Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 281659

Area Forecast Discussion
National Weather Service Green Bay WI
1159 AM CDT Tue Mar 28 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 326 AM CDT Tue Mar 28 2017

Forecast challenge today into Wednesday will center on cloud
forecasts with results on temperatures.

Upper system which produce extensive clouds and precipitation the
last few days continues to depart eastward while high pressure
builds into the the Great Lakes. Early this morning clouds have
decreased across the northwest half of the state. Guidance
indicates a good deal of sunshine in the forecast today, but
there is some potential of more clouds to temper max temps.
Satellite imagery shows mid to high level clouds in the high
pressure region over the Northern Plains approaching the area
today. In addition, back edge of clouds from the departing system
slowing down a bit over southeast Wisconsin due to a lingering
surface trough. Other issues with the potential of more clouds
deal with the cool air aloft with late March Sun, but model data
cu rule indicates little development. Cool flow off the Great
Lakes may also produce more lake clouds today into tonight.
Eventually expect clear skies to dominate later today into tonight
with upper ridge building into the area.

With some clearing early this morning, keeping an eye out for
widespread dense fog development. So far not widespread enough due
to probably to much of a northerly gradient. But that could
produce a period of a stratus deck this morning. Latest trends
suggest fog will be handled with a SPS this morning.

Mid to high level clouds will be on the increase from southwest
to northeast Wednesday ahead of another system approaching from
Central Plains.

.LONG TERM...Wednesday Night Through Monday
Issued at 326 AM CDT Tue Mar 28 2017

Extended models continue to show an active split flow across the
CONUS into next week. This pattern will continue to bring a series
of low amplitude/weak northern stream systems across southern
Canada, while a stronger southern stream sends systems that drop
from the Pacific NW to the southern Plains, then east-northeast
into the Mid-Atlantic states. This active pattern would bring
precipitation chances to northeast WI every two or three days with
precipitation type a problem at night as temperatures cool below
freezing. Temperatures should end up averaging slightly above
normal through the period.

There remains timing differences among the models with respect to
the onset of precipitation Wednesday night as a weak northern
stream shortwave trough attempts to interact with a southern
stream system moving into the Mid-MS Valley. There will also be a
dry east-northeast wind into northeast WI from a high pressure
situated over Ontario that may inhibit/delay the start time of the
precipitation. The NAM holds off the precipitation until Thursday
morning, while the GFS/ECMWF have high chance pops reaching
central/east-central WI shortly after midnight. The CMC is in
between with the leading edge approaching our southern counties by
daybreak. Have ended up keeping the previous forecast timing of
bringing chance pops to areas south of Highway 29 with
precipitation type as all snow as temperatures both aloft and at
the surface at or below freezing.

As the southern stream system slowly pushes toward the lower
reaches of the Ohio River Valley on Thursday, the accompanying
warm front is forecast to remain over northern sections of
Illinois and Indiana, thus the best frontogenetical forcing will
be over southern WI where heavier rains to fall. The northern
edge of the rain shield is still expected to impact central/east-
central WI, again mainly along/south of Highway 29. The snow will
mix with and eventually change to all rain by midday as surface
temperatures warm into the lower 40s south, middle 40s north.
Parts of central/east-central WI could see around one-half inch of
slushy accumulation between late Wednesday night/Thursday morning.

Due to the overall slow nature of this system, expect to see some
forcing/deformation zone of light precipitation linger through at
least Thursday evening, if not all Thursday night in the east. As
has been the case with previous systems, enough cool air at/near
the surface at night to have rain mix with or change to all snow,
although any accumulations appear negligible as the better
moisture to have shifted east by this time. Min temperatures will
drop into the middle to upper 20s north, lower 30s south. A weak
area of high pressure is forecast to move into the Great Lakes by
Friday and should bring decreasing clouds from northwest to
southeast during the day. Temperatures will range from the lower
40s near Lake MI (prevailing northeast wind), to the upper 40s
to around 50 degrees toward central WI.

Quiet conditions are expected Friday night with partly cloudy
skies and light winds. Some increase in clouds are forecast for
Saturday as a northern stream shortwave trough races across
northern sections of the Great Lakes. For now, models keep any
precipitation north of WI so have opted to run with the dry
forecast through the day. Temperatures will continue to slowly
moderate with readings on Saturday reaching the middle to upper
40s near Lake MI, lower 50s inland.

Clouds are progged to increase across most of WI once again
Saturday night ahead of the next weather system moving into the
southern Plains. The combination of weak WAA and the left exit
region of the upper jet may bring enough lift to generate light
rain primarily south and west of the forecast area. By Sunday,
this lift remains mainly focused to our south, but cannot
completely rule out a stray shower or sprinkles over parts of
central/east-central WI. Even if the precipitation stays away,
just the clouds alone could impact temperatures. Have kept
readings a touch below guidance and similar to values seen on

Questions remain as to the eventual movement of this latest
southern stream system headed into the new work week. The new GFS
keeps this system well south of WI, whereas the ECMWF lifts this
system northeast and spreads rain from the Upper Midwest to the
East Coast. Enough potential is there to at least mention small
pops for now and see how the models adjust in the coming days.

.AVIATION...for 18Z TAF Issuance
Issued at 1155 AM CDT Tue Mar 28 2017

VFR conditions expected at most locations through the taf period,
but with a couple exceptions. IFR/LIFR conditions will impact the
lakeshore areas through mid-afternoon, where very low stratus and
pockets of dense fog are moving southwest off the Lake. Some
concern that fog could reform along the lakeshore again late
tonight, but confidence is too low to include in the tafs. Fog
could also redevelop over north-central WI late tonight, but
again, confidence is low. Otherwise, mid and high clouds will
increase late tonight into Wednesday morning.




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