Special Weather Statement, NWS Green Bay, WI

Area Forecast Discussion
National Weather Service Green Bay WI
616 PM CDT Tue Jul 18 2017

Updated aviation portion for 00Z TAF issuance

.SYNOPSIS...
Issued at 210 PM CDT Tue Jul 18 2017

Warm and humid with periods of thunderstorms for the rest of the
work week and into the upcoming weekend.

A low amplitude band of westerlies will remain across the
northern CONUS and southern Canada into the weekend. Amplification
will begin during the weekend, and continue into next week. Though
medium range models differ on the details of the pattern during
that time, it seems likely that we'll head back into what has
been the favored pattern during the past several weeks. That would
be a regime with a ridge over the intermountain West and a trough
over the Great Lakes.

The frontal boundary crossing the area tonight is likely to play a
major role in our weather for the next several days, as it stalls
and stretches out along the southern edge of the westerlies. At
this point it seems unlikely to move far enough south to allow for
any prolonged dry periods. The resulting numerous opportunities
for precipitation are likely to result in above (north) to much
above (south) normal precipitation amounts. Humid conditions and
temperatures near to modestly above normal are expected through
the weekend, though readings could be held back on any days with
widespread clouds and precipitation. Drier conditions and
temperatures returning close to seasonal normals are likely early
next week as the upper flow begins to tilt northwest in response
to the redevelopment of an eastern NOAM upper trough.
&&

.SHORT TERM...Tonight and Wednesday
Issued at 210 PM CDT Tue Jul 18 2017

The latest RAP analysis and satellite/radar imagery show a cold
front moving southeast from southern South Dakota to far northwest
Wisconsin early this afternoon. Showers and storms continue to
develop ahead of the front with help from a old mcv as a result from
convection over the Plains last night.  The most widespread of
storms have occurred over Vilas county, and its possible that the
atmosphere may not have time to destabilize ahead of the front.  The
same cannot be said for further south and east where storms have
been more isolated/short-lived and further destabilization is
expected.  ML capes approaching a 1000 j/kg over central and parts
of far northeast WI where storms remain on track to develop in the
20-22z time period. 0-6km bulk shear of 25-30 kts will be sufficient
for severe storms to develop.  Damaging winds and large hail remain
the primary threats through tonight.

Tonight...As the mcv exits north-central WI by early this evening, a
cold front will be moving southeast over central and northeast
Wisconsin, and clearing east-central WI by midnight or shortly
thereafter.  Ahead of the front, mixed layer capes up to 1500 j/kg
will be gradually diminishing through the evening.  Therefore, the
best chance of severe storms will occur for the first few hours of
the evening over the southeast half of the forecast area. Some of
the higher res models keep some precip around into the overnight
hours, so kept a small chance going, but this may be too ambitious.
High pressure will build in across northern WI in the wake of the
front late tonight.  Guidance is not excited about fog chances, and
there may be just enough of a boundary layer wind to keep that from
happening.  Turning cooler and less humid behind the front, with
lows ranging from the mid 50s north to mid 60s south.

Wednesday...Will likely see some morning clouds over parts of
central and east-central WI through mid-morning before drier air
arrives with the southward moving surface high.  With plenty of sun
in the afternoon, raised temps a degree.  Highs in the low to mid
80s.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 210 PM CDT Tue Jul 18 2017

Confidence in forecast details beyond about 48 hours remains low
due to the flat upper pattern.

In addition to difficulties timing rain chances due to the low
amplitude pattern, we are having major difficulties discerning the
heavy rain/flooding potential associated with additional rounds of
convection. Several days ago, heavy rainfall and flooding seemed
like a realistic possibility given the proximity of the front.
Then there was somewhat of a model consensus to shove the front
just far enough south to keep the area safe from any flooding
rains. The overall trend during the past 24 hours was to edge
things north again, with much of the guidance suggesting a severe
MCS will roll from southeast Minnesota, across southern Wisconsin
late Wednesday night into Thursday. That may pose a risk of
damaging winds in the area (though agree with the SPC Day 2
Convective Outlook that the greatest severe threat will probably
be to our south). But at least central and east-central Wisconsin
seem likely to be affected by the northeast flank of the MCS. With
additional rain chances later this week and into the weekend, the
possibility of a flooding threat evolving across the area looks
higher than it did yesterday. Will attempt to detail the
situation along with the uncertainty in the HWO.

With little confidence in the forecast details, no significant
changes were made the forecast initialization generated from a
broad-based blend of guidance products.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 616 PM CDT Tue Jul 18 2017

A line of showers and thunderstorms continue to move through the
eastern TAF sites this evening ahead of an upper level
disturbance. Some of these storms may be strong to severe at times
with damaging winds and hail. Storms should exit the TAF sites in
a few hours. VFR conditions are expected for much of Wednesday as
high pressure moves through the area.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

SYNOPSIS.......Skowronski
SHORT TERM.....MPC
LONG TERM......Skowronski
AVIATION.......Kurimski

Hazardous Weather Outlook
National Weather Service Green Bay WI
217 PM CDT Tue Jul 18 2017

WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-191930-
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-
217 PM CDT Tue Jul 18 2017

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

.DAY ONE...Tonight

Scattered severe thunderstorms with damaging winds, heavy
rainfall, and localized flooding are possible through this
evening. A few of the storms could also produce large hail.

.DAYS TWO THROUGH SEVEN...Wednesday through Tuesday

Another round of storms is likely Wednesday night into Thursday,
especially across central and east-central Wisconsin. Heavy
rainfall is possible with the storms, and that again could lead to
localized flooding. The storms may also pose a risk of damaging
winds, though there is more uncertainty related to that aspect of
the forecast. Be sure to monitor later forecasts for updated
information.

Additional rounds of storms are possible at times during the rest
of the week and into the upcoming weekend. Some of the storms
could produce heavy rainfall and localized flooding, especially
over central and east-central Wisconsin. It is too early to
determine whether any of these thunderstorms may turn severe.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be necessary through this evening.

LMZ521-522-541>543-191930-
The Bay of Green Bay south of a line from Cedar River MI to Rock
Island Passage-Lake Michigan from Washington Island to Sheboygan-
217 PM CDT Tue Jul 18 2017

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

.DAY ONE...Tonight

Scattered thunderstorms are expected tonight. Some of the storms
could produce strong winds, heavy rainfall, and hail, and pose a
significant hazard to boaters.

.DAYS TWO THROUGH SEVEN...Wednesday through Tuesday

Thunderstorms are possible at times during the rest of the week
through next weekend. Humid air returning to the region could also
result in dense fog on the lake at times, which would pose a
navigation hazard to boaters.

MPC/Skowronski

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