Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 250901

Area Forecast Discussion
National Weather Service Green Bay WI
401 AM CDT Tue Apr 25 2017

Forecast discussion for routine morning forecast issuance

Issued at 358 AM CDT Tue Apr 25 2017

Still warm and mainly dry today, then a cooler and wetter weather
pattern will settle across the area for the rest of the week and
the upcoming weekend.

The large scale flow across North America will remain split during
the period. The southern branch of the split will be much stronger
and dominate most of the CONUS. A large scale trough will remain over
the southern Plains, with ridging near the coasts. The position
of the forecast area downstream from the mean trough position will
result in unsettled weather, and much above normal precipitation
for the period. Temperatures will remain well above normal today
(except near the Bay and Lake), then cooler air will gradually
spread east. That will eventually drop temperatures to below
seasonal normals on days with significant precipitation, and to
near normal otherwise.

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 358 AM CDT Tue Apr 25 2017

The overall forecast scenario the next couple days is rather
simple. Shortwaves ejecting out of the strengthening southern
stream upper trough over the Plains will bring several rounds of
precipitation to the area. The details of those (in terms of exact
timing and amounts) are much more elusive. The lead shortwave
that crossed the area overnight generated considerable radar
returns, but only managed to produce a couple sprinkles on surface
obs due to dry air at low-levels. The 00Z KGRB RAOB was very dry
between 500 mb and 850 mb. KDVN and KILX RAOBS were similar,
though not as drastic. So accepted the overall trend of the models
of showing minimal precipitation chances this morning. The dry
air may well hold off precipitation this afternoon as well,
especially over east-central Wisconsin. Satellite imagery suggests
some breaks in the clouds are likely as well. As a result, edged
temps upward everywhere except near the Lake and Bay, where
southeast winds will hold readings back.

A continued influx of moisture will eventually result in clusters
of showers spreading across the area tonight, though timing is
still uncertain. Followed the SWODY1 in keeping a mention of
thunder for much of the area.

A fairly strong frontal boundary will be across the area
Wednesday, as a well defined frontal wave lifts from central to
northeast Wisconsin. The east will still be in the warm sector, so
edged temps up there (again, except near the Lake and Bay). Also
maintained a greater chance of thunder in the east. That fit well
with the SWODY2 outlook from SPC. Training of convection could
result in a heavy rain risk across the area, and will mention that
in the HWO.

.LONG TERM...Wednesday Night Through Monday
Issued at 358 AM CDT Tue Apr 25 2017

Upper ridges over the eastern Pacific and western Atlantic will
keep a persistent trough from the Rockies to the Appalachians. A
series of systems are forecast to move through this trough and
bring several opportunities for precipitation to northeast WI
during the extended forecast. Depending on the synoptic set-up,
some of this precipitation could be heavy at times which would
aggravate already swollen rivers over northeast WI. Temperatures
will go below normal due to the clouds/precipitation, while
precipitation amounts to be above to well-above normal into early
next week.

The slow-moving cold front will edge across WI Wednesday night,
primarily due to an area of low pressure riding north along the
frontal boundary. Models differ a bit with exactly where this low
pressure will be during the night with the GFS almost 12 hours
faster than the NAM, with the ECMWF and CMC somewhere in between.
This is only a minor detail with plenty of gulf moisture
available, a mid-level shortwave trough becoming slightly
negatively-tilted over the Upper Midwest and the right entrance
region of the upper jet adding lift. All of this adds up to a wet
night across the forecast area with rain being the main
precipitation type. However, as surface temperatures approach the
freezing mark over parts of north-central WI later at night, the
rain could mix with or change to freezing rain. This will need to
be watched closely over the next 36 hours. Headed into Thursday,
the cold front exits WI to the east and the mid-level trough lifts
northeast through the western Great Lakes region. Expect mainly
rain (still a brief mix possible early north-central) to continue
through at least Thursday morning with a gradual decrease in
precipitation chances Thursday afternoon, especially over central/
east-central WI where a dry slot may rotate into southern WI. Much
cooler air mass to set-up over WI by Thursday with max
temperatures falling to below normal levels.

Northeast WI appears to reside between systems for Thursday night
and at least most, if not all of Friday depending on which model
is correct. Both the GFS and CMC agree in sending light rain into
central/east-central WI along a warm front lifting northward
toward WI. Meanwhile, the ECMWF hold this frontal boundary well to
our south on Friday with no precipitation close to northeast WI.
Have placed more faith in the GFS/CMC solutions based on the fact
that upper ridging to the east-southeast would prevent this front
from dropping too far south and additional mid-level energy moving
through the central CONUS large-scale trough would head in our
direction. Thus, will have precipitation chances return Friday
afternoon, especially across central/east-central WI. Temperatures
will make a modest recovery on Friday with readings only slightly
below normal.

The warm front is not expected to lift any farther north as a
surface wave develops along the front and any convection south of
the front will tend to pull the front farther south with time
Friday night. This scenario would keep precipitation chances in
the forecast across the southern half of WI, while northern
sections would be dry. Cannot rule out a little snow potentially
mixing with the rain as temperatures fall into the 30s. The latest
model output from 00z for Saturday now indicate that most of
Saturday could be dry as the next system organizes over the
southern Plains. May leave a small pop for central/east-central WI
Saturday afternoon as gulf moisture begins to surge north once
again. Northeast winds will continue to keep temperatures on the
cool side of normal with readings only in the upper 40s to lower

This next system is progged to lift northeast into the central
Plains/Mid-MS Valley Saturday night and reach the Midwest/southern
Great Lakes region on Sunday. A surface low will move northeast
and pull the old frontal boundary back to the north as a warm
front. A trailing strong mid-level shortwave trough is forecast to
reach the Upper MS Valley south to the central Plains by 00z
Monday. Much like the first system, there are model differences
with the track of the surface low and whether eastern WI could get
into the warm sector for possible thunderstorms. Majority of the
models favor the low passing to our east which would bring a cold
rain to much of northeast WI Saturday night and Sunday. We will
have to watch northern WI Saturday night/Sunday morning for a
rain/snow or all snow situation with temperatures hovering around
32 degrees.

Sunday night could be a tricky forecast as the mid-level trough
lifts into the western Great Lakes and the atmospheric column
cools as the main deformation zone lifts northeast into WI.
Depending on how fast surface temperatures fall, we could get at
least light snow accumulations over parts of the forecast area.
The cyclonic flow around the departed trough could linger light
precipitation into Monday, although amounts would be lighter.

.AVIATION...for 12Z TAF Issuance
Issued at 358 AM CDT Tue Apr 25 2017

The air mass across the area remains very dry at low-levels, so
expect VFR conditions to persist into tonight. Ceilings will
eventually come down once rain becomes more widespread tonight,
but guidance is probably still a little fast in lowering them.



SHORT TERM.....Skowronski
LONG TERM......Kallas
AVIATION.......Skowronski is the U.S. government's official web portal to all federal, state and local government web resources and services.