900
FXUS63 KGRB 202308
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
608 PM CDT Fri Sep 20 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Unseasonably warm temperatures will continue Saturday before
  temperatures return closer to normal Sunday into early next
  week.

- There is a Marginal risk of severe storms with strong winds and
  hail across central and north central WI during the late
  afternoon and early evening on Saturday.

&&

.DISCUSSION...
Issued at 237 PM CDT Fri Sep 20 2024

Short Term...Tonight and Saturday

Forecast concerns include fog potential tonight and a Marginal
risk of severe thunderstorms across central and north central WI
late Saturday afternoon.

A cold front had shifted east of the region this afternoon, as
evidenced by westerly winds and falling dew points at observation
sites. Dry conditions prevailed, and scattered cumulus formed
due to daytime heating. Temperatures ranged from the middle 70s
to lower 80s.

High pressure will bring clear to partly cloudy skies tonight,
with generally dry conditions anticipated. Some models show a few
light showers moving into north central WI late, perhaps the
result of weakening elevated convection that develops upstream of
the region on the nose of a low-level jet. Have opted to leave
these showers out of the forecast for now. Of greater concern is
the potential for some fog development, especially over far
northeast WI, where models are forecasting the lowest visibilities
and possibly some patchy dense fog. Lows should drop into the 50s,
except upper 40s in the typical cool spots in north central WI,
and around 60 in parts of Door County.

Saturday will start off mostly sunny and dry, but an approaching
cold front is expected to trigger thunderstorms in central and
north central WI during the late afternoon, mainly after 4 pm.
MUCAPE of 500-1000 j/kg, deep layer shear of 30-40 knots, and
0-3 km helicities of 150-200 support potential for a few strong
to marginally severe storms with gusty winds and hail. SPC has
expanded the Marginal risk into C/NC WI, and this seems
reasonable. high temperatures will be in the lower to middle 80s,
except mid to upper 70s near Lake Michigan.

Long Term...Saturday Night Through Friday

Primary period of focus for the extended forecast is Saturday night
through Sunday as a cold front crossing the region returns a chance
(50-60%) for showers and storms. There may be ongoing thunderstorms
at the start of the period as MUCAPE peaks around 1000-1200 J/kg
Saturday late afternoon. Additionally, forecast soundings show 30-
35kts of 0-6km layer shear during this time which may support
stronger storms capable of producing strong wind gusts, and hail.
Periods of moderate to heavy rain may also be possible as PWATS
climb to 1.2-1.4" inches during this time. Showers may linger into
Sunday afternoon as a short-wave trailing the initial cold front
clears the area. General expecting most places to see under 1" of
rain rain by Sunday evening, only 10-20% probs for greater than 1"
across central and east-central WI. Do expect the region to clear
out and turn dry Sunday night as dry continental air moves into the
mid-levels.

The passage of the cold front Saturday into Sunday will also usher
in more seasonal temperatures. Highs Sunday through the middle part
of next week are forecast to range from the middle 60s to low 70s
with less than a 10% chance to exceed 75 degrees. Overnight nights
lows will also trend about 5 to degrees cooler than the last several
days. Statistical guidance is showing the chance for lows Monday
morning to drop into the middle to upper 30s across the Northwoods.
Will need to monitor this trend for possible frost/freeze headlines.

The forecast in terms of precipitation chances during the early to
middle part of next week is rather uncertain. The latest LREF run
has trended drier than previous runs. This is in large part due to a
southward trend of a low pressure system that is forecast to eject
out of the central Plains early next week. However, there is chance
(20-30%) that eastern WI sees a few showers as moisture wraps around
the northern part of the low late Monday into Tuesday. Another
system is then forecast to dive south toward the Ohio Valley
Wednesday into Thursday which may produce a glancing shower or two,
but generally expecting dry conditions during the middle of the
week.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 608 PM CDT Fri Sep 20 2024

Diurnally induced SCT-BKN cumulus clouds, mainly across north-
central Wisconsin early this evening, will dissipate shortly after
sunset. Mostly clear skies are then expected during the overnight
period. This, combined with light winds, should allow
patchy/areas of MVFR/IFR fog to form overnight, especially across
far NE WI. Any fog should mix out early Saturday morning, with VFR
conditions returning regionwide. A cold front will bring a chance
of showers and thunderstorms late Saturday afternoon into
Saturday night, at the very end of the TAF period.

&&

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

&&

$$

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