Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 261023 AAA
AFDGRB

Area Forecast Discussion...UPDATED
National Weather Service Green Bay WI
523 AM CDT Wed Apr 26 2017

Updated to add short term portion for morning forecast issuance

.SYNOPSIS...
Issued at 520 AM CDT Wed Apr 26 2017

Much cooler and wetter the next couple days. More rain likely
this weekend.

Split flow will be maintained across North America throughout the
forecast period. The southern branch of the split will be much
stronger and dominate most of the CONUS, with ridges initially
near both coasts and a deep trough over the southern Plains. The
features will remain in place for about the first half of the
period, after which a slow progression is expected.

This is a wet weather pattern for the area, with significant
precipitation expected today into early Thursday. A second round
of significant precipitation is expected as a strong cyclone
ejects out of the Plains trough this weekend. Between the two
events, well above normal precipitation amounts are expected
across the whole area. Temperatures across the east will still be
mild today, but otherwise daytime highs will generally run 8 to 15
F degrees below normal for most of the period, with some
moderation possible late. Much of the cooling will be due to
clouds and precipitation, so the departure for nighttime lows
won`t be nearly as great.
&&

.SHORT TERM...Today...Tonight...and Thursday
Issued at 520 AM CDT Wed Apr 26 2017

Lots of details to work out again this morning. The overall
scenario is for a strengthening frontal boundary to shift east
across the area, though its eastward progress will be held up as
a series of waves ride northeast along the front. The wave lifting
up through the area this morning will dissipate later today, with
a stronger wave expected tonight into early Thursday as a much
stronger shortwave swings around into a negative tilt as it ejects
out of the southern Plains upper trough. Added resolution into
the PoPs with the idea there would be a round of rain early this
morning, another batch over mainly the east this afternoon, and an
even more significant surge of precipitation tonight with the
main shortwave. But given the pattern with a constant influx of
moisture however, the added resolution went more into creating
mins and maxes of PoPs rather than wet and dry periods.

The cold front that dropped into the far north is likely to
gradually lose its identity today as it merges with the main
boundary to the south and east. Temperatures across north-
central Wisconsin were colder than on any of the guidance
products, but luckily still a degree or two above freezing. Expect
only a minimal recovery across the far north today. Probably the
biggest forecast issue is what temperatures will be when the main
surge of precipitation reaches the area tonight. Trended a little
warmer than the guidance as it seems the main frontal wave may
track a little farther west than shown on the models. This meant
confining sub-freezing temperatures to mainly Vilas county. Even
with slightly sub-freezing air temperatures, not sure what the
icing potential will be given how warm the ground likely is after
some sun and very warm temps yesterday. Another factor is forecast
soundings looking favorable for sleet. That could end of being of
more significance in generating hazardous travel conditions that
freezing rain since it can lead to slick roads even if the ground
is still relatively warm. Given the uncertainty in exact
temperature forecast and precipitation type, the best course of
action seemed to be to defer the final decision on a headline to
the day shift.

The precipitation is likely to still be across much of the area
early Thursday, though it will diminish during the morning. West
winds will probably become gusty by afternoon as the primary cyclone
consolidates off to our northeast over Ontario.

.LONG TERM...Thursday Night Through Tuesday
Issued at 352 AM CDT Wed Apr 26 2017

After a break in the precipitation chances at the end of the week,
attention will be focused on the Sunday-Monday time frame as a
strong, nearly stacked system to move from the southern Plains
through the Great Lakes region. Depending on the eventual track of
this system, locally heavy rains may occur, accumulating snow may
occur or a mix of rain/snow may occur. Even after this system
departs, there are questions with another weaker system to
possibly approach toward the middle of next week. Temperatures are
expected to run below normal through the extended forecast.

