River Flood Warning 30, NWS Green Bay, WI

Area Forecast Discussion
National Weather Service Green Bay WI
1046 PM CDT Sat Apr 15 2023

Updated aviation portion for 06Z TAF issuance

.SYNOPSIS...
Issued at 353 PM CDT Sat Apr 15 2023

Strong storm system to bring rain and thunderstorms tonight, then
wintry precipitation (possibly heavy in central and north-
central Wisconsin) by Sunday night and Monday.

An upper trough shifting east across the CONUS will strengthen
considerably as upstream ridging builds in response to a powerful
Pacific trough approaching the West Coast. The pattern will
amplify further into an omega blocking pattern by mid-week. Then
the strong trough comprising the western leg of the block will
shift into the area late in the week.

Two strong cyclones will affect the area, the first the next
couple days and another with the trough arriving late in the week.
That will result in much above normal precipitation, which will
lead to a continuation of river flooding issues. Temperatures will
drop to considerably below normal by Monday, then recover back
closer to seasonal normals for the rest of the period.
&&

.SHORT TERM...Tonight Through Monday
Issued at 353 PM CDT Sat Apr 15 2023

No change to the Winter Storm Watch.

Another intense spring cyclone will bring the full gamut of
weather to the area the next 48 hours. Sunshine acting on the
already warm air mass across the area allowed surface temperatures
to warm into the 70s to low 80s at all but lakeshore locations.
Increased moisture compared to the past few days allowed the
convective temperature to be reached, and scattered showers and
thunderstorms were developing across much of the area. Low-level
lapse rates AOA 10K/km combined with inverted-V soundings suggest
gusty winds will be the main threat. Most gusts will be sub-
severe, but an isolated severe gust can't be ruled out--especially
with cell mergers. The storms will generally be confined between
a cold front working into the area from the west and more stable
air flowing inland from Lake Michigan.

The storms should weaken/exit the area by mid-evening. There will
then be a lull in the showers during the late evening into the
overnight hours, but another band of showers should spread north
across the area later tonight. Those should pose less risk for
wind and hail, but pose a greater risk of locally heavy rainfall.

Temporal details of the rest of the precipitation forecast
quickly become far less certain. The front will probably reach
eastern Wisconsin and then shift back west a bit as mid-level
flow backs around to the SSE ahead of a strong shortwave wrapping
around the base of the approaching large-scale trough. There will
be periods of showers and possibly some thunderstorms through
Sunday as the primary shortwave closes off and pulls the upper
trough around into a negative tilt, but it won't rain all the time.
The center of the surface circulation will probably elongate over
eastern Wisconsin/western Lake Michigan Sunday afternoon into
Sunday night.

Crashing upper heights as the cyclone becomes vertically stacked
across the area along with some low-level CAA in the northern
flow on the western flank of the cyclone will cause the rain to
change to wintry precipitation. P-Type late Sunday into Sunday
evening looks to be mainly sleet for a time over north-central
Wisconsin. It will change to snow, but shortly thereafter a large
mid-level dry slot will wrap into that area from the east. That
will result in a loss of ice nuclei and a transition to mainly
FZDZ/FZRA for a time. The amount of mixed precip will greatly
affect snow totals, and the potential for heavy snow over all but
far NW Vilas county may have decreased a bit from what was
previously expected.

Central Wisconsin is another story. That area will probably
remain in the deep moisture and strong lift on the SW flank of the
cyclone. The potential for heavy snow in this area has increased.
Our new deterministic snowfall forecast reflects these changes.
However, since the snow is still out in the third and fourth
forecast periods and other changes could still occur, opted to
leave the watch and the rather open ended snowfall wording in the
watch message intact for now. That can be fine-tuned when an
advisory/warning decision is made. Lingering warmer air in far NE
Wisconsin will probably be the last to get dislodged, so snowfall
totals in this area will probably be the lowest.

