641
FXUS63 KGRB 161952
AFDGRB

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

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Isolated severe storms possible into this evening over central,
  east-central, and far northeast Wisconsin. Strong wind gusts,
  small hail and torrential rainfall are the main hazards.

- Locally heavy rainfall in excess of an inch possible will
  accompany any slow moving thunderstorms. The heavy rain could
  produce localized flooding in urban, low-lying, and poor
  drainage areas.

- Additional showers and storms on Saturday, especially eastern
  WI, then 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 251 PM CDT Fri Aug 16 2024

Short Term...Tonight and Saturday

Forecast concerns include precipitation trends, and potential for
severe weather and heavy rainfall.

Numerous showers and scattered thunderstorms were ongoing over
much of the forecast area this afternoon, as forcing associated
with a western WI short-wave encountered a moist (PWATs around 1.5
inches) and unstable (SBCAPE of 1500-2500 j/kg) air mass. So far,
thunderstorms have remained below severe limits, as deep layer
shear was only 10-25 knots (strongest southeast) and wet-bulb zero
heights were high (10.5-11.5k ft AGL). Pockets of heavy rainfall
have been reported, and the threat of continued heavy rainfall and
localized urban flooding will remain the primary concern through
this evening. An isolated severe storm with damaging winds is
still possible through about sunset.

There will be a decrease in convection as the short-wave shifts
east this evening, but showers and isolated storms will likely
persist over far NE WI; especially Door County, through the night.
Low clouds and patchy fog are expected overnight, though surface
and boundary layer winds should increase enough to prevent dense
fog formation. Lows will be in the lower to middle 60s.

On Saturday, the upper low will shift east of the region, but
short-wave energy will continue to wrap around its western
periphery. This will combine with cyclonic north flow and
modest daytime instability of 500-1000 j/kg to support showers
and a few thunderstorms, especially during the afternoon.
Shower coverage will be less over the far west; farther removed
from the upper low. Highs will in the 70s.

Long Term...Saturday Night Through Friday

Low pressure crossing northern WI this evening will only shift
across lower Michigan by Saturday evening. Some showers and
thunderstorms over eastern WI will diminish by late evening. Fog
on Saturday night probably ends up limited due to steady north
winds developing in wake of the low. Still looks like Small Craft
Advisories may be needed late Saturday night into Sunday Night.
However, the north wind direction will result in an offshore flow
and minimal wave action so will not need any Beach Hazards
Statements.

Thereafter, northwest flow will be present over the western Great
Lakes through the middle of next week before the upstream ridge
moves overhead late in the week. Still some hints a cold front may
try to drop across late in the week. Overall though precip chances
look low (staying less than 20 percent for any time frame). Temps
will not be too far from normal with highs upper 70s to lower
80s. Lows will start out 40s north to mid 50s south, then gradually
increase into the 50s to lower 60s by late in the week.

&&

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