859
FXUS63 KGRB 061147
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
647 AM CDT Sat Jul 6 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Flooding will continue along the Little Wolf River downstream of
  the Manawa Dam. Do not drive across flooded roadways if
  traveling through this area. Other rivers and streams will
  continue to run above bankfull due to recent rainfall.

- Isolated showers and thunderstorms are possible this afternoon.
  Any storms will be slow moving and could produce locally heavy
  rainfall. Most areas will remain dry though.

- A few thunderstorms on Sunday afternoon could be strong with
  gusty winds, small hail and heavy rain. At this time severe
  weather is not expected. On and off chances for showers and
  thunderstorms will continue Monday through Wednesday. Although
  the severe weather threat remains low, locally heavy rainfall
  will be possible.

- A warmer/drier summer pattern may be in store toward the end of
  the week as upper-level flow flattens out. Multiple consecutive
  days of dry weather may occur.

&&

.DISCUSSION...
Issued at 355 AM CDT Sat Jul 6 2024

Short Term...Today...Tonight...and Sunday

Today and tonight...Low that caused the heavy rain and flooding
on Friday is heading across the lower Great Lakes early this
morning. Ridge of high pressure is drying conditions out. With the
dry air and clear skies, temps have fallen as low as the upper
40s north- central. Patchy ground fog where temps are the coolest
and where rainfall yesterday has kept ground saturated.

Other than some scattered-broken stratocu early on, skies will be
mostly sunny this morning. Daytime heating will then result in
building cumulus by midday as convective temps remain quite low.
With highs in the upper 70s to around 80 degrees inland, upwards
of 1000 J/kg MLCAPE will build by early afternoon. Though forcing
is weak there is a hint a weak mid-level wave will move in during
peak heating to trigger scattered showers and storms. Most of that
activity would be from central to north-central WI, but there are
still hints that lake breezes could also generate isolated
storms. Overall, there will be plenty of dry time today, but
cannot rule out a pop-up shower or storm from these mechanisms. No
severe weather with very weak shear, but that also means storm
motions will be slow again as MBE vectors are hardly 5 kts. Unlike
yesterday though PWAT is quite low, barely over 1 inch. Locally
heavy downpours will be possible given the slow moving nature of
the storms. Any showers fade quickly this evening with loss of
daytime heating. Rest of tonight will be dry. Risk of ground fog
looks lower tonight as min temps will mainly stay above the
crossover temp. MOS guidance not picking up on any fog either,
even as typical favored spots such as RHI.

Sunday...Shortwave trough over the plains shifts to northern MN
while sfc low follows similar track. Weak front with the system
will still be lagging behind, entering MN mid to late afternoon.
A pre-frontal surface trough tied to weaker lead shortwave will
bring some showers with minimal instability for thunder through
the morning. Greatest chances for showers through noon will be
over central WI. This activity makes it to northeast and east-
central WI early afternoon. One idea is for the convection to
strengthen as daytime heating (highs mid 70s west to around 80
east) boosts MLCAPES to at least 1000J/kg. Another possibility is
these initial showers weaken while moving through, then with
heating that occurs later there could be resurgence of more sfc
based showers and thunderstorms mid to late afternoon. Given
MLCAPEs 1000-1500J/kg and our area being within modest mid-level
jet (enhancing effective shear to at least 30 kts) there is a risk
for at least isolated stronger storm could occur with gusty winds
and hail both possible, along with heavy rain.

Long Term...Sunday Night Through Friday

Focus for the first part of the extended remains on periods of
heavy rain/storms Sunday through Tuesday. Best potential looks to
be Sunday afternoon through Monday as a surface low tracks across
the upper Mississippi Valley and through the Great Lakes. An end
to the current wet pattern may then be in sight toward the end of
the week as upper-level flow flattens out and high pressure makes
its way across the US/Canada border.

Sunday evening through Tuesday... A persistent troughing regime
will bring several chances for rain/storms to the forecast area
as weak impulses circulate through the broader synoptic pattern.
The first of these impulses is progged to arrive Sunday afternoon,
which may be enough to generate some storms with help from low-
level moisture transport and accompanying instability (nearing
2,000 J/kg). Additional support may be provided by placement under
the left exit quad of an upper-level jet. Some strong to
marginally severe storms may develop Sunday afternoon and evening
given decent deep-layer shear exceeding 30 knots. Otherwise, more
diurnally-driven showers/storms will be possible Monday through
Wednesday. Storm coverage will likely be more widespread on Monday
due to low convective temperatures (mid to upper 60s).

Rest of the extended... A warmer/drier summer pattern may be in
store toward the end of the week as ridging over the west coast
shifts east and flattens the upper-level flow. Expect temperatures
during this time to be stagnant in the upper 70s to low 80s,
running slightly above average for this time of year.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 647 AM CDT Sat Jul 6 2024

Some IFR-MVFR stratocu may impact Door County and along the
lakeshore through mid morning (including MTW). Otherwise, VFR
based cu will blossom with daytime heating late morning into the
mid afternoon. A pop-up shower or storm could form out of these
clouds, but have only included at RHI where models have the best
consensus it could occur. Most locations will stay dry today
though.

Quiet tonight with light southerly winds and increasing mid and
high clouds. Showers will begin to move into central WI around
daybreak on Sunday and spread to all the TAF sites by late morning
or afternoon hours. Mainly VFR though pockets of MVFR is possible
over central and north-central WI. Greatest chance for thunder on
Sunday will be during the afternoon.

&&

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

&&

$$

DISCUSSION.....JLA/Goodin
AVIATION.......JLA