Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 231128 AAB
AFDGRB

Area Forecast Discussion...UPDATED
National Weather Service Green Bay WI
628 AM CDT Mon Oct 23 2017

New Information added to update section

.UPDATE...
Issued at 627 AM CDT Mon Oct 23 2017

Middle clouds have made it a little farther east than anticipated.
As a result, visibilities in central Wisconsin have come up. Will
adjust the Dense Fog Advisory accordingly.
&&

.SYNOPSIS...
Issued at 506 AM CDT Mon Oct 23 2017

Some dense fog this morning, otherwise today will be the lull
before a major autumn storm buffets the area with wind Tuesday.

The primary variation in the longwave pattern during the period
will be amplitude, as a ridge will remain along the West Coast
while a downstream trough set-up near 85W.

The unseasonably mild air is gone and won`t be coming back any
time soon. Temperatures will still be modestly above normal today,
then readings will oscillate between a little above to a little
below normal for the rest of the period. The proximity of the
longwave trough to the forecast area will result in several
opportunities for precipitation, the best of which will be
tomorrow. Amounts are likely to end up above normal for the
period.
&&

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 506 AM CDT Mon Oct 23 2017

Plenty of significant weather to deal with in the short term. The
first issue is fog. Despite a cirrus shield holding over eastern
Wisconsin, light winds and moisture from yesterday`s rains allowed
dense fog to form. The fog is generally east of a line from KY50
to KAUW to KIMT, and perhaps a little patchier over the western
edge of the advisory and in the lakeside counties. Conditions
looked bad enough on the webcams to warrant expanding the initial
advisory westward. Will continue to monitor the webcams and
visibilities, but do not anticipate further changes to the areal
extent of the advisory. Timing is another story. Although the
advisory currently runs through 900 am, conditions are far from
ideal for a quick burn-off of the fog this morning. Winds will
stay light, cirrus will remain over the top, and the late October
sun angle is pretty low. Would not be surprised if the day shifts
needs to extend it a few hours, perhaps only for areas near the
middle of the advisory area.

A second forecast issue today is whether or not the rain band
associated with the warm conveyor belt waves back into the far
southeast part of the forecast area later today. Most of the
guidance keeps it east, but it will be a close call. The rain
spreading east from Minnesota will slow as it near the area since
we are in a col in the upper wind field.

The next issue is the potential for strong winds on Tuesday. The
key will be to what degree the remnants of yesterday`s system
(which separated from the main flow and dug into the lower/middle
Mississippi Valley) phases with a strong jet roaring eastward
from the Pacific into the northern Plains. Models have toyed with
phasing these systems for several days, but there was never a
clear consensus among the models or good run-to-run consistency
of any individual model (except for the Canadian hemispheric which
was pretty insistent on total phasing). The differences are now
gone with the latest runs of all the models showing the total
phasing of the flow tonight and tomorrow. That fits very well with
observational data as 00Z upper air charts indicated a large area
of substantial upper height rises off the West Coast. That will
turn the Plains jet southeast. Meanwhile, water vapor imagery
indicates that the Mississippi Valley shortwave is swinging around
into a negative tilt and beginning to eject northeastward. That
will cause the surface low currently over southeast Illinois to
track NNE into Lower Michigan. Given excellent upper support,
rapid intensification of the cyclone is likely, hence the
potential for strong winds.

After relatively light winds today, winds will increase
substantially tonight. The strongest winds will likely occur on
Tuesday, when gusts around 45 mph are likely. A Wind Advisory
will likely be needed, but will allow the day shift one last look
to line up the details and clear through the Dense Fog Advisory
before issuing it. Destabilization of low-levels as colder air
crosses the northern Bay of Green Bay will result in some mixing
into 60+ kt winds aloft, so issued a High Wind Watch for the Door.
A key factor with winds in the Door will be the exact track of
the cyclone. Some of the models swipe it back westward over/near
the northern tip of the Door as it wraps around the tightening upper
system. That could result in a lull in the winds. Even if it
occurs, the lull would be brief and timing of it uncertain, so did
not attempt to work it into the forecast.

Rains is also expected to be pretty widespread on Tuesday as the
cyclone deepens to our east. The boundary layer may cool enough
for some snow to mix with the rain over the far north late in the
day. Max/Min temperature grids were taken from hourly grids
generated primarily from raw model output. That technique seems to
work well with strong synoptic systems such as this when
advection is the primary process driving temperature changes.

.LONG TERM...Tuesday Night Through Sunday
Issued at 333 AM CDT Mon Oct 23 2017

Models have come into better agreement with a strong fall storm
moving away from the region Tuesday night. Core of strongest winds
aloft still noted across northeast Wisconsin at 7 pm Tuesday.
Thus, the strong winds will continue into early Tuesday evening
especially across Door County where winds could still gust close
to 60 mph. The winds should gradually diminish later in the
evening into early Wednesday morning, although gusts to around 35
mph are still possible across northern Door County. Across the
north Tuesday night, some light snow is expected at times with a
light dusting of snow possible across Vilas County.

On Wednesday, winds will continue to subside as high pressure
builds into the region. High temperatures will be below normal for
late October, something that we haven`t seen since the first half
of September. The next system approaches from the northwest
Thursday afternoon and moves across the area Thursday night. Most
of the precipitation should fall as rain, though could end as a
little snow across the north. Upper low on the ECMWF swings
across the area on Friday, bringing continued chances of showers.
May be a little snow early in the day across the north. 500mb
trough lingers across the western Great Lakes region next weekend,
thus unsettled conditions are possible at times. Confidence is
low in the timing of features in the mean flow. Below normal
temperatures are expected for much of the period.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 506 AM CDT Mon Oct 23 2017

Fog will be the major aviation forecast issue through at least
mid-morning. Winds along with MVFR or IFR ceilings will become an
issue overnight as the strong cyclone approaches from the south.
&&

.MARINE...
Issued at 333 AM CDT Mon Oct 23 2017

A strong fall storm is expected to move across the Great Lakes
region Monday night into Tuesday night. The latest model runs
have come into better agreement, thus high confidence that a high
end gale event will occur from late Monday evening into Wednesday
morning. The strongest winds are expected on Tuesday into Tuesday
evening. Storm force gusts are possible Tuesday into Tuesday
evening across the coastal waters of northern Door County. It is
possible that this area may need to be upgraded to a storm warning
later today or tonight. Hazardous conditions for small craft are
possible Thursday and Friday.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning for WIZ011>013-
020>022-031-037>040-045-048>050-073-074.

High Wind Watch from Tuesday morning through Tuesday evening for
WIZ022.
&&

$$

UPDATE.........Skowronski
SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Eckberg
AVIATION.......Skowronski
MARINE.........Eckberg



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