Area Forecast Discussion
Issued by NWS Green Bay, WI

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918
FXUS63 KGRB 210006
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
606 PM CST Mon Nov 20 2017

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Tuesday
Issued at 236 PM CST Mon Nov 20 2017

The main forecast focus to be on potential lake effect snow
showers across north-central WI as a strong cold front sweeps
through later tonight and turns the winds to the northwest.

The 19z MSAS surface analysis indicated a clipper low pressure
system near Lake Winnipeg with a cold front extended south-
southwest into the central Plains. Meanwhile, high pressure was
situated over the Mid-Atlantic states. A broad southwest flow
aloft existed between these two systems and that had helped boost
temperatures in northeast WI into the 40s.

Models remain on track with the movement of the clipper low across
southern Ontario tonight, which will drag the cold front across
northeast WI after midnight. Limited moisture availability should
allow for a dry frontal passage with the main impacts being a wind
shift to gusty west-northwest direction and the development of
strong CAA. Trajectories will not become favorable for lake
effect over north-central WI until almost sunrise, thus have
maintained the limited accumulation potential. Min temperatures
to range from the lower to middle 20s north-central WI, to the
middle 30s near Lake MI. It should be noted that actual min
temperatures may not be reached until after sunrise as the colder
air sweeps into the region.

The passage of a shortwave trough/embedded shortwave will help to
veer the winds more to the northwest on Tuesday and allow for a
better chance of lake effect snow showers to impact north-central
WI. 8H temperatures drop to around -15C over western Lake Superior
which would provide delta-T values of around 20. Inversion heights
are relatively low and trajectories are not perfect, thus
accumulation numbers for Vilas County should be limited to an inch
or two in the snowbelt. Winds will still be gusty, especially
during the morning hours, so there may be some blowing/drifting of
the new snow. The rest of northeast WI will see more clouds in the
morning with gradually more sun in the afternoon as the eastern
fringes of high pressure centered over the Plains begins to send
drier air into WI. Tuesday is expected to be a colder/blustery day
overall with max temperatures only in the middle to upper 20s
north-central, middle to upper 30s east-central WI. Wind chills by
the end of the day could drop to around 10 above northern WI,
teens to around 20 degrees elsewhere.

.LONG TERM...Tuesday Night Through Monday
Issued at 236 PM CST Mon Nov 20 2017

Northwest upper flow is forecast to prevail through
the end of the week. This sort of pattern favors mostly dry and
cooler than normal weather.

There could be some leftover lake effect snow showers Tuesday
night, but they should be minimal. A cold front moving through the
region Friday should bring mild temperatures and rain showers and
be followed by blustery and colder conditions for Saturday and
Sunday with lake effect snow showers in far northern Wisconsin.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 548 PM CST Mon Nov 20 2017

A large band of cirrus clouds will continue to move across the
forecast area ahead of a cold front tonight. This front should
reach RHI/AUW/CWA shortly before midnight, then GRB/ATW/MTW
overnight. A secondary front with even colder air will move
through Tuesday morning, with an area of BKN-OVC stratocumulus
clouds arriving in its wake. While most of the forecast area
should have VFR ceilings, parts of north central WI will see MVFR
conditions, especially in association with scattered lake-effect
snow showers in the morning and early afternoon. MVFR conditions
may impact RHI for a few hours, especially during the late morning
and early afternoon. Strong northwest winds will gust to 25 to 30
knots behind the secondary front.

LLWS will continue tonight, as southwest to west winds increase
to 40-50 kts just above the surface. The LLWS should weaken at
RHI/AUW/CWA around 08z-09z/Tues, and at GRB/ATW/MTW around
11z-12z/Tues.
&&

.MARINE...
Issued at 236 PM CST Mon Nov 20 2017

Pressure gradient is forecast to tighten between high pressure
over the Mid-Atlantic states and the clipper low pressure moving
across southern Canada tonight. A Small Craft Advisory is in
effect starting this afternoon. A few Gale force gusts are
possible tonight and Tuesday as winds become northwest behind the
passage of a cold front. However, winds are not expected to last
long enough to issue a Gale Warning at this time. Winds are
expected to drop below small craft criteria by Wednesday as high
pressure moves through the western Great Lakes.
&&

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

$$

SHORT TERM.....Kallas
LONG TERM......RDM
AVIATION.......Kieckbusch
MARINE.........Kallas



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