Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 040524
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1124 PM CST Sun Dec 3 2017

Updated aviation portion for 06Z TAF issuance

.SYNOPSIS...
Issued at 218 PM CST Sun Dec 3 2017

Warm and becoming windy with periods of rain tonight through
Monday, very windy Monday night, then much colder for the rest of
the week.

Amplification of the large scale flow is now underway. During the
next several days, a sharp ridge will build along the West Coast,
forcing a deep trough near 90W. This round of amplification should
peak by the start of the weekend. Though perhaps losing a little
amplitude thereafter, the high amplitude pattern is likely to
persist for at least an additional week.

Temperatures will remain 20-25 F degrees above normal through
tomorrow, then drop back to 5 to 10 F degrees below normal by
mid-week. The below normal temperatures will linger for the
remainder of the forecast period, and quite possibly to the middle
of the month. The bulk of the precipitation will occur as a
strong cyclone crosses the region the next couple days. The system
will access Gulf moisture and should be a pretty good
precipitation producer. But having the dry slot drive right
through the forecast area will temper amounts. Once that system
departs, there are likely to be several additional light
precipitation events as shortwaves rotate through the deep eastern
North America upper trough. It`s reasonable to expect that most
of the area to end up with near or slightly above normal
precipitation amounts for the period.
&&

.SHORT TERM...Tonight and Monday
Issued at 218 PM CST Sun Dec 3 2017

The latest RAP analysis and satellite/radar imagery show flattened
ridging across the western Great Lakes early this afternoon, ahead
of a deep trough digging over the western conus. The jet stream is
already spreading a thick layer of mid and high clouds across the
northern Mississippi Valley ahead of this trough. With broad
southerly winds developing across the Plains, the leading edge
of Gulf moisture is lifting north into northern Missouri. As the
deep trough and associated strengthening surface low move east,
forecast concerns revolve around precip trends, chances for
thunderstorms, and strong winds on the Bay and Lake.

Tonight...Low pressure will emerge from the Rockies and move east
across South Dakota/Nebraska. Ahead of the low, mid and high
clouds are anticipated to lower some, while Gulf moisture surges
northeast across the area during the late evening and overnight
hours. The latest guidance and progged soundings suggest that
precip may start out as a drizzle over north-central WI late in
the evening, before increasing saturation depth changes precip to
rain showers overnight. Showers to become more widespread
overnight, with elevated instability increasing late, and think
there could be a small chance of a thunderstorm over central WI
and the southern Fox Valley after 3 am. Temps are expected to
remain relatively steady this evening, and then rise overnight.

Monday...The low pressure system will strengthen as it swings
northeast towards western Lake Superior. With elevated instability
increasing up to 500 j/kg and pwats above 1 inch, expect showers
and a few thunderstorms to remain widespread during the morning,
particularly over northern WI. The mid-level dry slot then lifts
into the area during the afternoon, which supports ending of the
heavier shower activity, but plenty of low level moisture should
maintain a light shower/drizzle scenario. It will become warm and
windy, with some gusts between 30 to 40 mph possible over the Fox
Valley and Lakeshore. Highs ranging from the low to mid 50s.

.LONG TERM...Monday Night Through Sunday
Issued at 218 PM CST Sun Dec 3 2017

An intense cyclone will track from the western tip of Lake
Superior early Monday evening, to central Ontario by mid-day
Tuesday. A cold front wrapping around the system will surge across
the area early Tuesday evening. Timing of the FROPA is now
forecast to occur a little more quickly than the models indicated
yesterday. Regardless, the main impact will be wind. Expect a
surge of 45-55 mph wind gusts along and immediately behind the
front. This could result in some downed trees and power lines,
especially across the north where coniferous trees are more
prevalent. Will continue to highlight potential wind impacts in
the HWO.

Although there are still some model differences with the track of
the cyclone, the consensus is now that the track will be far
enough west and the movement of the system quick enough that the
cusp of the comma head will lift through to the west of the
forecast area rather than pivot back into north-central Wisconsin.
That will limit the snow potential across the north. But even
just an inch of snow could make for difficult travel conditions
across the north Tuesday given the wind.

Though timing remains elusive, there will likely be additional
light precipitation episodes during the remainder of the period as
shortwaves rotate through the long-wave trough entrenched across
the region. Temperatures will be cold enough for snow, and a light
snow cover could gradually begin to become established across the
region. Temperatures, especially at night, may require downward
adjustment in future forecasts if a snowcover becomes established.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1124 PM CST Sun Dec 3 2017

VFR conditions will deteriorate overnight as cigs lower to mvfr
levels and rain becomes widespread as a low pressure system
approaches from the west. Widespread rain will continue during the
day on Monday as conditions lower to IFR. Conditions are expected
to improve a bit to MVFR Monday evening as the low lifts to the
north and south to southwest surface winds gust to 30 to 35 knots.
LLWS conditions expected tonight and Monday. The only reason LLWS
does not remain Monday evening is that surface winds will increase
close to the levels in the lower atmosphere.
&&

.MARINE...
Issued at 218 PM CST Sun Dec 3 2017

A deepening low pressure system is expected to track across the
northern Great Lakes region later tonight through Tuesday. South
winds will be on the increase late tonight into Monday with gale
force gusts starting on Monday morning on Lake Michigan. Then
subsidence in the dry slot Monday night will produce westerly gale
gusts with the potential of storm force winds toward the open
waters. These conditions will persist into Tuesday before
gradually tapering off.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....MPC
LONG TERM......Skowronski
AVIATION.......Kurimski
MARINE.........MPC



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