.LONG TERM...Monday Night Through Saturday
Issued at 353 PM CDT Sat Apr 15 2023

An active weather pattern is in the offing through the long-term
portion of the forecast and beyond. As surface-based low pressure
departs the region Monday night, high pressure ridging will spread
in for only a short time. By Thursday, another significant low
pressure system will gather strength over the High Plains and
spread northeast into our area. Chances for precipitation exist
from Wednesday through Saturday. Temperature profiles primarily
support rain, but some snow may be possible, especially for
northern WI. Impacts from any snow would be limited as daily high
temps will warm into the 40s and 50s.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1046 PM CDT Sat Apr 15 2023

Additional showers and scattered thunderstorm activity
will arrive overnight and continue Sunday as an upper level low
pressure system approaches slowly from the Missouri River Valley.
Ceilings and visibilities will lower to IFR from west to east
overnight and remain there through Sunday night in most places.
Rain will change to snow from west to east Sunday night, with
some accumulation likely west of a MNM to GRB to OSH line.
Significant snow accumulations are possible at
D25,LNL,EGV,ARV,RHI,TKV,RRL,AIG,AUW,CWA,MFI,ISW,STE and Y50
by Monday morning.
&&

.HYDROLOGY...
Issued at 353 PM CDT Sat Apr 15 2023

Widespread 1 to 2 inch precipitation totals are expected tonight
through Monday. As long as those totals hold, that won't be
sufficient to bring about a flash flood threat. But it will likely
exacerbate the ongoing river flooding on our larger rivers as
they will need to move another significant influx of water.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday afternoon through Monday evening
for WIZ005-010-018.

Winter Storm Watch from Sunday evening through Monday afternoon
for WIZ030-035-036.
&&

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......ML
AVIATION.......RDM
HYDROLOGY......Skowronski

Hazardous Weather Outlook
National Weather Service Green Bay WI
244 PM CDT Sat Apr 15 2023

WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-161945-
Vilas-Oneida-Forest-Florence-Northern Marinette County-Lincoln-
Langlade-Menominee-Northern Oconto County-Door-Marathon-Shawano-
Wood-Portage-Waupaca-Outagamie-Brown-Kewaunee-Waushara-Winnebago-
Calumet-Manitowoc-Southern Marinette County-
Southern Oconto County-
244 PM CDT Sat Apr 15 2023

This Hazardous Weather Outlook is for north-central and northeast
Wisconsin.

.DAY ONE...Tonight

Scattered thunderstorms are possible through early evening, and
again overnight. A few of the evening storms could be strong and
produce hail to 1/2 inch in diameter and wind gusts to around 40
mph, mainly across central and east-central Wisconsin.

Runoff from snow melt will continue to cause high water levels and
strong flows on rivers and streams, along with low-land flooding.

.DAYS TWO THROUGH SEVEN...Sunday through Saturday

River flooding and strong flows will continue across much of
central and far northeast Wisconsin through the week. Potential
for even more extensive river flooding will increase later this
week and into next week as precipitation persists in the forecast.

Hazardous travel is possible late Sunday into Monday due to heavy
snow. Amounts are still uncertain, but significant accumulations
of 6 inches or more are possible across central and north-central
Wisconsin. Wind gusts up to 45 mph will create blowing and
drifting snow and possible power outages.

A few thunderstorms are possible Sunday. These storms are not
expected to be severe.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to report any hail or wind gusts over 40
mph to the National Weather Service in Green Bay.

LMZ521-522-541>543-161945-
The Bay of Green Bay south of a line from Cedar River MI to Rock
Island Passage-Lake Michigan from Washington Island to Sheboygan-
244 PM CDT Sat Apr 15 2023

This Hazardous Weather Outlook is for portions of northwest Lake
Michigan and the waters of Green Bay.

.DAY ONE...Tonight

A few thunderstorms are possible late this afternoon into tonight.

.DAYS TWO THROUGH SEVEN...Sunday through Saturday

A few thunderstorms are possible Sunday. These storms are not
expected to be severe.

Gusty winds and high waves could produce hazardous boating
conditions Sunday evening into Tuesday morning.

Skowronski/ML

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