Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 220448
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1048 PM CST Tue Feb 21 2017

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Wednesday
Issued at 240 PM CST Tue Feb 21 2017

The main forecast challenges take place on Wednesday, between max
temperatures and extent of precipitation chances. If the rains
were to hold off, we would again be flirting with record
temperatures.

The 20z MSAS surface analysis showed a weak ridge of high pressure
that extended from the central Plains northeast into the western
Great Lakes. A weak area of low pressure was situated over
southeast ND with a warm front stretched southeast from the low
pressure into the Midwest. The visible satellite imagery picked
up on high clouds ahead of the warm front, making a move toward
WI.

Initially, we will see the increase in high clouds through early
this evening as the warm front pushes through. After the front
departs, skies should begin to clear. However, expect to see an
increase in mid/high clouds once again mainly focused over the
northern sections of the forecast area later tonight in
association with the approach of a cold front and mid-level
shortwave. Central/east-central WI will see some clouds this
evening, but overall, skies will be mostly clear for the overnight
hours. There is some concern about the possibility of a stratus
deck or patchy fog to develop over the region, but there appears
to be enough boundary layer wind to prevent either from happening
tonight. Do believe fog will exist over the nearshore waters with
dew points in the middle 30s and lake temperatures in the middle
to upper 30s. Therefore, have added areas of fog to the NSH. Min
temperatures will range from the middle 30s north, upper 30s to
around 40 degrees south.

The cold front/shortwave tandem push east into the western Great
Lakes on Wednesday and bring at least a possibility of light rain
to northeast WI. Models are showing decent lift with the cold
front, but the better forcing passes mainly to our north. Thus,
will focus small chance pops north (mainly along/north of Hwy 8)
with a slight chance pop down to around Hwy 29. Max temperatures
for Wednesday will be tricky depending on precipitation/cloud
trends. For now, have temperatures reaching the lower to middle
50s north-central WI, to the middle 60s across southwest sections
of the forecast area (south of a ISW-PCZ-OSH line).

.LONG TERM...Wednesday Night Through Tuesday
Issued at 240 PM CST Tue Feb 21 2017

A weak low pressure system will track to the north Wednesday
night, bringing a small chance of precipitation to the northern
cwa. Temperatures appear to be warm enough for all rain, however a
wintry mix cannot be ruled out before this round comes to an end
late in the night. Thursday morning looks dry, however the large
low pressure system approaching from the plains could bring some
rain to the southern cwa during the afternoon hours.

A large low pressure system takes aim for the western Great Lakes
region Thursday night, lasting through Saturday as this slow
moving system affects the region for several periods. There are
several things to note regarding the 12Z suite of models regarding
this system. First is the NAM which appears to be an outlier as it
is much quicker than any of the other models, clearing out
precipitation by Friday afternoon. The more significant thing to
note is the continued northward progression of the system with the
other models, namely the GFS and ECMWF models. These new runs
suggest the well advertised dry slot will play a larger role over
a larger area, holding down snowfall totals across the area, most
notably east-central and portions of northeast Wisconsin. Current
runs suggest the heaviest snow will be across north-central and
much of central Wisconsin, with a fairly sharp drop off from about
a Wausaukee to Wisconsin Rapids line. Areas north of this line
are most at risk of receiving 6 inches or more of snow, mainly
from Thursday night through Friday night, with totals much less
than that as you head east given the dry air which will also act
to produce a wintry mix as ice crystals may have issues reaching
the ground at times. The dry slot will introduce an increased risk
for freezing rain across areas previously thought to be all snow
across central and north-central Wisconsin. Although there was
some consideration regarding headlines the uncertainty posed by
the changing model solutions, the periods the event is forecast,
5th through 7th periods, the continued northward progression of
the system and associated dry slot, as well as the long duration,
on the order of 36 hours, decided to hold off with this forecast
cycle. The changing paths of this system pose significant
challenges in nailing down the swath of heavy snow and mixed
precipitation, as a further jog to the north would limit heavy
snow to mainly north-central Wisconsin with areas probably
struggling to approach headline criteria if the storm moves
further north.

Behind this system a colder airmass will settle in across the
western Great Lakes region, with temperatures close to or below
normal through Monday. Lake effect snow showers could affect far
north-central Wisconsin at times during this period. Another low
pressure system is then slated to affect the western Great Lakes
region Monday night and next Tuesday. There is quite a bit of
model discrepancy regarding the evolution of this system,
therefore confidence if fairly low in that portion of the
forecast. That being said there are some indications that
temperatures could rise to well below normal levels Monday night
and next Tuesday.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1048 PM CST Tue Feb 21 2017

VFR conditions are expected to continue tonight,
though there could be patchy ground fog late tonight over snow
covered areas. There could also be some fog near the Lake Michigan
shore. Marginal low level wind shear possible later tonight with
southwest winds around 30 knots at 1000 ft agl.

Wednesday will start out with VFR conditions but low clouds and
light rain may arrive during the late afternoon or early evening
north of a Wausau to Sturgeon Bay line. Should remain dry and VFR
further south.
&&

.HYDROLOGY...
Issued at 240 PM CST Tue Feb 21 2017

Minor river flooding will remain a problem tonight due to rain
last night and continued melting snow from northern WI working
downstream. Expect to see most of the rivers and streams that are
above flood stage to begin falling later tonight. Please see the
latest hydrologic products for more details on specific rivers and
current stages. We are also watching the late winter storm for
later this week which could raise river levels once again.
&&

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

$$

SHORT TERM.....Kallas
LONG TERM......Kurimski
AVIATION.......RDM
HYDROLOGY......Kallas



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