Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 192322

Area Forecast Discussion
National Weather Service Green Bay WI
622 PM CDT Wed Jul 19 2017

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Thursday
Issued at 244 PM CDT Wed Jul 19 2017

Quiet weather through the evening hours as a surface
high over Lake Michigan moves slowly east. The forecast is more
difficult after that, as there are very large differences in the
computer model placement of the axis of heaviest precipitation
late tonight in association with a convective complex that should
move across WIsconsin. The NAM, Canadian and ECMWF place the heaviest
precipitation across our forecast area, while the GFS has it mostly
to our south.

We think the GFS might be most correct, as the convection over
southern Minnesota at 20Z may inhibit the northward return of the
surface warm front, and may keep deeper moisture from moving
north. WPC is also siding with the GFS, and it also has support
from the CIPS analogs from Saint louis University. If this
reasoning is correct, the severe threat and heaviest rains should
stay south of here tonight, though we should still get a half
inch or more of rain. If the northern track should be correct,
there could possibly be some urban and small stream flooding.

Showers should end early Thursday, with some clearing by midday.
Even though a weak surface front will have moved through, it will
just as warm tomorrow as today, with slightly higher humidity.

.LONG TERM...Thursday Night Through Wednesday
Issued at 244 PM CDT Wed Jul 19 2017

Focus of this forecast mainly revolves around thunderstorm chances
and severe potential on Friday and Saturday.  No clear cut model of
choice in this forecast cycle, but will put slightly more weight into
the ecmwf.

Thursday night through Saturday...With seasonably fast west to east
flow in the jet stream present along the US-Canadian border, will
see a stalled front return northward over the northern Plains on
Thursday night, where thunderstorms are likely. Over Wisconsin, weak
high pressure will provide a mostly clear and seasonable night.
Quiet conditions will give way to more active weather as the front
continues to lift northward and approach central WI by Friday
afternoon.  With ample forcing over the boundary via low level jet
and the region in the right rear quad of the jet streak,
thunderstorms look to return to central and north-central WI late on
Friday afternoon, and the rest of the area on Friday night.  Highest
pwats and instability favor the possibility of heavy rainfall over
central and east-central WI during this time.  However, questions
remain in regards to the degree of instability and overall severe
threat.  Progged soundings indicate relatively high freezing levels
and skinny cape profiles that suggests a severe threat that is quite
low.  An inch or two of rain is not out of the question though.

Attention then turns to the possibility of thunderstorms on Saturday
afternoon and evening.  A rather potent shortwave trough will be
diving southeast over Lake Superior and northern WI during this
time.  It appears that there could be enough instability for a
chance of severe weather over northern WI, and 0-6km shear of 40 kts
is more than sufficient.  This area is already highlighted in the
day 4 SPC convective outlook.

Rest of the forecast...Following the passage of the shortwave, high
pressure builds in for Sunday and will stick around through early
next week.  Other than a few showers sticking around for Sunday
beneath upper troughing, looks like a several day period of quiet
and seasonable weather.  As the high departs, return flow will bring
the next chance of storms by midweek.

.AVIATION...for 00Z TAF Issuance
Issued at 622 PM CDT Wed Jul 19 2017

The northern fringed of a thunderstorm complex will track through
the extreme southern cwa early this evening. At this time it
appears it should stay south of GRB and may only affect ATW and
MTW. Otherwise additional showers and thunderstorms should arrive
from west to east after midnight and bring MVFR and local IFR
conditions. Low level wind shear possible after midnight with
light south surface winds at south winds around 30 knots at 1000
ft agl. Flight weather should improve quickly Thursday, with VFR
by the midday hours.



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