377
FXUS63 KGRB 180921
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
421 AM CDT Sat May 18 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Thunderstorms are likely late this afternoon across central and
  north-central Wisconsin. There is a chance of thunderstorms in
  east central and far northeast Wisconsin this evening. Some
  storms may become strong to severe with damaging winds and hail
  the main threat.

- Chances for showers and thunderstorms continue through mid-week.
  While too early to determine severe potential, confidence is
  increasing in locally heavy rain on Tuesday.

&&

.DISCUSSION...
Issued at 420 AM CDT Sat May 18 2024

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

Scattered light showers and sprinkles are possible in far north
central Wisconsin early this morning due to elevated instability.
They should end shortly after sunrise.

It will be a summer like day today as gusty southerly winds
provide good mixing of the lower atmosphere. Mixing to 900mb or so
will result in highs in the 80s in the forecast area, though it
will be cooler near Lake Michigan. A scattered to broken line of
thunderstorms is expected to develop ahead of a cold front that
will move into central and north central Wisconsin in the late
afternoon. 1000 J/KG of CAPE and an inverted V sounding could make
for a few strong or severe storms with damaging winds and hail.
The activity should should diminish as it moves into the Fox
Valley and lakeshore counties in the early evening and encounters
lake stabilized air in the low levels and loss of surface heating
as the sun sets.

Whatever convection makes it to the lakeshore should move out over
the lake by midnight, with clearing skies and slightly cooler and
drier air arriving overnight. Lows will be near normal. Sunday
should be very pleasant, with light winds, mostly sunny skies and
low humidity. Highs will be about 5 to 8 degrees above normal.

Long Term...Sunday Night Through Friday

As previously mentioned, focus for the extended remains on
rain/storm chances through mid-week brought on by an active
synoptic pattern. Large-scale ridging across the eastern CONUS
is expected to put much of the Midwest under a southwest flow
regime through mid-week, which is conducive to warm air advection
and moisture transport up from the south. Hence, periods of
moderate to heavy rainfall with potentially high QPF will be
something to look out for.

Monday precip chances... A robust trough situated across the
western CONUS will eject a shortwave up into the western Great
Lakes, bringing a fast-moving round of rain to the forecast area
Monday morning. During this time, a warm front will stall to our
south, providing some lift via isentropic ascent and a surge of
warm air advection. Additionally, an open Gulf will bring a swath
of deep moisture up into portions of central and east-central
Wisconsin Monday afternoon, placing a corridor of 1.5+" of PWAT
across the Fox Valley. Models seem to be targeting central
Wisconsin to receive the bulk of the precip with this system (~0.6"
to 0.8" QPF) due to showers being more prolonged over that area.
Most convective elements look to remain to our south along the
warm front, but cannot rule out a rumble of thunder along/south of
Highway 29.

Tuesday/Wednesday precip chances... As Monday`s system departs to
the east, attention then turns to a more dynamic system that
treks from the central Plains to the upper Mississippi Valley by
Tuesday evening. Current model guidance suggests that this system
has the potential to bring heavy rain to much of the forecast
area Tuesday through Wednesday morning, although it is still too
soon to discern exact amounts given significant model spread this
far out. With plentiful Gulf moisture and dewpoints in the low to
mid 60s, suspect that QPF will trend on the higher end of what
ensembles are currently showing. This being said, forecast
confidence is high enough to warrant a day 4 marginal to slight
risk of excessive rainfall across the CWA. A first glance also shows
that thunderstorms may be possible across our southern counties.
However, instability in the warm sector will struggle to make it
this far north and east, hindering severe potential for the time
being. Will continue to monitor convective elements as medium-
range models come into play. Overall, biggest things to look out
for will be changes in QPF and risk for flooding/flash flooding.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1043 PM CDT Fri May 17 2024

VFR conditions prevailed during the evening hours. Mainly VFR
conditions are expected tonight into Saturday morning with only
some mid clouds expected at times. Exception will be near Lake
Michigan (possibly impacting MTW/2P2/3D2/SUE) where dense fog and
low clouds are expected overnight into Saturday morning. Areas of
dense fog have persisted across parts of southern Lake Michigan
this evening, and this area is expected to shift/expand northward
overnight. Some uncertainty to just how fast this will occur and
if it will push inland at all. No evidence of the fog expanding
north as of yet and models have backed off on the inland extent,
so have backed off the arrival of the lower visibilities and
removed the low ceilings. Low chance (under 30%) that some patchy
ground fog may form further inland. Fog trends will need to be
monitored overnight.

A few high based showers or sprinkles are possible across central
and north-central WI overnight into early Saturday but should
have no impact on ceilings/visibilities. A broken line of
showers and isolated/scattered storms is expected to push west to
east across the area Saturday afternoon and early evening,
arriving at AUW/CWA/RHI in the 22z to 00z timeframe and
GRB/ATW/MTW between 01-03z. Will include thunder in the TEMPO
groups for central and north-central WI, but hold off further east
as instability will be lower/decreasing as the activity pushes
east and/or the storms may stay just to the north of these sites.

Light winds are expected the rest of the night. Low-end LLWS will
be possible at RHI overnight as winds at 2000 ft increase to
30-35 kts. South winds will increase Saturday morning, gusting to
25 kts by late morning, with some higher gusts possible in the
afternoon.

&&

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

&&

$$

DISCUSSION.....RDM/Goodin
AVIATION.......Bersch