676
FXUS63 KGRB 161751
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1251 PM CDT Fri Aug 16 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Locally heavy rainfall in excess of one inch will be possible
  this afternoon into this evening. The heavy rain could produce
  localized flooding in urban, low-lying, and poor drainage areas.

- Strong storms, and perhaps a few severe storms, are possible
  over central, east-central, and far northeast Wisconsin this
  afternoon into early this evening. Damaging winds, hail and
  torrential rainfall are the main hazards.

- 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 419 AM CDT Fri Aug 16 2024

Short Term...Today...Tonight...and Saturday

Shower/storm/fog trends, and heavy rainfall and severe
thunderstorm potential, are the the main forecast concerns today
into Saturday.

Rain & Storm Chances / Heavy Rain & Severe Wx Potential:

The unsettled period of weather will continue into Saturday as
vertically stacked low pressure slowly tracks from near Duluth
early this morning to northeast WI afternoon/evening to Lower MI
Saturday afternoon. Latest wave of showers and a few storms
formed a little after midnight along/ahead of some moisture
convergence and shortwave energy, with 1200-1600 J/kg of MUCAPE to
work with. Heavy rain, gusts to ~30 mph and small hail (but high
wet-bulb zero heights will limit hail production) look to be the
main hazards with this activity as it spreads northeast through
the early morning hours, with some other spotty/lighter showers
possible. Then as low pressure and next shortwave pushes into the
area, another round of showers and storms is expected to develop
across the region. SB/MUCAPEs should remain/climb to 1500-2000
J/kg by late morning, with shear remaining fairly weak (up to 25
kts). While widespread severe storms are not expected, can`t rule
out an isolated severe storm or two with damaging winds being the
main threat, with some small hail possible in the strongest cores.
Best chance for a severe storm looks to be across central and
eastern WI from 1-7pm, where the higher dewpoints, better
shear/instability resides. SPC has placed most of this area in a
Marginal Risk for severe weather.

While we won`t highlight as the better threat appears to be just
to our west, will need to monitor for the threat of some funnels
or non-supercell tornadoes as the upper low moves in. Some issues
about how much low-level instability we can get due to cloud
cover, especially over the north. Better low-level vorticity also
looks to remain our west (closer to the surface low), and by the
time it gets here we should lose the better low level lapse rates
/CAPE.

Heavy rain will be the other main hazard from the showers/storms
today into this evening. PWATs of 1.3-1.7" will remain in place,
along with warm/deep cloud depths and tall/skinny CAPE, will
provide for very efficient rain makers. Localized flooding will be
possible, especially in flood prone and urban areas (due to high
rain rates), and where any storms move over the same location.
Slowest moving storms will be across northern WI. WPC has extended
the Marginal Risk for excessive rain back into all of northeast WI.

As the low slowly spins into Lower Michigan, cyclonic flow will
bring wrap-around showers, clouds and possibly a few storms
across the area later tonight into Saturday, with the greatest
coverage across eastern WI. Locally heavy rain and localized
flooding will be the main threat.

Fog Trends:

Dewpoints in the 60s to around 70, recent rains and relatively
light winds have allowed fog/mist to develop across much of the
area overnight. This will continue into the morning hours, with
visibilities improving after sunrise. Some dense fog is expected,
especially where any breaks in the clouds occur and where the
heavier rain fell yesterday. But think the threat for widespread
dense fog will remain on the low side due to the low stratus deck
and showers pushing through to keep things stirred up. But will
need to watch for the stratus deck lowering toward sunrise.
Dewpoints won`t be as high as tonight, but patchy fog is expected
again, especially where the heavier rain falls today.

Temperatures / Humidity:

After a warm and muggy night, temps don`t look to climb much today
due the clouds/precip expected. However, with such a warm start,
it won`t take much warming to get highs in the mid 70s to low 80s
across most of the area. Warmest temps will be in east central WI
where the best chances for breaks in the clouds later this morning
will be, with some mid 80s possible if the breaks are more
persistent. Lows tonight should range from the upper 50s in north
central WI to the mid 60s across eastern WI. CAA, clouds and
precip will hold highs on Saturday mainly in the 70s, with a few
upper 60s possible in far north central WI. It will humid today,
especially in eastern WI where dewpoints will be around 70.
Slightly less humid conditions arrive tonight into Saturday, but
dewpoints will remain the 60s, keeping a touch of humidity in the
air.

Long Term...Saturday Night Through Thursday

Relatively low impact weather remains expected late this weekend and
for much of next week.

With the large upper low continuing to spin across the central Great
Lakes, deep cyclonic flow combined with ample moisture in the column
will lead light showers continuing over at least eastern WI on
Saturday night.  The shower chance will finally depart on Sunday
morning as the system slides further east.

Thereafter, northwest flow will be present over the western Great
Lakes through the middle of next week before the upstream ridge
moves overhead around next Thursday and Friday.  Models had been
advertising a front moving into the region during the Wednesday-
Thursday time frame, but confidence in this scenario has lowered
over the past 24 hours.  In general, precip chances look low next
week.

In terms of hazards, north flow on the backside of the departing low
pressure system could lead to 25 kt winds on the Bay and Lake from
Saturday night through Sunday night.  Confidence is only medium that
a small craft advisory will be needed.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1237 PM CDT Fri Aug 16 2024

Showers and thunderstorms were developing over the region due to
increasing daytime instability (CAPE of 1500-2500 j/kg) and the
approach of a well-defined short-wave trough over western WI.
The first line of storms is poised to impact AUW/CWA at TAF
issuance, with potential for torrential rainfall and gusts to
35-40 knots. The storms will continue to shift east through early
evening, so have framed out TEMPO groups for the most likely
period of storms at each TAF site. Flight conditions will vary
between VFR and MVFR this afternoon and early evening, except for
local IFR/LIFR vsbys in heavy rainfall during storms.

Expect shower activity to wind down early this evening, though
showers and isolated storms may continue to impact eastern WI,
and especially Door county, through the rest of the TAF period.

Look for MVFR ceilings to gradually lower to IFR/LIFR later
tonight, and vsbys to drop to MVFR/IFR in fog. Not really
expecting dense fog at this time, as N-NW winds will be picking up
a bit later tonight into Saturday morning. The very low ceilings
should start to rise during by late morning, with most locations
improving to MVFR toward the end of the TAF period.

&&

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

&&

$$

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