An initial system will continue to lift north farther into Ontario
Thursday night, leaving very weak surface ridging extended from
the northern Plains to the Great Lakes. Expect skies to at least
become partly cloudy with winds gradually diminishing into the 5
to 10 mph range. The departed system will pull colder air into the
region with 8H temperatures down to the zero to -8C range by 12z
Friday. This would send min temperatures into the middle 20s
north-central WI, lower 30s east-central WI. Dry weather would
continue into Friday, however there is a small threat of light
rain into central/east-central WI Friday afternoon as a weak
surface wave moves along an old frontal boundary, a mid-level
shortwave ejects northeast out of the broad central CONUS upper
trough and WI to reside in the left exit region of the upper jet
for additional lift. Meanwhile, northern WI will remain partly
cloudy through the day, but still cool by late April standards.
Look for max temperatures to be in the upper 40s to around 50
degrees north, lower to middle 50s south.

The light rain chances will linger into Friday evening as the
surface low/shortwave move toward the eastern Great Lakes. High
pressure is then forecast to build into WI later Friday night and
allow skies to become partly cloudy. Winds are forecast to shift
to the north-northeast and continue to funnel cool air into WI.
Min temperatures to range from 25-30 degrees north, lower to
middle 30s south. High pressure centered over western Ontario will
continue to send a cool, dry northeast winds into northeast WI on
Saturday and help to at least temporarily stall precipitation from
returning to the area. There is no doubt that Saturday will be the
nicer of the two weekend days, although clouds are expected to
increase through the day and max temperatures to be about 10 to 15
degrees below normal.

Attention then turns toward the southern Plains where a strong
upper trough to reside with a strengthening area of low pressure
starting to lift north into the mid-MS Valley Saturday night. A 50
knot southerly low-level jet ahead of this system will not only
quickly pull gulf moisture northward toward the Great Lakes, but
also lift a warm front north into northern IL by 12z Sunday. A
push of WAA into southern WI will allow for precipitation to
steadily lift north with precipitation chances increasing,
especially over central/east-central WI after midnight. Residual
cooler air over northern WI and parts of central WI may have this
precipitation fall as a rain/snow mix or even all snow. By Sunday,
the upper trough is projected to close off into an upper low over
the Midwest and the strong low pressure to move into eastern IA or
western IL. The warm front is forecast to push toward the WI/IL
border with the models showing a broad mid-level frontogenetical
signal over most of WI. Any snow or mixed precipitation will
quickly transition to all rain Sunday morning with rain likely
over all of northeast WI by Sunday afternoon. Combination of
clouds and precipitation will make it difficult for temperatures
to get out of the 40s on Sunday.

The system is progged to become vertically-stacked as it tracks
northeast into southwest WI Sunday night and northern sections of
the Great Lakes on Monday. Considering this system is stacked, the
GFS may be a bit hasty in pulling the system away on Monday and
prefer the slower movement like the ECMWF indicates. Assuming the
system follows this path, this would keep any convection and
heaviest rains to our east Sunday night, while the main
deformation zone with higher snow amounts would be to our
northwest. Nevertheless, we will still see some decent rain totals
over eastern WI with the potential for a light accumulation over
north-central WI later Sunday night into Monday morning. Too early
yet for specifics, so stay tuned to later forecasts! The chance
for rain would then continue for the rest of Monday as cold air
aloft wraps into WI behind the system and brings some instability
to the region.

Precipitation chances could even linger into Monday evening
(albeit very light) before finally tapering off later Monday
night. Big differences among the models with the handling of the
next system beginning next Tuesday. The ECMWF is much weak and
farther south than the GFS which is farther north and much wetter.
For now, will follow the consensus solution which favors the ECMWF
and keeps northeast WI dry on Tuesday.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 520 AM CDT Wed Apr 26 2017

Low ceilings were developing in central and north-central
Wisconsin behind the remnants of the weakening cold front.
Ceilings in the east were holding up much better. Suspect the low
ceilings will be slow to shift east today as the frontal boundary
reorganizes from SSW-NNE across the area. Ceilings across the
entire area should settle into IFR or LIFR tonight as the
precipitation increases in advance of the primary shortwave
heading into the region.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......Skowronski



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