808
FXUS63 KGRB 191949
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
249 PM CDT Thu Sep 19 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- There is a marginal risk of severe storms in north-central and
  central Wisconsin this evening. Damaging winds are the main
  threat, with small hail and locally heavy rain also possible.

- A pattern shift is expected this weekend, with highs dropping
  closer to normal Sunday through the middle of next week.
  Confidence is low on the timing/location and rainfall amounts
  Sunday into early next week.

&&

.DISCUSSION...
Issued at 249 PM CDT Thu Sep 19 2024

Short Term...Tonight and Friday

Forecast concerns include precipitation trends and severe
thunderstorm potential tonight, and possible fog late tonight
into early Friday.

Scattered showers and thunderstorms continue to percolate over
north central WI, on the leading edge of the moisture and
instability gradient. This activity has been slow to advance
eastward due to lingering dry air in the mid-levels. The main show
was firing up along the MN/IA state line, where strong convection
was developing along the cold front, in an area of strong
instability (CAPE around 2000 j/kg) and deep layer shear (40 kts).
In general, the CAMs have been showing a slightly later arrival
time for this main band of covection; around 00z-01z in our far
western counties. This narrows the window of opportunity for
severe weather over NC/C WI, as MUCAPE around 1000 j/kg quickly
wanes after sunset. CAMs do suggest that convection will organize
into a bowing line, and there should be enough instability and
DCAPE (currently 1000-1200 j/kg just upstream of the CWA) to
support a short period of strong to marginally severe wind gusts
in our far western counties between 7 pm and 10 pm. Much of the
forecast area should receive a quarter to half inch of rain, with
locally higher amounts up to an inch possible in NC/C WI. The
convection is expected to weaken as it shifts east during the late
evening and overnight hours. A few models suggest a weaker
secondary area of convection developing overnight as the cold
front front arrives, but confidence is low, so did not go any
higher than chance pops with this feature. If this occurs, any
lingering precipitation chances should end as the cold front
shifts east Friday morning. Dry conditions are anticipated in the
late morning and afternoon, along with at least partial clearing.
Lows tonight should be in the lower to middle 60s. Highs on Friday
should reach the middle 70s to lower 80s.

Fog may develop across the northwest half of the forecast area
late tonight into early Friday morning, but there should be enough
wind to prevent dense fog from occurring.

Long Term...Friday Night Through Thursday

Focus of the long-term forecast mainly revolves around shower and
storm chances later Saturday afternoon through Sunday as a cold
front sweeps across the state. There is more uncertainty with the
forecast early next week as ensembles struggle to resolve a pattern
featuring several short-waves.

Friday night through Sunday...Anticipate the week ending on a dry
note as a weak ridge moves over the region late Friday into
Saturday. The well above normal temperature trend will continue into
Saturday with LREF probs for highs greater than 80 degrees in the 50-
70% range across much of the area.

Ensembles show an upper-level trough riding along the US/Canada
border arriving to the western Great Lakes late Saturday. WAA ahead
of this trough may drive scatter rain showers late Saturday
afternoon and evening, with higher chances for precipitation (40-
60%) arriving along an attendant cold front late Saturday evening
into Sunday. Given the expected overnight frontal passage the severe
weather threat is low, however, forecast soundings do show a rather
dynamic wind field, 0-6km wind shear around 35-40 kts, MUCAPE values
around 1000 J/kg, and PWATs between 1-1.4". With this environment
storms may be able to produce briefly gusty winds, small hail,
locally heavy rain, and frequent lightning.

Sunday`s rainfall totals are currently expected to range from around
0.25-0.75" with only a 20-30% at most locations to see greater than
1". However, a few ensemble members do point to about a 10% chance
for greater than 1.5" at locations that see the heaviest rain. Even
the lower end of these expected totals will be welcome as parts
northeast WI were placed in a D-1, moderate drought, recently due to
lack of rainfall over the past month.

Monday through the end of the extended...The feature to watch for
the early part of next week is a low pressure system that is
expected to eject out of the 4-corners region sometime Sunday and
arrive in the Upper-Midwest Monday. The majority of LREF ensemble
members have the thunderstorms and precipitation associated with this
low staying south of the region, however, the more aggressive members
show about a 10% for greater than an inch of rain in central WI
Monday. The range in the potential rainfall is likely a function of
how far north the low allows a plume of Gulf moisture to make it.
The generally unsettled pattern will likely persist through Tuesday
and into Wednesday as another pair of disturbances are forecast to
cross the western Great Lakes.

Temperatures are forecast to become more seasonal during this period
with highs ranging from the middle 60s to low 70s each day, and a
less than 10% chance for highs above 75 degrees anywhere in the CWA.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1241 PM CDT Thu Sep 19 2024

Scattered showers and a few thunderstorms were trying to edge into
NC WI mid-day, but were not making much progress due to lingering
dry air. Although a few showers may brush past RHI this afternoon,
VFR conditions are expected to prevail through the afternoon. The
main band of showers and thunderstorms will arrive in NC/C WI
after sunset, then track east, reaching the eastern TAF sites
during the late evening and overnight hours. Models are hinting at
a secondary period of showers and storms along the cold front
later tonight, but overall confidence is greatest in the first
band. Expect some fog and low clouds to form across the northwest
half of the forecast area late tonight into early Friday, with
possible IFR/LIFR conditions at RHI/AUW/CWA. Showers should taper
off by early Friday, with at least partial clearing and a return
to VFR conditions occurring from west to east during the mid to
late morning.

&&

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

&&

$$

DISCUSSION.....Kieckbusch/GK
AVIATION.......Kieckbusch