Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 252025

Area Forecast Discussion
National Weather Service Green Bay WI
325 PM CDT Tue Jul 25 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Wednesday
Issued at 325 PM CDT Tue Jul 25 2017

Radar echoes that were in western Wisconsin this morning
continued to move southeast into the forecast area today but
showers/sprinkles gradually died off in the much drier air mass in
central and eastern Wisconsin (as seen on the 12Z GRB sounding).
A separate area of showers and storms across northwest Wisconsin,
were the MUCAPE was mainly in the 500-1000 J/kg and there was
little or no CIN, was moving east-southeast. This will impact
parts of central Wisconsin, but with little or no CAPE in eastern
Wisconsin it should die off before reaching the Fox Valley.

An approaching frontal boundary combined with mid level short
wave energy will help to bring thunderstorms to the area for
tonight. The storms would impact northern Wisconsin first before
moving to the southeast during the night. SPC day 1 outlook has a
marginal risk of severe storms across much of the area (generally
west of the Fox Valley) and a slight risk in the western reaches
of Oneida County and Vilas County. Though there is some CAPE in
the western part of the area, and there is a decent amount of
surface to 3 km shear in western Wisconsin according to
mesoanalysis, there is not significant boundary and hires models
did not even forecast the convection that is currently moving into
central/north central Wisconsin. Would think the main threat is
late tonight with the surface boundary approaching.

As the front continues north to south on Wednesday the showers and
storms are expected to continue. Models do seem to shift the
focus more to the west of the area during the day, so confidence
in PoP forecast for Wednesday is below average.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 325 PM CDT Tue Jul 25 2017

Storms likely to be ongoing at the start of Wednesday evening
over most locations as a cold front drops southward over the area.
Instability and shear parameters may be enough for strong to
severe storms over parts of central and east central wisconsin in
the evening hours before the front drops south of the area
overnight. High pressure builds into the region from the north to
decrease clouds.

Medium range progs hint of a weak short wave trough dropping into
at least northern Wisconsin Thursday afternoon. Height 500 temp
data shows the NAM is very quick with dropping the cold core of
air aloft into central Wisconsin Thursday afternoon, while the
ECMWF and GFS still have the cold core over the Minnesota
Arrowhead late Thursday afternoon. Will confine any small pops to
northern wisconsin at this time with daytime heating. As heights
increase over the Plains States Thursday night into Friday, the
short wave trough is progged to turn into a nearly closed low over
northern Lake Michigan by Friday morning before drifting into
Lower Michigan in the afternoon. Will continue a dry forecast with
surface dewpoints in the lower 50s under the surface high
pressure regime.

Upper ridge then nudges into the area for the start of the
weekend while the surface high remains settled over the Great
Lakes Region. Progs attempt to drop a cold front into the area
later in the weekend with a northern stream short wave trough.
But coverage appears minimal and timing challenging, so will
handle with lower end pops for now.

.AVIATION...for 18Z TAF Issuance
Issued at 102 PM CDT Tue Jul 25 2017

Isolated/scattered showers or sprinkles are expected across central
and east central Wisconsin during the early part of the afternoon
as convection from the west moves into a much drier air mass.
There were some MVFR ceilings from stratocumulus that moved in
from the south, but this should either scatter out or the bases
should rise enough to reach the VFR category early this afternoon.
Otherwise, timing of convection is the biggest/most important
issue. Showers and some thunderstorms should mostly hold off until
evening, with more widespread thunderstorms along with MVFR/IFR
conditions later in the night. There were some differences in PoPs
among the models for Wednesday morning as frontal boundaries push
through the area. Have left thunderstorms out of the forecast
after about 12Z or 14Z due to the uncertainty factor and the
desire to avoid having thunder mentioned in the forecast for more
than the 6 or 8 straight hours already in the TAFs.



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