Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 131918

Area Forecast Discussion
National Weather Service Green Bay WI
218 PM CDT Wed Sep 13 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Thursday
Issued at 217 PM CDT Wed Sep 13 2017

Primary issue for the rest of this afternoon as well as into
Thursday will be the fog coverage and intensity.

Water temperatures this afternoon along the Lake Michigan
shoreline were in the mid 60s while surface temperatures just to
the west were approaching 80. Surface dewpoints were in the lower
60s over East Central Wisconsin. Healthy inversion continues to
produce dense fog along the lake michigan shoreline this afternoon
and the slight southeast flow continues to drift the fog
northward. Have expanded the dense fog advisory for the nearshore
waters. As the inversion weakens toward evening anticipate fog to
decrease some.

But the potential for fog will return overnight. Light winds and
clear skies under a diminishing ridge of high pressure may again
produce fog across the area, especially over eastern Wisconsin
within the ridge. A little more mixing to the west due to an
approaching frontal system may produce less fog with respect to
duration and coverage.

Fog expected to diminish Thursday morning with further mixing and
then return to above normal temperatures and perhaps some
increase with high level clouds from the west.

.LONG TERM...Thursday Night Through Wednesday
Issued at 217 PM CDT Wed Sep 13 2017

Main focus will be on precipitation chances through the weekend.
Then deciphering model differences in shower chances and
temperatures next week.

Continued chance of showers across northern Wisconsin at the
start of the period, Thursday evening through Friday morning. This
is due to the surface front that is stationed north of Wisconsin.
Models still differ on whether any pcpn will reach this far
south, but with the boundary hanging around they cannot be ruled
out. Fog should be less of a concern by Friday morning through
the weekend as winds pick up.

As a ridge builds across the area on Friday afternoon this will
push the surface front north. Dry weather is then expected
through Saturday afternoon. At the same time, southerly winds
will be on the increase. This will boost temps and dew points even
a few more degrees above normal. Saturday looks rather humid for
mid- September with dew points in the 60 to 65 degree range.

Models agree on bringing a surface cold front through the area
Saturday evening into Sunday. There is slightly more agreement on
precip timing, with the mostly likely time frame from 00Z to 18Z
Sunday. Showers and thunderstorms are possible as the front comes
through. It is too early to determine if severe storms are
possible. Right now, there is some instability and shear, however
the timing is not ideal for stronger storms.

GFS, ECMWF, and Canadian show surface high pressure and mid-
level ridging immediately following the front passage, and then
diverge from there. These difference seem to stem in how `Jose`
is handled. The EC is much slower in pushing this system back into
the Atlantic, and as a result, the mid-level ridge continues to
build across the Great Lakes and the trough in the western CONUS
deepens. This scenario would keep Wisconsin dry through the
forecast period. The GFS and Canadian are more progressive with
bringing Jose out into the Atlantic, which allows for a more
active pattern across Wisconsin with various waves bringing
chances for showers through the week. Therefore, have continued
with a blended model solution until details become more clear.

Temperatures will be well above seasonal normals through Saturday,
with highs generally in the low to mid 80s (70s along Lake
Michigan) and lows in the upper 50s to mid 60s. Temperatures fall
back some after the cold front passes on Sunday, but still remain
above normal. Temperatures beyond Monday are more uncertain, once
again due to the model differences, but look to remain above
normal. The question will be how much above normal.

.AVIATION...for 18Z TAF Issuance
Issued at 1150 AM CDT Wed Sep 13 2017

The last band of inland IFR/LIFR conditions due to
fog from Shawano to Lake Poygan dissipated late morning. More
persistent band of fog along the Lake Michigan shore will linger
this afternoon. MTW may see a period of IFR vsbys due fog this
afternoon. Otherwise besides the localized fog over eastern
Wisconsin, VFR conditions will dominate this afternoon into
evening. Clear skies and light winds will produce another night
of fog, especially over eastern Wisconsin within a narrowing
ridge of high pressure. A bit more boundary layer wind due to an
approaching frontal boundary from the Northern Plains may diminish
the amount of time or intensity of the fog over central and north
central Wisconsin. The fog should dissipate again around 14z



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