837
FXUS63 KGRB 051645
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1145 AM CDT Fri Jul 5 2024

Updated aviation portion for 18Z TAF issuance
and new information added to update section

.UPDATE...
Issued at 1145 AM CDT Fri Jul 5 2024

An area of low pressure will be tracking east across east-central
Wisconsin to central Lake Michigan this afternoon. Showers and
thunderstorms continue in an area of 925-850mb moisture
convergence and north of the surface low. A cluster of showers and
storms have been slow moving over Waupaca county this morning,
resulting in 2 to 4 inches of rain since around 8 am this morning.
Street flooding, water rescues, and evacuations have occurred near
the city of Manawa.

The short term guidance have indicated the threat of heavy
rainfall will spread southeast across east-central Wisconsin
though late afternoon. Both the RAP and HRRR indicate areas of 2+
inches of rain will be possible. Considering 3 hour flash flood
guidance is under 2 inches, have issued a Flash Flood Watch
through 4 pm. The area south of Appleton to Manitowoc will have
the highest risk of flooding, but radar indicates the threat could
impact areas south of Green Bay as well.

&&

.KEY MESSAGES...

- Areas of showers and some thunderstorms are expected today. The
  primary threat from storms will be locally heavy downpours.

- River and stream levels will remain elevated into the weekend.

- On and off chances for rain/storms continue through early next
  week as upper-level flow remains unsettled.

&&

.DISCUSSION...
Issued at 302 AM CDT Fri Jul 5 2024

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

Shower and thunderstorm coverage into this evening is primary
focus in the short term. Mid-level low is spinning over southern
MN while weak sfc low is over southwest WI. Arc of showers and
isolated thunder is crossing northeast WI with area of steadier
and heavier showers over northern WI into central MN. Thus far
this system has drastically underperformed in our CWA in terms of
rainfall amounts and heavy rainfall potential, so that limits the
flooding risk going forward. Think lighter QPF thus far is due in
part to limited instability to boost rainfall rates by time the
showers reached our area last evening and also PWATS barely up to
1.50 inches which is only up to 125 percent of normal. Do expect
showers to increase this morning into the afternoon as the mid-
level low tracks across the area.

Thunder has been quite limited so far but do expect an uptick in
thunder toward daybreak this morning as theta-e advection and
elevated instability combine ahead of the mid-level low and sfc
low. Best chance for thunder impacting more of the area though
will occur late morning into the afternoon as the lift from the
mid-level low and steeper lapse rates/MLCAPEs building to 800J/kg
interact with some daytime "heating" (relative term as highs today
will be below normal in the mid 60s north to the lower 70s
south). Following MLCAPEs, expect thunder potential all areas
through midday then shifting to mainly south of highway 29 the
rest of the afternoon. Wind fields are not strong with the system
as mid-upper jets are displaced to the east and southeast over the
central Great Lakes. Thus effective shear is weak so am not expecting
any severe weather. Heavy downpours would be the main threat from
storms as PWATs don`t change much from what they are early this morning.

Lingering showers end early, then it turns mainly dry tonight. Cool
advection in wake of the trough and departing sfc low will keep
skies partly to mostly cloudy. Hint of some sprinkles at times
northeast WI, ending by daybreak Sat. Majority of Sat looks dry,
but with approaching shortwave trough and weak warm air advection
resulting in MLCAPEs reaching 500-1000J/kg, did include small
chances for showers and storms most areas during peak heating.
Also is signal for convergence along Michigan lake breeze which
could also support pop-up shower or storm later Saturday, very
isolated coverage. That said even with these small chances for
rain, plenty of dry time is expected on Saturday with highs
bouncing back into the mid 70s lakeside to low 80s inland.

Long Term...Saturday Night Through Thursday

Not many changes to the extended as an active pattern remains
through the middle of next week. Next chances for moderate rain
will arrive Sunday afternoon, becoming more robust into Monday
morning. There is still uncertainty regarding rainfall amounts,
although current ensemble guidance seems to hone in on east-
central Wisconsin to receive the bulk of the precip. Definitely
worth keeping an eye on given that east-central Wisconsin is
currently vulnerable to flooding given recent bouts of heavy rain.

Weekend precip chances... As upper-level ridging over the Pacific
struggles to build onshore, much of the central CONUS will be
left under a persistent troughing regime through mid-week. This
pattern will maximize opportunities for perturbations/weak
impulses to bring chances for rain and storms to the forecast
area. Best chances look to be Sunday afternoon through Monday
morning as a surface low and attendant cold front track across
the upper Mississippi Valley. Moisture content appears to be
favorable for periods of moderate to heavy rain during this time
as a surge of Gulf moisture places a narrow corridor of 1.75 inch
PWATs (~125% of normal) along the lakeshore. Some diurnally-driven
storms will also be possible Sunday afternoon and evening as
resultant instability exceeds 1,000 J/kg. However, the big
question will be whether or not the bullseye of highest rainfall
misses off to the east over the lake.

Rest of the extended... Rain/storm chances will continue through
Wednesday as several embedded impulses circulate through a
longwave trough. An end to the current wet pattern may then be in
sight toward the end of the extended as long-range models show
upper-level ridging settling in across the CONUS. This may signal
the beginning of a warmer/drier summer pattern, although only time
will tell.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1145 AM CDT Fri Jul 5 2024

A slow moving low pressure system will continue to move east
across east-central Wisconsin through this afternoon. Widespread
MVFR and IFR ceilings will persist into the afternoon,
particularly over east-central Wisconsin where showers and
thunderstorms will be widespread through 21-22z. Locally heavy
rainfall will be possible. Improving ceilings will occur this
evening as the low pressure system departs and north winds bring
in drier air

With clearing skies and a moist boundary layer, ground fog is
likely to develop after midnight across north-central Wisconsin.
Locally dense fog is possible, including at RHI.

Convective clouds are likely to develop by late morning across the
entire area. A few showers may pop near the end of the taf period,
but the likelihood of showers impacting the taf sites is low.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 7 PM CDT this evening for WIZ037>039-045-
048>050.
&&

$$

UPDATE.........MPC
DISCUSSION.....JLA/Goodin
AVIATION.......MPC