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000
FXUS63 KGRB 181745
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1245 PM CDT Mon Sep 18 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 355 AM CDT Mon Sep 18 2017

After a brief cool down today, temperatures will return to
unseasonably warm levels for the rest of the week and the upcoming
weekend.

The large scale pattern with a western trough and an eastern ridge
across North America will remain pretty much intact throughout the
forecast period. Though a slow eastward progression of the trough
may begin late, the primary change will be amplitude. The pattern
will gradually gain amplitude during the work week, then remain
highly amplified into next weekend.

The strengthening southwest upper flow between the trough out to
the west and the ridge just to our east will feed very warm air
from the Plains into the area. So after seasonable temperatures
with a Canadian air mass today, readings will begin heading upward
Tuesday. Temperatures 10-20F degrees above normal are expected
for most of the rest of the period. The most significant
precipitation event will likely be Wednesday and Wednesday night
as a weakening frontal system crosses the area. Although a few
locations will probably get decent rains during the next week, it
looks like amounts at most locations will be AOB normal for the
period despite the warm moist air residing across the area.
&&

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 355 AM CDT Mon Sep 18 2017

Patchy fog was across the north early this morning. Some
sprinkles/light showers were also moving toward that area. The
precipitation was falling from high-based clouds. Will carry low
PoPs to about mid-morning to account for what`s on radar now.

Otherwise, models have backed down on the precipitation with the
incoming shortwave. Forcing still looks multifaceted but on the
weak side. The main limiting factor appears to be dry air at low-
levels. Scaled back a little on PoPs, with the best chance being
across the west and north. The ECMWF and Canadian NH were about
the only models still getting precipitation into the east, but
even they appear to dry the precipitation up as it arrives. Kept
low PoPs in the east since the ECMWF is generally one of the top
performing models. Thereafter, kept Tuesday dry though there was
some support for precipitation developing over southeast Wisconsin
possibly clipping east-central Wisconsin during the afternoon.

Stayed pretty close to a broad-based blend of guidance products
for temperatures.

.LONG TERM...Tuesday Night Through Sunday
Issued at 355 AM CDT Mon Sep 18 2017

The large-scale mean flow will remain highly-amplified through
next weekend, consisting of an eastern Pacific upper ridge, a
western CONUS upper trough and an eastern CONUS upper ridge. This
pattern would keep northeast WI under a southwest flow aloft with
the crux of the forecast dependent on the location of the upper
ridge in proximity of the Great Lakes. Models still differ in this
regard and this would be the difference between mild/unsettled
versus warm/drier pattern. Temperatures will be above normal
through the week, it is just a matter of how far above normal.

Shower/thunderstorm chances look to gradually increase later
Tuesday night as a warm front lifts north into WI and we see a
surge of WAA spread into the region. The better chance of seeing
precipitation to be over central WI where the stronger lift to
exist, along with better instability. Thicker clouds and a
southeast wind around 10 mph will bring a mild night to the
forecast area with min temperatures in the middle to upper 50s
north, generally around the 60 degree mark south. A cold front is
then forecast to move into the western Great Lakes on Wednesday,
reaching central WI by 00z Thursday. Anticipate showers and
thunderstorms to be on the increase through the day and we will
have to watch these storms as the models indicate MUCAPES at
around 2K J/KG and bulk shear in the 30-40 knot range. Moisture
will not be a problem with dew points rising through the 60s and
PW values reaching around 1.75". Therefore, a severe threat cannot
be ruled out as long as the timing of the cold front does not
change. Max temperatures on Wednesday will be warm with middle 70s
north-central/near Lake MI, lower 80s south.

This cold front is progged to slow down Wednesday night as it
begins to encounter the stout upper ridge axis sitting over the
eastern Great Lakes. Expect additional showers/thunderstorms
across northeast WI and any severe threat may turn into a locally
heavy rain event as moisture pools along the frontal boundary.
Something to watch over the next couple of days. Min temperatures
to range from around 50 degrees over parts of north-central WI, to
the lower 60s across east-central WI. The cold front essentially
stalls or even begins to head back north as a warm front on
Thursday as the prevailing southwest winds aloft continues into
WI. Since this boundary to still be in our general vicinity,
cannot rule out a few showers or thunderstorms somewhere across
the forecast area. Otherwise, expect a warm/somewhat humid day
with max temperatures in the middle 70s near Lake MI, upper 70s
north and mainly the 80-85 degree range south.

As more energy digs into the upper trough, the amplified pattern
gets even stronger with the end result being a blocking pattern by
the end of the week. This would be synoptically similar to what
we just experienced with a quasi-stationary front located to our
north and a cold front stuck well to our west over the Plains.
Under this scenario, temperatures would be well-above normal once
again with readings in the middle to upper 70s near Lake MI, lower
to middle 80s elsewhere on Friday. This blocking pattern would
continue into next weekend, although the upper ridge may tend to
weaken a bit on its western fringe by next Sunday. If this were to
occur, a small chance of showers/thunderstorms may approach
north-central WI as the cold front slowly edges east into the
Upper MS Valley. Otherwise, the unseasonably warm/muggy conditions
will persist through Saturday with a slight cool down on Sunday
with more clouds around.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1242 PM CDT Mon Sep 18 2017

Still some locations with MVFR ceilings around midday today, but
would expect conditions to become VFR within the first couple of
hours of the TAF valid period. Radar mosaic and MRMS showed a
northeast to southwest area of showers from far western Upper
Michigan to eastern Kansas. This area of rain will gradually make
its way eastward as a mid level short wave moves through northern
Wisconsin this afternoon/evening. There were a variety of ceilings
upstream from the forecast area, but have kept VFR conditions
going in the TAFs. Any locations that receive a significant amount
of rain could end up with some MVFR or IFR fog overnight, but it
should dissipate shortly after sunrise. Otherwise VFR conditions
should prevail through Tuesday.
&&

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

$$

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