133
FXUS63 KGRB 200914
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
414 AM CDT Thu Jun 20 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- There is a chance of thunderstorms in central and east central
  Wisconsin this afternoon and tonight. Severe storms are
  unlikely.

- Thunderstorms with gusty winds and locally heavy rain are
  possible Friday and Friday night. The rain could produce small
  stream and urban flooding.

- Strong to severe thunderstorms with damaging winds, hail and
  heavy rain are possible Saturday. Heavy rains may cause rivers
  to rise rapidly.

&&

.DISCUSSION...
Issued at 412 AM CDT Thu Jun 20 2024

Short Term...Today...Tonight...and Friday

The right entrance region of an upper jet was producing light
rain showers across east central Wisconsin at 09 UTC this morning.
This jet will exit the forecast area this morning. Another jet
streak will approach central and northcentral Wisconsin during the
afternoon, and could produce scattered showers and isolated
thunderstorms. A surface high centered over Lake Superior will
bring a northeast flow of cooler air today with high temperatures
five to ten degrees below average on the first day of summer.

There is a chance of a shower or thunderstorm tonight in association
with weak upper support, but most areas will likely remain dry.
Cloud cover will limit how cool it will get, but temperatures
should still drop to normal levels for this time of the year.

The likelihood of showers and thunderstorms will increase Friday
ahead of a warm front moving north from the Illinois border.
There will be a lot of moisture in the atmosphere above the stable
surface layer, so locally heavy rain is possible with the
thunderstorms. Clouds, rain and an east northeast will make for a
cool day with highs in the middle 60s to low 70s.

Long Term...Friday Night Through Wednesday

Main focus of the extended period will be the potential for heavy
rain and stronger thunderstorms Saturday as a low pressure system
moves across the region. Confidence in the flood risk is
increasing, especially for low-laying and urban areas.

Friday Night through Saturday...Moderate to heavy rain will
likely be ongoing Friday night with widespread isentropic ascent
over the region as a warm front slowly lifts north. The heaviest
band of rainfall fall Friday night into Saturday morning will
likely be focused across northern WI along the noise of
developing 30-40kt low- level jet. Periods of heavy rainfall are
progged to continue into Saturday afternoon as models show a
deepening low pressure system propagating across the Dakotas
toward the Minnesota Arrowhead. The axis of heaviest rainfall
during this period will likely be focused along the axis of
greater frontogenetic lift which is currently forecast to develop
generally along a line from east-central MN to central WI. In
combination with the abundant lift, the expected heavy rain is
the result of an anomalously moist air mass moving over the
region. Medium range guidance is showing PWATS ranging from 2-
2.5 inches which is in the 99th percentile of the CFSR
climatology database. In terms of total QPF Friday through
Sunday, ensemble models are showing a 15-40 percent chance for
greater than 2.5 inches of rain with the highest chances in
central and north-central WI. Deterministic models are more
bullish with the GFS, ECMWF, and Canadian all showing high end
amounts of 3-4 inches.

There is more uncertainty with the severe weather potential
Saturday, compared to the heavy rain potential. A few stronger
discrete cells may develop over central WI Saturday morning,
however the better chance for stronger to severe thunderstorms
looks to come Saturday afternoon and evening as a cold front
sweeps across the region. However, with relatively modest low and
mid-level lapse rates forecast sounding show tall skinny CAPE
profiles with MUCAPE values up to about 500 J/kg in central WI
which is not necessarily an environment that would favor quickly
accelerating updrafts. Effective shear values of 35-45 kts are
expected along the cold front which could help to sustain any
updrafts that are able to develop. If stronger storms are able to
develop strong wind gusts and hail would likely be the main
concern.

Sunday through Wednesday...Once the cold front passes through the
region the risk for heavy rain will diminish, however, a few
showers and storms may linger into Sunday as moisture wraps
around around the departing low. Ensemble models then favor a high
pressure system taking over Monday returning dry conditions to
the region. The upper- level is then forecast to flatten out
toward the middle of next week allowing a few chances for showers
and storms to cross the area Tuesday and Wednesday. Temperatures
are expected to start warming back up next week with highest on
Tuesday expected to peak in the middle to upper 80s.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1045 PM CDT Wed Jun 19 2024

Cold front has settled south and east of the area so stronger
showers and thunderstorms have ended for the night. Still could be
waves of lighter showers crossing the area at times overnight
into Thursday with best chances late tonight into midday Thursday
over east-central WI. At this point, it does not look like the
showers on Thursday will be widespread. Meanwhile it will remain
mostly dry at AUW/CWA/RHI until late Thursday afternoon as a warm
front shifts in from the southwest and an upper disturbance crosses
the northern plains.

Conditions will be mostly VFR this evening, but will trend MVFR
overnight into much of Thursday at east-central WI terminals as a
cooler/moist NE flow develops to the north of the front. Cigs will
drop to IFR near the lakeshore at times with some fog as well,
including at MTW. Cigs will improve to VFR late on Thursday.

&&

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

&&

$$

DISCUSSION.....RDM/GK
AVIATION.......JLA