Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 151637

Area Forecast Discussion
National Weather Service Green Bay WI
1137 AM CDT Fri Sep 15 2017

Updated aviation portion for 18Z TAF issuance

Issued at 410 AM CDT Fri Sep 15 2017

Continued very warm and mainly dry for a couple more days, then a
little cooler by the start of the work week.

The upper flow across North America will continue to evolve toward
a higher amplitude regime, with troughing in the west and ridging
in the east. The amplification will occur in two main phases. The
first is occurring now as a shortwave digging south from western
Canada phases with the remnants of a cut-off that were off the
California coast. Brief flattening of the flow will occur as the
bulk of that energy ejects toward southwest Ontario. Then a
second, stronger (almost late fall/early winter-like) round of
amplification will occur as a Pacific jet tops a broad, strong
ridge near the dateline and digs into the western North America.
Once that occurs, the deep re-energized western trough will
persist through the end of the forecast period and beyond.

Backing upper flow downstream from the developing trough will
continue to steer a very warm air mass from the Plains into the
area, keeping temperatures significantly (10-18 F degrees) above
normal through the weekend. Readings will briefly drop back close
to normal next week as a Canadian air mass moves through, but
quickly rebound to above normal as the southwest upper flow
returns and persists. Precipitation chances with the passage of
the cold front during the weekend do not look as robust as they
once did. That`s because the bulk of the upper forcing will pass
through to our northwest and the cold front will cross the area
during the night. Rain chances during the latter part of the
period are tough to gauge as they will depend on the exact
position and structure of the western trough and downstream
eastern ridge. They could be considerable since Gulf moisture will
likely be available. But if the trough stays too far west and the
area is under strong ridging, it may be tough to generate much

.SHORT TERM...Today...Tonight...and Saturday
Issued at 410 AM CDT Fri Sep 15 2017

Scattered high-based convection occurring within weak isentropic
lift at the downstream edge of the elevated mixed layer continued
to form west of the area early this morning. Some of it could
bleed into the west, so carried low PoPs to account for it.
Something similar could happen again late today into this evening,
but conditions look even less favorable so just brushed low PoPs
across north-central Wisconsin during that time period.
Otherwise, the main area of convection will continue to be located
along and north of the strengthening frontal boundary draped just
north of the area.

The high temperature forecasts are a blend of guidance products
and persistence, and changed very little from the previous
forecast. Increasing dew points and fairly strong boundary layer
winds will likely keep temps from falling too far tonight.

.LONG TERM...Saturday Night Through Thursday
Issued at 410 AM CDT Fri Sep 15 2017

The mean flow to generally consist of an upper trough over the
western CONUS and upper ridging over the eastern CONUS during the
next seven days. There are two systems of interest that may bring
precipitation back to the area. The first system is well agreed
upon by the models and sends a cold front through WI Saturday
night into Sunday morning. After a dry start to the new work week,
the forecast becomes more uncertain mainly due to the unknown
movement of a Pacific tropical system. The GFS lifts the remains
of this system northeast, eventually reaching the Great Lakes
around Wednesday with a good swath of precipitation. The ECMWF/CMC
keep the tropical system near Baja CA and keeps the upper ridge in
our vicinity, thus a dry forecast. This conundrum will be a
problem in the days to come.

The chance for showers/thunderstorms will increase Saturday night
as the cold front moves east, reaching central WI by 12z Sunday.
The threat of any severe storms appears minimal due to a lack of
instability, stronger shear trails the cold front and the better
mid-level forcing associated with the upper trough stays well to
our north and west. However, due to the available moisture and
lift from the front, locally heavy rain/small hail/brief gusty
winds will be possible. Min temperatures to be in the upper 50s to
around 60 degrees north-central WI, generally lower to middle 60s
elsewhere. Shower/thunderstorm chances to be more focused over
eastern WI Sunday morning until the cold front exits the state.
Models then bring an area of high pressure across the Upper MS
Valley Sunday afternoon, sending a drier/cooler air mass into WI.
Look for decreasing clouds from west to east during the day with
most locations mostly sunny by mid-afternoon. Max temperatures to
range from the middle 60s to around 70 degrees north-central, to
the middle 70s across eastern WI.

This area of high pressure slides into the western Great Lakes
Sunday night, but is rather weak. Models are already sending some
higher-level moisture back into WI after midnight from WAA
occurring over the Plains/Midwest. It will still be a cooler night
compared to Saturday night with min temperatures in the lower to
middle 40s north-central, lower to middle 50s east-central WI. As
the surface high shifts east on Monday, the main concern would be
the location of the old cold front which will have stalled
somewhere over central IL and turned into a warm front farther
west from the Midwest into the central Plains. The GFS tries to
bring some moisture back into WI by Monday afternoon with a small
chance of showers/thunderstorms. The ECMWF/CMC keep enough dry air
in place to prevent any precipitation from reaching northeast WI.
Prefer to run with persistence here and keep the dry forecast
intact for Monday. The cool start to the day will likely bring max
temperatures back to near normal levels with upper 60s north,
upper 60s to lower 70s south.

A new upper trough is progged to dig southeast toward the
northern/central Rockies early next week and provide for a broad
southwest flow aloft into the Great Lakes. This should aid in both
lifting a warm front northward into WI, as well as weak shortwaves
moving northeast at periodic intervals. Cannot rule out at least a
small chance of showers or thunderstorms later Monday night into
Tuesday, but plenty of dry hours are expected. Onset of WAA will
also draw warmer/more moist air mass into WI with max temperatures
on Tuesday in the lower 70s north/near Lake MI, middle to upper
70s south.

The forecast for the middle of next week remains muddled due to
the unknown influence of Pacific tropical system `Norma`, the
location of the upper ridge over the eastern CONUS and the eastern
extent of height falls from the western CONUS upper trough. The
GFS remains consistent in drawing the remains of `Norma`
northeastward toward the Great Lakes, while also sending a cold
front eastward to help focus showers/thunderstorms into WI
Wednesday afternoon/night. Meanwhile, the ECMWF keeps `Norma`
parked near Baja CA and keeps the upper ridge centered just to our
east with little in the way of precipitation Wednesday or
Thursday. Low confidence on how this will all play out as far as
precipitation trends go, but with the prevailing southwest flow
aloft, potential would exist for spotty chances for showers and
thunderstorms. One thing for sure is that temperatures will remain
about 10 degrees above normal for both Wednesday and Thursday.

.AVIATION...for 18Z TAF Issuance
Issued at 1137 AM CDT Fri Sep 15 2017

Mainly VFR conditions to continue this afternoon into
Saturday as the region is between high pressure drifting east and
a frontal system slowly approaching from the Northern Plains.
Surface winds have been slow to pick up today and with increasing
925 mb winds to 35 kts tonight, will add a period of low level
wind shear to the TAFS overnight. Even though upper heights
increase tonight, a few of the meso models develop a brief period
of convection over central and north central Wisconsin late
tonight as the LLJ works into the area.

Beach Hazards Statement until 8 PM CDT this evening for WIZ022-


SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......TDH is the U.S. government's official web portal to all federal, state and local government web resources and services.