Mississippi Valley.  Light rain chances are expected
to redevelop over Minnesota within the deformation zone of the upper
trough and north of the surface low track on Wednesday night before
tracking east towards central and east-central Wisconsin late on
Wednesday night through much of Thursday before exiting Thursday
evening.  Rainfall amounts of a tenth to a quarter inch appear
possible.

Another chance of rain will arrive on Friday night into Saturday
when a sharp upper trough digs southeast across the western Great
Lakes.  There will be a slight chance of thunderstorms, but meager
instability means no chance of severe weather.

Temperatures:  Colder temperatures are forecast to arrive on
Thursday which will persist into the weekend.  Low temps are
forecast to become cold enough for frost over north-central WI on
Thursday night. But northwest winds 5 to 10 mph may limit coverage
to sheltered areas.  Patchy frost could also develop over north-
central WI on Saturday night.

Gusty wind potential:  Breezy northeast to north winds could produce
gusts to 30 mph on late Wednesday night into Thursday. There is a 60-
80% chance of gusts greater than 25 mph over the Fox Valley and
Lakeshore south of Sturgeon Bay.  Conditions hazardous to small
craft will be possible.

Gusty north winds will also be possible on Saturday that could lead
to conditions hazardous to small craft.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 645 AM CDT Tue May 7 2024

A band of moderate showers will overspread the forecast area from
SW to NE this morning through midday, with flight conditions
deteriorating to MVFR. The steady showers will only last 3 to 4
hours at any given location before moving out, but MVFR conditions
will persist in some areas after the rain departs. Showers will
linger into tonight across northern WI, along with potential for
flight conditions to drop to IFR. Farther south, scattered
showers and storms may redevelop in the late afternoon and early
evening, followed by improving flight conditions overnight.

Gusty SE winds and some LLWS will occur across the region this
morning.

&&

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

&&

$$

DISCUSSION.....Kieckbusch/MPC
AVIATION.......Kieckbusch