776
FXUS63 KGRB 271731
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1231 PM CDT Sat Apr 27 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Southwest winds could gust to 45 mph or more in the Fox Valley
  and Manitowoc and Kewaunee counties this afternoon.

- It remains possible that conditions will support a few strong to
  severe storms this afternoon into this evening across northeast
  Wisconsin, with the greatest risk of severe storms from 3 pm to
  9 pm south of highway 29 from the Fox valley to the lakeshore.
  There is still a lot of uncertainty in the coverage of strong or
  severe storms.

- Several systems will bring periods of showers and thunderstorms
  tonight through mid-week. Periods of moderate to heavy rain
  Sunday into Sunday night could result in isolated flash flooding
  in urban and low lying areas. Rivers will be on the rise through
  the middle of next week, with a few of them reaching bankfull
  stage.

- Conditions hazardous to small craft are expected this weekend
  on the Bay and Lake. A few gale force gusts are possible this
  afternoon, especially on the southern end of the bay.

&&

.DISCUSSION...
Issued at 414 AM CDT Sat Apr 27 2024

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

Wind and convection trends today are primary concerns.

Deep sfc low around 995mb is lifting into southwest MN with sfc
warm front as close as northern IA and far southwest WI. Warm air
advection aloft and very limited elevated instability resulted in
bands of showers lifting across the area since last evening. More
recent activity has carried isolated thunder as well as the
instability aloft is building in from the southwest. Clusters of
convection lifting across the bay and Lake Michigan and over
central WI will lift north next few hours leaving rest of the
morning mainly dry as sfc warm front mixes northeast battling
cool, moist SE flow off northern Lake Michigan and the bay.

Eventually by early afternoon as sfc low tracks over Lake Superior
effectively the warm sector will be over most if not all of the
area. Soundings continue to show well mixed sfc-H85 layer leading
to SW gusts 45-50 mph for at least a few hours over east-central
WI, maximized across the Fox Valley to the southern shore of Green
Bay. Gusts of 40-45 mph will likely make it into portions of
Marinette and Oconto counties this afternoon, but left the going
wind advisory in place where confidence was highest winds will
shift to SW and mixing will be strongest. Though there is still
some concern about S flow holding down temps near the immediate
lakeshore and downwind of the bay, continued to favor continuity
and model/MOS consensus in running high temps well into the 70s
for all but far north-central WI.

These temps along with dewpoints to around 60 by mid to late
afternoon will result in building MLCAPES upward of 1000-1500J/kg,
but the greatest instability still looks to be locked up
southeast of a line from Wood County into Brown/Kewaunee counties
and even more so into the far southern fringe of our cwa. As has
shown up last two days, even with the forecast instability,
convective initiation is uncertain given H8-H7 capping still
forecast in shortwave ridging and mid-level drying between
negative tilted shortwave trough lifting the sfc low to our
northwest and north and next shortwave trough well upstream over
the high plains. This even as there will be a cold front crossing
the area late this afternoon (north-central) to early evening
(east-central). Models are very underwhelmed in additional
convection this afternoon especially northwest of the Fox Valley
into northern WI. Still some isolated convection showing up in
various model runs over far south portion of cwa late today, after
21z. Real tough situation because as was mentioned in previous
discussion, if convection can form, it would certainly have the
potential to become severe with more than sufficient instability
and strong effective shear over 40 kts. All modes of severe
weather could occur. But the main question is will there be any
storms in our area at all or will they develop just to the south?
We will continue to message the conditional threat as our area is
still within Day1 marginal and slight risks from SPC. But,
considering parts of our area (north-central) now have limited if
any pops, it is becoming more of a challenge.

Late tonight, next round of leftover showers and some storms will
approach from the southwest. These showers will gradually develop
into more widespread area of moderate rain on Sunday spreading
from southwest to northeast. Rain totals on Sunday will be from a
half inch to an inch. The rain will be supplemented by a cool,
moist northeast wind flowing in off Lake Michigan and the bay as
the wave of low pressure will remain to the southwest. Temps were
lowered as a result and it seems like some areas over north-
central WI and closer to the bay and lake may spend most of the
day with readings below 50.

Long Term...Sunday Night Through Friday

500mb ridging across the mid-Atlantic states and a closed low
across the northern plains will result in a moist southwest flow
pattern Sunday night into Monday. At 00z Monday, the surface warm
front will be located along the Wisconsin/Illinois border. A low
level jet is expected to increase to 40 to 50 knots Sunday
evening, resulting in showers and increasing chances for
thunderstorms as the better elevated instability lifts northward
into the area. With precipitable water values of 1 to 1.5 inches,
locally heavy rainfall is a distinct possibility. Localized flash
flooding, especially in urban areas will be highlighted. The
steadier rain should taper off to scattered showers and storms
after 06z Monday. A few strong or severe storms are possible in
the late evening and overnight hours as reflected in the Storm
Prediction Center Day 2 Outlook that has a Marginal Risk across
portions of central Wisconsin east into Winnebago County.

The chances of thunderstorms continue on Monday until the cold
front moves east of the area. Current thinking is the cold front
should move east of the lakeshore counties around 20z. May need
to watch to see if there would be some stronger storms across far
eastern Wisconsin with the better chance to warm up. Dry
conditions are expected Monday night and for most of the day on
Tuesday. The next system will bring chances of showers and storms
Tuesday night into early Wednesday morning. The forecast becomes
muddled Wednesday afternoon into Thursday due to timing
differences of the next feature, which the timing has been
inconsistent the last few night. Mainly dry conditions are
anticipated for Friday.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1230 PM CDT Sat Apr 27 2024

Windy conditions will continue at the TAF sites this afternoon
with southwest winds gusting to 30 to 40 knots at times. Winds
will slowly ease this evening below 20 knots then increase again
late tonight into Sunday morning as they turn to the northeast and
gust to around 20 knots.

Isolated showers and storms may redevelop after 22z, mainly south
and southeast of GRB and ATW as a cold front moves through the
area. After these showers and storms end this evening, most of the
night should stay mainly dry.

Ceilings will be IFR near the Lake Michigan shore, although
satellite imagery shows the low stratus is dissipating from south
to north. MVFR ceilings are expected across north-central
Wisconsin with VFR across most of central and east-central
Wisconsin. Other than RHI, most sites will be VFR this evening
then trend towards MVFR late tonight into Sunday morning. Another
low pressure system will bring steady rain to the area late
tonight into Sunday morning and last through Sunday night.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 4 PM CDT this afternoon for WIZ030-031-
035>040-045-048>050.
&&

$$

DISCUSSION.....JLA/Eckberg
AVIATION.......Kurimski