Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 202242

Area Forecast Discussion
National Weather Service Green Bay WI
542 PM CDT Fri Oct 20 2017

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Saturday
Issued at 247 PM CDT Fri Oct 20 2017

Mild forecast temperatures tonight into Saturday and precipitation
trends for Saturday the focus.

Upper ridge continues to drift eastward while a long wave trough
works into the Western States. Surface dewpoints were gradually
increasing from the southwest despite a very dry airmass aloft and
some potential of mixing this afternoon. Developing LLJ was
lifting a warm dry air mass into the region on southwest winds.
Only high level clouds noted over the region with some mid level
clouds with weak returns showing up over Missouri. These mid level
clouds may work into western Wisconsin overnight, with a few
sprinkles possible toward daybreak. Progs have backed off on this
light pcpn for early Saturday, so left dry, but later shifts may
still need to consider adding a light pcpn mention for the morning

Progs continue to back off on the arrival of the showers for
Saturday, due to the initially dry air mass to overcome and a
later arrival time of the cold front. But still left a chance
over western areas due to somewhat unstable mid level lapse rates
and the boundary layer was gradually moistening up. Total totals
are progged to increase to the lower 50s Saturday afternoon which
include parts of northeast Wisconsin, but will keep the east dry
due to the air mass is expected to lag behind with saturating.

.LONG TERM...Saturday Night Through Friday
Issued at 247 PM CDT Fri Oct 20 2017

Model guidance agree on the development of a highly-amplified mean
flow consisting of an east-central North American upper trough,
sandwiched between strong upper ridges over the West Coast and the
western Atlantic. While there may be a brief break in this pattern
during the middle of next week (the upper trough shifts to eastern
North America), another upper trough is forecast to settle over
east-central North America by the end of the week. One forecast
issue is whether the initial upper trough will close off into an
upper low and if so, where? Another concern focuses on model
timing with several cold fronts that will pass through northeast
WI next week. The unseasonably warm weather is expected to end on
Tuesday as temperatures settle closer to normal for the rest of
the week. There is potential that max temperatures next Friday
could be 30 degrees colder than today.

The cold front is forecast to push east Saturday night, reaching
central WI toward daybreak. Meanwhile, the trailing upper trough
will only reach the Upper MS Valley by 12z Sunday. Plenty of low-
level moisture will be place over northeast WI as the front
arrives with dew points in the 50s to around 60 degrees and PW
values from 1.25-1.50 inches. Instability appears to be marginal
at best, but there will be strong shear (0-6km > 50 knots) along
and behind the front. Add in the lift from the cold front and the
right entrance region of the upper jet and you have a set-up for a
round of showers with a chance of thunderstorms Saturday night.
Due to the strength of the shear, a few of these storms could
become strong to severe, especially over central WI Saturday
evening where a marginal risk of severe storms exists. Strong to
damaging winds would be the primary threat. Min temperatures will
still be mild with readings in the lower to middle 50s central WI,
upper 50s to around 60 degrees eastern WI.

The cold front completes its passage across the rest of WI Sunday
morning, followed by the arrival of the weakening upper trough
Sunday afternoon. Showers should be ongoing over most of the
forecast area Sunday morning and gradually taper off from west to
east during the afternoon. In fact, central WI will see decreasing
clouds in the afternoon as cooler/drier air begins to overspread
the region. Have kept thunder out of the forecast for Sunday as
instability is nil, even though shear remains strong. Temperatures
will be interesting as central WI should see readings recover a
bit with the afternoon sun, while eastern WI could see
temperatures falling in the afternoon behind the cold front.

The clearing skies will continue through the evening, however the
next cold front is forecast to already be knocking on our door
(over the Upper MS Valley) later Sunday night, thus some clouds
could reach north-central WI during the pre-dawn hours. A
noticeable drop in temperatures can also be expected as readings
dip into the upper 30s to around 40 degrees north, lower to
middle 40s south. This latest cold front sweeps across WI on
Tuesday, but moisture appears to be seriously lacking so it is
conceivable that we could see a dry frontal passage. The concern
here is the trailing upper trough that will be digging southeast
into the western Great Lakes Monday afternoon. There may be enough
interaction between a surge of colder air and mid-level energy
within this trough, to kick off a few showers in the afternoon.
Have kept low pops in the forecast for now. Max temperatures on
Monday are expected to be above normal (one last time) with upper
50s to around 60 degrees north-central WI, mainly lower 60s

Model differences become prevalent starting Monday night as the
upper trough continues to strengthen over east-central North
America. The models disagree on where cyclogenesis will develop
near the Gulf Coast states before lifting northeast. The GFS shows
a later starting time for cyclogenesis, thus the resultant surface
low tracks farther east (up the Appalachians). The ECMWF has an
earlier start time, thus taking the surface low toward the eastern
Great Lakes. The latest CMC favors the ECMWF, so will keep a
chance of showers in the forecast for both Monday night into
Tuesday as the surface low continues to track northeast, a
cyclonic flow develops and colder air continues to get pulled into
WI. Max temperatures Tuesday to only reach the upper 40s to around
50 degrees central WI, mainly lower 50s eastern WI (near normal
for late October).

Models do agree on bringing a weak surface ridge into WI Tuesday
night, followed by a temporary change in the mean flow from
northwest to west on Wednesday. Even with rising upper heights,
weak low-level WAA and some mixed sunshine, temperatures are
expected to be similar to Tuesday with upper 40s to lower 50s
across northeast WI.

After a quiet Wednesday night, the next cold front is progged to
approach the western Great Lakes on Thursday. Models do not agree
on the timing of this front (GFS/CMC faster), thus impacting
precipitation chances over northeast WI. For now, have followed
the consensus solution, which brings a chance of showers generally
north and west of a line from Wisconsin Rapids to Wausaukee
Thursday afternoon. Max temperatures for Thursday to be in the
lower to middle 50s.

Due to the highly-amplified mean flow across the CONUS, this
latest cold front will be a slow-mover and could linger
precipitation chances Thursday night. If this does happen, the
rain could mix with or change to all snow across northern and
parts of central WI. Can not even rule out small precipitation
chances into Friday if this front slows down enough. Another surge
of cold air behind the cold front could only allow max
temperatures on Friday to reach the middle 40s central, upper 40s
for eastern WI.

.AVIATION...for 00Z TAF Issuance
Issued at 541 PM CDT Fri Oct 20 2017

VFR conditions are expected tonight through most of
Saturday, with mostly middle and high clouds. Low level wind shear
is expected tonight through 15z Saturday as south winds increase
to around 35 knots at 1000` with light south winds at the surface.
Clouds will increase Saturday afternoon with showers and
scattered thunderstorms arriving from west to east Saturday night
ahead of a slow moving cold front.



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