189
FXUS63 KGRB 051918
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
218 PM CDT Wed Jun 5 2024

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Thunderstorms are expected to impact the region from late this
  afternoon through late this evening. The strongest storms are
  expected mainly between 5 pm and 9 pm and may produce isolated damaging
  wind gusts. But the threat of most storms will be strong, gusty
  winds in excess of 40 mph and small hail.

- Conditions hazardous to small craft will be possible at times
  from Thursday through Friday.

- There will be a chance of showers at times from Thursday through
  the weekend. While low chances of thunderstorms may develop, the
  severe weather risk will be low.

&&

.DISCUSSION...
Issued at 218 PM CDT Wed Jun 5 2024

Short Term...Tonight and Thursday

The latest RAP analysis and satellite imagery show a cold front
moving across northern Lake Michigan early this afternoon.
Precipitation along the front and associated with a shortwave
aloft has exited northeast Wisconsin. Strong isolation behind the
back edge of the cloud line continues to take place over western
and central Wisconsin where 0-3km surface based capes (sb cape) are
rapidly rising to around 100 j/kg. Will be monitoring western
Wisconsin for storm intensification over the next 1-2 hours which
resides within the left exit region of a 100 kt jet streak and
just ahead of a potent shortwave trough. As storms push east,
potential for strong to severe thunderstorms is the main forecast
concern.

Severe weather chances: Thunderstorms are expected to grow
upscale from mid to late afternoon before entering north-central
and central Wisconsin around the 5 pm hour. Despite modest
instability (500-800 j/kg sb cape), wind fields aloft are forecast
to significantly strengthen in the 700-500mb layer into the 60-70
kt range. Both the HRRR and RAP indicate that mid-level winds at
15 kft could approach around 80 kts over central WI by around 22z.
This is upwards of 99th percentile in the NAEFS climatology. With
low level lapse rates around 9 C/km and downdraft capes around
700 j/kg, potential is certainly there for damaging wind gusts out
of the strongest thunderstorms. Think the highest potential will
be across central and east-central Wisconsin from about 5 pm to 9
pm when the overlap of max instability and jet stream winds will
occur.

While a lesser concern, the strongest updrafts could also produce
hail. Considering the low cape in the hail growth zone, it will
likely take a rotating updraft for severe hail to develop.
Effective shear is 25 to 30 kts so severe hail remains more
marginal and the focus will be on damaging straight line winds.

Late tonight through Thursday: A lull in the precipitation is
expected overnight. Deeper low and mid-level moisture will return
towards sunrise and will likely see light rain showers spread from
northwest to southeast over the course of the morning across the
region. That doesn`t bode well for thunderstorm chances and will
remove the mention. Will also lower high temps due to cloud cover
and light precip. Gusts to 30 mph will also be possible.

Long Term...Thursday Night Through Wednesday

Extended period will primarily be dominated by a near stationary
cut- off low that is forecast settle over the eastern Great
Lakes/SE Ontario. Cyclonic flow around the low will lead to low
to moderate end PoPs (20-30%) at times this weekend. In addition,
to the broad rain chances there will be a chance for a few
embedded thunderstorms, mainly Saturday.

Thursday night through Sunday... A few showers may linger into
early Thursday night as the initial trough pushes off to the
east. Expect dry conditions overnight through Friday as drier air
filters into the mid-levels. Winds may become breezy Friday
afternoon as the pressure gradient tightens, northwest winds may
gusts 20-25 mph.

Chance for precipitation returns Saturday as a second low
pressure system moves across the MN Arrowhead and phases with the
stalled low off to the east. There is a low end chance (15-25%)
for a few embedded thunderstorms to develop late Saturday morning
through the afternoon. With only marginal instability (MUCAPE 300-
500 J/kg) and weak shear expected thunderstorms to be weak and
pulsy. One caveat here is if clouds clear out for Saturday
morning and and low-level lapse rates steepen enough to get a few
accelerating updrafts. In terms of rainfall amounts Saturday QPF
looks low, with most locations expected to only see up to a tenth
or two.

Another chance for rain showers is forecast for Sunday as a piece
of jet energy is progged to phase with the stalled low. Don`t
expect more than a few rumbles of thunder with these showers as
instability will be lacking.

Rest of the extended...The stubborn low is forecast to finally
release control of the region Monday as ensembles show a quick
round of ridging returning drier conditions to the region. There
are some signals that the next chance for showers and storms may
come Tuesday as the ridge departs the region.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1144 AM CDT Wed Jun 5 2024

Light rain is exiting northeast Wisconsin along with the
associated cold front. MVFR cigs hang back into the Fox Valley and
west of the Bay, but should see improving cigs by 18-19z. A period
of clearing skies is then expected until late afternoon.

Scattered showers and storms are expected to spread west to east
across much of the area after 4-5 pm this afternoon, then end in
the late evening or early overnight. These thunderstorms will
have high bases (generally above 5-6 kft), with dry air below.
Conditions will be favorable for locally strong downdrafts, with
winds in excess of 40 kts in the strongest of storms. Small hail
is also possible, but not as high of a concern as winds.

A period of LLWS is expected as winds at 2000 ft increase to
35-40 kts overnight.

MVFR cigs in scattered showers will push southeast across the area
on Thursday morning. Think the MVFR cigs will remain northwest of
the Fox Valley. Gusty winds to 25 kts will also be possible on
Thursday.

&&

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

&&

$$

DISCUSSION.....MPC/GK
AVIATION.......MPC