Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 121750
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1250 PM CDT Tue Sep 12 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 341 AM CDT Tue Sep 12 2017

Mild and dry weather to continue for several more days.

The main westerlies across North America have pulled almost
totally into Canada, leaving weak upper flow across most of the
CONUS. That will persist for a couple more days, then a large
scale trough will gradually develop over the intermountain states.
Ridging will increase across the forecast area late this week and
early this weekend as the trough forms out west. Energy ejecting
out of the trough will probably begin to affect the area during
the weekend or next week.

Temperatures will remain considerably above normal through the
weekend, then drop back closer to seasonal normals next week.
Several more dry days are expected, with the only real opportunity
for precipitation during the period coming late in the weekend as
some upper level energy and a frontal system cross the area. So
amounts will probably end up below normal.
&&

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 341 AM CDT Tue Sep 12 2017

Some fluctuations in visibility will continue, but overall,
visibilities across north-central Wisconsin were low enough to
support a dense fog advisory early this morning. Went a couple
hours past sunrise since very weak flow aloft means we`ll likely
need to burn through the fog without much help mechanically
mixing it out.

Conditions may be even more favorable for fog tonight as near
calm conditions will exist up through 850 mb. Carried patchy/areas
of fog across the whole area, but will leave it to later shifts
to decide if we`ll need another dense fog advisory.

Otherwise, the main forecast issue is temperatures. The remnants
of a frontal boundary will continue sliding across the area this
morning, but there won`t really be a change in air mass.
Temperatures aloft will be a bit warmer than yesterday, and 850
mb temperatures across north-central Wisconsin may edge up a
couple more degrees Wednesday. For maxes, mixed together the
broad-based blend of guidance products and the recent top
performers. Used a similar approach for mins, though lowered
readings a little more in the climatological cool spots.

.LONG TERM...Wednesday Night Through Monday
Issued at 341 AM CDT Tue Sep 12 2017

The remnants of `Irma` are forecast to move from the Ohio Valley
to the New England states during the Wednesday night-Friday night
time frame. Meanwhile, a positive-tilted upper trough from central
Canada to the central Rockies is progged to lift northeastward
this weekend and help to push a cold front into the Great Lakes.
This cold front will be our next weather maker, bringing a chance
of showers/thunderstorms to northeast WI Saturday night into
Sunday. A prevailing southwest flow aloft ahead of the upper
trough will keep temperatures above normal through Sunday, before
cooling down closer to normal early next week.

The remains of `Irma` are forecast to be in the upper reaches of
the Ohio Valley Wednesday night with a movement to the northeast.
While `Irma` will not affect our weather, there are two concerns
for Wednesday night. The first concern is the very slim chance for
a shower across far northern WI in associated with a warm front.
Right now, the GFS is the only model suggesting this possibility,
thus will keep the forecast dry for now. The other concern will be
fog potential with mostly clear skies, light winds and
temperature/dew point values close to one another. May need to hit
the fog a bit harder in the forecast. Min temperatures to mainly
be in the lower to middle 50s north, middle to upper 50s south.

Northeast WI to remain between `Irma` to our east and the upper
trough/cold front to our west through Thursday. While north-
central WI could see more clouds than the rest of the forecast
area, do not see any reason to deviate from the dry forecast under
mostly sunny to partly cloudy skies. The prevailing southerly
winds will continue to slowly advect more moisture into WI with
dew points reaching the upper 50s to lower 60s. Max temperatures
to continue running above normal with lower to middle 70s near
Lake MI, middle to upper 70s elsewhere.

As the cold front slowly edges closer to WI Thursday night, expect
to see more clouds slide toward central WI, however any
precipitation would remain to our north and west. A mild night on
tap with a bit more clouds and a bit more wind than previous
nights. Look for readings to only drop into the middle 50s north,
55-60 degree range south (about 10 degrees above normal). The
digging of the upper trough over the Rockies is actually going to
raise upper heights over the Great Lakes region on Friday and
briefly stall the cold front over the Upper MS Valley. This would
bring a continuation of partly cloudy skies central WI, mostly
sunny skies eastern WI with temperatures almost 10 degrees above
normal. This would bring max temperatures into the lower to middle
70s near Lake MI, but middle 70s to around 80 degrees over the
rest of northeast WI.

After a quiet Friday night, the models begin to lift the upper
trough into the northern/central Plains on Saturday. There are
some questions as to how far east the cold front will move during
the day, but the bottom line is that the boundary would remain to
our west and keep any precipitation away. If we can get enough
sunshine to hold, parts of northeast WI could see max temperatures
reach the lower 80s which is more typical of late July.

Even though the best forcing associated with the upper trough is
progged to stay well to our northwest Saturday night, the cold
front will push east into western WI after midnight and then trek
into eastern WI on Sunday. Expect to see shower/thunderstorm
activity gradually overspread the forecast area and we may need to
watch these storms on Sunday with the models showing moderate
instability and 30-35 knots of shear. More clouds with
precipitation will impact temperatures with readings on Sunday
only in the upper 60s to lower 70s north, lower to middle 70s
south. The chance for showers/thunderstorms are expected to
continue over eastern WI Sunday evening until the cold front
clears the area. High pressure is then forecast to build into the
region later Sunday night into Monday, bringing dry/cooler
conditions to start the new work week. Max temperatures on Monday
will be closer to normal for mid-September with readings mainly in
the 65-70 degree range.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1249 PM CDT Tue Sep 12 2017

excellent flying weather will continue through the
evening hours. Areas of ground fog are likely again tonight, with
conditions below minimums between 08z and 13z possible at some
locations. Whatever fog does materialize should dissipate by 14z
with VFR conditions Wednesday afternoon and evening.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......RDM



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