Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 191059

Area Forecast Discussion
National Weather Service Green Bay WI
559 AM CDT Thu Apr 19 2018

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Friday
Issued at 317 AM CDT Thu Apr 19 2018

A drier and continued cool air mass will filter into the area on
north winds this morning. Upper ridge with surface high pressure
will build into the area for the rest of the week with clear
skies and moderating temperatures. As a result, forecast
temperatures will be the primary issue with respect to the
lingering deep snowpack.

Brisk north winds early this morning are helping to prevent
temperatures falling much so far, so most areas will begin with
a relatively warm start from 25 to 30, followed with plenty of
sunshine today. Satellite imagery does show a bit more cirrus over
the Northern Plains which may drift into western Wisconsin today.
Despite plenty of clouds on Wednesday, many locations were able
to get into the lower 40s, especially across northern wisconsin.

Clear skies and diminishing winds tonight offer the potential of
fog and falling temperatures. Noticed some diurnal cold spots over
western Wisconsin early this morning where winds subsided. As far
fog, surface dewpoints remain in the 20s and melting snow likely
to get absorbed into the snowpack for now. Will add a patchy
mention for now.

More sunshine is expected for Friday, but likely a cooler start
in the morning may yield similar max temperature results compared
to today.  Some progs suggest a weak frontal boundary drops into
northern wisconsin or far northeast wisconsin Friday afternoon,
but will be dry and perhaps producing only a subtle wind shift at
this time.

.LONG TERM...Friday Night Through Wednesday
Issued at 317 AM CDT Thu Apr 19 2018

Mean flow to be semi-amplified and split with several shortwave
troughs moving across the CONUS. The first of these troughs is
forecast to pass to our south this weekend and not have an impact
on our weather. The second shortwave trough is expected to follow
a cold front into the Great Lakes Tuesday night and bring our next
chance for precipitation which would carry over into Wednesday.
The bigger story however will be the much anticipated warm-up
which will continue through at least Tuesday before cooler air
arrives behind the cold front and knocks temperatures a bit below
normal. A concern that will need to be watched into next week will
be the rising rivers and streams as the large snowpack over the
area quickly begins to melt.

High pressure is forecast to extend from near Hudson Bay south
through the Great Lakes into the southeast CONUS Friday night.
Meanwhile, a system is expected to mover southeast into the
central High Plains and spread precipitation into the Plains later
Friday night. Some high clouds from this precipitation will make a
push across the Upper Midwest into WI, so eventually skies will
become partly cloudy over the forecast area. Have continued the
trend of going below guidance for temperatures due to the melting
snowpack. Look for min temperatures to range from the middle 20s
north-central WI, to around 30 degrees east-central WI.

The combination of the high pressure and the approach of an upper
ridge will keep quiet conditions intact headed into the weekend.
The aforementioned system is progged to drop into the southern
Plains, therefore the high clouds are expected to be pulled
farther south, leaving mostly sunny skies across northeast WI.
Light south-southeast winds and warm late April sun angle will
continue to chip away at the snowpack with max temperatures able
to reach the middle to upper 50s for most locations. The exception
to be on the north side of the Bay and near Lake MI where readings
to only reach the upper 40s to lower 50s.

The western Great Lakes region will remain on the western fringe
of the high pressure on Saturday night with the upper ridge
overhead. Mostly clear skies and light winds to prevail with min
temperatures to range from the middle to upper 20s north, upper
20s to lower 30s south. On a side note, cannot rule out some
patchy fog developing overnight as the melting snow adds moisture
to the lower levels of the atmosphere. Not much changes headed
into Sunday as the initial system to be moving across the Gulf
Coast states and high pressure to stretch from the western Great
Lakes to the New England states. Anticipate another day with
plenty of sunshine and temperatures around normal. This would be
middle to upper 50s inland (few localized 60 degrees), but only
around 50 degrees lakeside.

Tranquil conditions to persist through Monday as the high pressure
begins to lose its influence over our weather. Nevertheless, no
precipitation yet with south-southwest winds pulling warmer air
into the region. Monday could be the warmest day of the extended
forecast with max temperatures in the upper 50s to lower 60s
inland, lower to middle 50s along Lake MI. After a quiet Monday
night, attention turns to an approaching cold front/trailing
shortwave trough for Tuesday. This cold front sweeps into
northeast WI by late Tuesday afternoon, while the shortwave trough
reaches the Upper MS Valley. The models indicate most of the
associated `rain` showers to be post-frontal, thus eastern parts
of the forecast area may not see any precipitation until early
Monday evening. More clouds and increasing threat of rain showers
will keep temperatures close to Monday`s readings despite the mild
start to the day. This would have max temperatures in the lower to
middle 50s near Lake MI, middle to upper 50s north-central WI and
upper 50s to lower 60s elsewhere.

As the cold front departs Tuesday night, the shortwave trough
moves into the western Great Lakes with enough mid-level forcing
to keep chance pops in the forecast. As temperatures cool through
the night, anticipate the rain to mix with or change over to all
snow over all but east-central WI where temperatures should remain
warm enough to keep precipitation type all rain. Models do differ
with respect to the shortwave trough on Wednesday. The ECMWF
favors a progressive movement of the trough, while the GFS and GEM
favors the trough to close off into an upper low over the Great
Lakes. The ECMWF solution would linger precipitation over east-
central WI into Wednesday morning before clearing things out. The
GFS/GEM solution would keep precipitation chances in the forecast
for all of Wednesday. For now, have kept a small pop in the
forecast through all of Wednesday. North-northwest winds and CAA
will end the warm-up with max temperatures around 50 degrees
north-central/near Lake MI, middle 50s elsewhere.

.AVIATION...for 12Z TAF Issuance
Issued at 559 AM CDT Thu Apr 19 2018

VFR conditions to prevail today into tonight as high
pressure settles into the region. Patchy fog may develop tonight
due to the light winds and clear skies.

Issued at 317 AM CDT Thu Apr 19 2018

Warming temperatures are expected to produce significant melting
of the current snowpack over northeast WI. This will lead to
runoff and a gradual rise of area rivers and streams. Early
indications are for the Wolf River to reach bankfull this weekend
and reach flood stage around next Tuesday. People living near
rivers and streams should keep a close eye on the anticipated
river rises and keep up to date on the latest river forecasts.



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