Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 221959

Area Forecast Discussion
National Weather Service Green Bay WI
259 PM CDT Sat Jul 22 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Sunday
Issued at 259 PM CDT Sat Jul 22 2017

Chance for, and strength of, thunderstorms late this afternoon and
tonight are the main concerns in the short term.

Fog from early this morning dissipated and low clouds were
gradually breaking up throughout the day. This allowed instability
to increase across the area during the late morning and afternoon.
Mesoanalysis as of 19Z had MUCAPE values generally in excess of
1000 J/kg range across the western part of the forecast area and
CIN values across north central Wisconsin were 25 J/kg or less.
Nothing had developed in the area or nearby as of 19Z, but there
was no shortage of boundaries, with a stationary front along the
Wisconsin/Illinois border, another across Lake Superior and Upper
Michigan, and a trough in eastern Minnesota. Wind directions
varied considerably across the area but speeds were light,
resulting in weak convergence at several locations. Dew points
were generally in the mid 60s to around 70.

Models were showing the surface low near the Minnesota/Manitoba
border passing north of the area and dragging a trough/cold front
through the state as a 500mb trough moves across Wisconsin with a
500mb low passing to the north of the state. These features
should be sufficient to get thunderstorms into the area later
today/tonight. Latest versions of the HRRR and RAP showed
development in northern Wisconsin around 23Z, just to the west of
the forecast area. Convection then moves into north central
Wisconsin with a line developing to the southwest as it makes its
way across the rest of the area during the late evening/nighttime

SPC day 1 convective outlook had most of the area in a slight
risk of severe with a marginal risk for Door County and the rest
of the lakeshore due to "robust speed shear."

Convection is forecast to exit the area overnight but there are
still some slight chance/low end chance PoPs in the far east and
north in cyclonic flow around the departing system.

.LONG TERM...Sunday Night Through Saturday
Issued at 259 PM CDT Sat Jul 22 2017

High pressure regimes will dominate much of the weather next week
with the exception of a frontal system with upper support from
Tuesday evening into Wednesday evening. Temperatures expected to
mostly be near seasonable levels.

Moist cyclonic flow with the departing deep upper trough or
nearly closed low may keep a few showers around Sunday evening
until daytime heating wanes. Clearing skies later Sunday night
with cooler overnight low temperatures in the 40s north to lower
and mid 50s central, may lead to fog formation by early Monday
morning. Otherwise a cool comfortable and drier air mass will
filter into the area for the start of new work week as a high
pressure system drifts into the western Great Lakes region.

Next chance for precipitation and possible strong to severe storms
during the Tuesday evening into wednesday evening time frame. A
strong short wave trough, near the Canadian Pacific coast line
this Saturday afternoon as per water vapor loop, is progged to
drop over the Great Lakes region late Tuesday into Wednesday. The
850 warm air return and possibly the start of the convection
begins to reach north central Wisconsin Tuesday afternoon and
evening. The short wave trough then drags a cold front across the
state on Wednesday, dropping the front south of the forecast area
later Wednesday evening.

Another high pressure system will then build into the area for
the remainder of the work week and perhaps remain settled over
the region into the start of next weekend.

.AVIATION...for 18Z TAF Issuance
Issued at 1238 PM CDT Sat Jul 22 2017

Areas of fog dissipated this morning and low clouds were slowly
breaking up too. Think that the low clouds will become scattered
this afternoon. Loss of the MVFR and lower end VFR ceilings will
allow the atmosphere to more easily destabilize during the
afternoon into early evening. Convective development is a concern
for later in the afternoon with increasing instability, an
abundance of boundaries, an approaching surface system, and a mid
level trough passing through Wisconsin. The best chances for
storms look to be in the late afternoon/early evening across north
central and central Wisconsin, and late evening/overnight farther
to the east. Expect MVFR or IFR conditions with thunderstorms and
some MVFR fog is likely later in the night. Should be VFR after
fog dissipates in the morning.



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