Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 291735
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1235 PM CDT Sat Apr 29 2017

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 416 AM CDT Sat Apr 29 2017

Mainly dry today, then chilly, windy, and wet for the rest of the
weekend.

The large scale upper features across the CONUS haven`t moved much
in the past week. Ridges were off the West Coast and along the
East Coast, and a strong trough was over the southern Plains. That
pattern will hold a little longer, then undergo some progression
during the upcoming work week. It looks like the features will
shift about 180 degrees downstream before locking into place
again late next week.

A strong cyclone ejecting out of the southern Plains upper trough
will result in another widespread soaking rainfall across the
area tonight through Monday night. The additional rain chances
during the remainder of the forecast period appear to be
lighter/more scattered-type rain events. But amounts for the
period should end up above normal from the first event alone.
Temperatures will remain below normal into the start of the
upcoming work week, then return near or a little above normal for
the second half of the week.
&&

.SHORT TERM...Today...Tonight...and Sunday
Issued at 416 AM CDT Sat Apr 29 2017

The main forecast issue was figuring out the specifics of the
precipitation chances and amounts as models exhibited
considerable differences on the specifics of the rainfall. Today
will be mainly dry, but rain will be edging north and could reach
the far southern part of the area late. The GFS and ECMWF
suggested a slug of precipitation would surge through east-
central Wisconsin this evening, then shift off to the east later
in the night. Some scattered/lighter precipitation could push
north in the wake of that. The ECMWF and GFS stood alone and yet
prevailed with their handling of the band of rain that clipped
east-central Wisconsin late Friday/Friday evening, so leaned
heavily toward their handling of the incoming precipitation
event. Tried to mimic their scenario with the PoPs tonight, with
likely/categorical PoPs over east- central Wisconsin for a time
during the evening, tapering back to chance PoPs later in the
night. At the same time, spread chance PoPs northward over the
western part of the area. Evaporational cooling of dry air at the
leading edge of the preciptation could result in some snow or a
rain/snow mix across central, northeast, and north-central
Wisconsin. Given the current timing of the northward advance of
the precipitation, do not expect any accumulation. However, if
the northward advance occurs significantly faster than expected
then some minor accumulations will be possible north of highway
29.

The model differences in precipitation continued Sunday, with the
hi-res WRFs suggesting little precipitation during the day, except
for a surge into central Wisconsin late in the afternoon. Stayed
closer to the ECMWF/GFS which were wetter and focused the higher
amounts in east-central Wisconsin. Increasing northeast winds off
the lake and bay will make for a very raw day in the east.

.LONG TERM...Sunday Night Through Friday
Issued at 416 AM CDT Sat Apr 29 2017

Models are finally showing an eventual change to the overall mean
flow from a central CONUS trough/upper ridges on either side, to a
western CONUS upper ridge/eastern CONUS upper trough toward the
latter half of next week. Main concern remains the stout system
that will affect northeast WI from Sunday night into Tuesday.
Locally heavy rainfall will be possible which would keep area
rivers/streams near bankfull or even surpass flood stage in some
instances. Precipitation chances then become more spotty as the
upper trough shifts east across the region. Temperatures are
expected to remain below normal through at least Thursday, before
readings get to normal (if not slightly above) by next Friday.

This strong system is still forecast to become vertically-stacked
over the Midwest Sunday night with the warm front reaching
southern WI, a surge of gulf moisture into WI with PW values
between 1.00-1.50" and a surge of synoptic lift/mid-level forcing
to generate a swath of locally heavy rain over northeast WI.
Models do show a dry slot rotating northward after midnight and
should start to taper the rain intensity during he pre-dawn hours
of Monday morning. The main concern will continue to be river/
stream levels and potential for flooding as grounds are already
saturated and river levels are already high to begin with.
Depending on exactly how much rain falls, a flood watch is not out
of the question.

The rain showers will continue through Monday as the system tracks
northeast through the heart of WI. Additional QPF to range from
around one-tenth of an inch east of the Fox Valley to around one-
third of an inch over north-central WI. Of course, any additional
precipitation will only serve to keep flooding concerns in the
forefront of the forecast with rivers/streams expected to rise
further. Max temperatures on Monday to range from the middle 40s
north-central, to the lower 50s east-central WI.

As the system heads toward eastern sections of Lake Superior
Monday night, northeast WI to get stuck under the cool, cyclonic
flow which would keep at least scattered showers in the forecast.
One noticeable difference will be some snow mixing with the rain
across northern and parts of central WI later Monday night as
cooler air begins to wrap into WI. We will have to watch this
precipitation type closely, but for now, no appreciable snow
accumulation is expected. Cannot rule out scattered showers into
Tuesday as the cool, cyclonic flow lingers over the region. Any
mixed precipitation will transition to all rain by late morning as
surface temperatures slowly rise. It would appear that north-
central WI would have the highest chance of seeing measurable
precipitation due to lake enhancement from gusty northwest winds.
Max temperatures should be similar to Monday with lower to middle
40s north-central, lower 50s east-central WI.

Precipitation chances should finally diminish Tuesday evening as
the cyclonic flow weakens and a modest area of high pressure moves
into the western Great Lakes. Wednesday appears to be a dry day as
the surface high drifts east-southeast across WI toward MI/IL. At
least a mix of sun and clouds are expected and this will help
boost temperatures into the lower to middle 50s north/lakeshore,
middle to upper 50s south. This is still a good 5 degrees below
normal, but an improvement from earlier in the week.

There continues to be a concern regarding precipitation chances
later Wednesday night into Thursday as a mid-level shortwave
trough dives southeast toward/into WI, accompanied by a weak
clipper-type area of low pressure. There appears to be sufficient
mid-level forcing and modest instability by Thursday to at least
warrant a small chance pop across the forecast area. Have added a
bit more cloud cover as well to the forecast on Thursday and kept
max temperatures in the middle 50s north, upper 50s to around 60
degrees south. This little system could linger over the area into
Thursday night, before pulling away to the south-southeast on
Friday. Northeast WI should see high pressure move into the region
on Friday and allow for sunshine to return with temperatures
actually reaching or even possibly going above normal.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1235 PM CDT Sat Apr 29 2017

Generally VFR conditions can be expected into the evening hours
before ceilings and visibilities gradually become MVFR from
southeast to northwest. IFR ceilings should develop in east
central Wisconsin later in the night as a surface low moves from
the Panhandle region to the central Plains. Expect MVFR and IFR
conditions to prevail through at least Sunday morning with the
approach of the low. The exception is the northern part of the
forecast area which remain VFR overnight with MVFR conditions
developing Sunday morning.
&&

.MARINE...
Issued at 416 AM CDT Sat Apr 29 2017

Increasing northeast winds along Lake Michigan will likely produce
high waves this weekend. Gale force gusts are likely Sunday
afternoon through Sunday night.
&&

.HYDROLOGY...
Issued at 416 AM CDT Sat Apr 29 2017

Above normal rainfall for April continues to keep river levels
and flows above normal. Rainfall from earlier this week has
prompted river flood warnings for part of the Wolf river south of
Shawano. Flows along the Menominee also remain above normal, with
flood advisories in effect for portions of the river and possible
flood warnings for minor flood stage this weekend. The incoming
storm will be capable of producing another widespread inch of
rainfall over two days. Locally higher amounts to 2 inches are
possible. This rainfall will continue to keep river running above
normal for the rest of April into early May. The above normal
river levels and flows will likely begin to affect recreational
users and interests. Be sure to check for and heed any no wake
postings.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......MG
MARINE.........Kallas
HYDROLOGY......Kallas/Skowronski


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