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000
FXUS63 KGRB 201719
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1219 PM CDT Tue Jun 20 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 343 AM CDT Tue Jun 20 2017

an upper jet moving through cyclonic flow will produce scattered
showers and isolated thunderstorms today, especially across the
southern part of the forecast area. A few of the storms could
produce small hail and gusty winds. Highs today will be a few
degrees below normal due to clouds and scattered showers.

Surface high pressure moves into Wisconsin tonight and should
permit for clear skies and cooler than normal temperatures. Ground
fog is possible in central and northcentral Wisconsin late
tonight.

Wednesday looks to be partly sunny and pleasant as the surface
high drifts off to the east.

.LONG TERM...Wednesday Night Through Monday
Issued at 343 AM CDT Tue Jun 20 2017

Persistent upper ridging to remain locked in place from the
eastern Pacific to the southwest CONUS through at least Saturday
before starting to weaken in response to an upper trough digging
southeast toward British Columbia. There will also be an upper
ridge across the western Atlantic, leaving a progressive upper
trough from the northern Rockies (Friday) to the Great Lakes
(Sunday).This pattern will continue to send periodic systems
across northeast WI, thus timing of these systems continues to
be the main forecast challenge. Temperatures are expected to be
slightly above normal through Friday, then settle below normal
next weekend/Monday as the upper trough arrives.

Showers/thunderstorms are expected to sweep across northeast WI
Wednesday night as a cold front moving into the region, aided by
WAA on a 40 knot southwest low-level jet. While instability
appears marginal at this time, there is strong 0-6km bulk shear
with values ranging from 35-60 knots (highest values north).
Therefore, we will have to watch for a few stronger to severe
storms possible and SPC has placed much of the area in a marginal
risk Wednesday night. Min temperatures to range from the middle to
upper 50s north, upper 50s to lower 60s south.

The cold front is forecast to slow its forward speed and
eventually become more orientated west to east as it encounters
both the upper ridge over the southeast CONUS and a tropical
system over the Deep South. Exactly where this front is situated
on Friday will determine how much precipitation we will see,
primarily over central/east-central WI. The higher shower/
thunderstorm potential will be over about the southern third of
the state, while northern WI should see improving conditions with
a little afternoon sunshine. Max temperatures to be in the middle
70s north-central WI, upper 70s to around 80 degrees elsewhere.

This frontal boundary is expected to slip a little farther south
Thursday night, but not far enough where our southern counties can
avoid a continued chance of mainly rain showers as daytime heating
wanes. Meanwhile, northern WI appears to sit under mostly clear to
partly cloudy skies with a light west wind. Would not be surprised
to see a few locations drop into the upper 40s north-central.
Otherwise, look for min temperatures in the lower 50s north,
middle 50s to around 60 degrees south. Much of Friday appears dry
at the moment as northeast WI to reside between systems. There are
subtle differences among the models with regards to the strength
of a shortwave trough moving across the northern Plains into the
Upper MS Valley by 00z Saturday. The GFS is much more aggressive
with this system compared to the ECMWF, so we will have to see how
this plays out. Max temperatures for Friday should be close to
seasonal normals with lower to middle 70s north, middle to upper
70s south.

This initial shortwave trough to then move across WI Friday night
without much fanfare as forcing/instability are weak and moisture
is lacking. Attention then turns to the main upper trough forecast
to dig across the northern Rockies into the northern Plains on
Saturday. While better forcing to be located well to our west,
onset of WAA with marginal instability could bring showers/storms
back into the forecast, especially by Saturday afternoon.
Increasing clouds with the precipitation threat will limit diurnal
warming with max temperatures Saturday only in the upper 60s to
around 70 degrees north/lakeshore, 70-75 degree range south.

Best chance of seeing showers/thunderstorms appears to be Saturday
night into Sunday as this upper trough shifts east across the
Great Lakes region. That being said, it will not rain all the time
with dry hours in between. Temperatures through the end of the
weekend will remain cool with temperatures similar to Saturday. By
early next week, northeast WI to find itself on the cooler,
cyclonic side of this upper trough which will bring conditions
similar to current conditions of this week. Widely scattered
showers Sunday night will give way to a chance of showers/
afternoon thunderstorms on Monday as instability increases due to
daytime heating. Perhaps a slight uptick on temperatures by Monday
with most locations in the 70-75 degree range (a few upper 70s
possible west of the Fox Valley).
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1210 PM CDT Tue Jun 20 2017

Scattered showers and a few thunderstorms will come to an end
later this afternoon or early this evening, mainly across
northeast and east-central Wisconsin. High pressure will bring
clear skies during the late evening and overnight hours. Latest
numerical guidance would not suggest any fog tonight, but the
combination of clear skies and light winds would allow for some
patchy fog to develop. Any fog should burn off by 13z on
Wednesday. Attention then turns to another complex of
thunderstorms that should arrive late Wednesday afternoon or more
likely Wednesday evening across central Wisconsin, then move
across northeast Wisconsin later Wednesday evening into early
Thursday morning. Strong to severe thunderstorms with large hail
or strong damaging winds are possible. Stay tuned!

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....RDM
LONG TERM......Kallas
AVIATION.......Eckberg