Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 181745
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1245 PM CDT Wed Apr 18 2018

Updated aviation portion for 18Z TAF issuance

.UPDATE...
Issued at 1022 AM CDT Wed Apr 18 2018

The regional radar mosaic indicates that the northern edge of the
snow band will remain farther south than earlier expected. In
addition, the latest model runs have also trended south, so the
Winter Weather Advisory for our southern tier of counties has
been cancelled. A dusting to an inch of snow is still possible in
the far southern part of the forecast area during the late
afternoon and early evening.
&&

.SHORT TERM...Today...Tonight...and Thursday
Issued at 356 AM CDT Wed Apr 18 2018

Primary forecast issue is the weather system tracking south of the
area this afternoon into this evening. Early this morning, the
system was producing snow along with thunderstorms over northwest
IA and just southeast of FSD. Forecast problem is the track of
the band of moderate snowfall and resulting headlines associated
with this system.

Consensus of progs, including various meso models, are in good
agreement with the band of higher snowfall shifting south of the
area, with 2 inches across the far southern portion of the
forecast area for the highest amounts. Have to be careful of
dropping a convective type system too quickly. Appears the south
trend is enough to remove Portage and Wood Counties from the
advisory. For the rest of the advisory area, will reduce the time
period but still include the afternoon commute period. Will also
mention that the focus is for far southern areas of the forecast
area, or south of highway 21. A little more southern trend and the
advisory can be completely removed.

System departs quickly tonight with drier air pouring in on north
winds. Do have some gusty winds over eastern Wisconsin, but
anticipate the snowpack should be compacted enough to prevent any
blowing or drifting snow issues.

And finally forecast max temperatures on Thursday will have some
challenge. Guidance numbers were in the lower 50s on Thursday with
abundant sunshine and warm start near 30 Thursday morning. Will
go with a tad lower numbers due to the snow cover and low surface
dewpoints.

.LONG TERM...Thursday Night Through Tuesday
Issued at 356 AM CDT Wed Apr 18 2018

A much quieter and warmer stretch of weather to take over
northeast WI starting late this week and continuing into early
next week. Two separate upper ridges to be the main reason as a
closed upper low passes well to our south (this weekend) and the
mean flow to be near zonal. Our next chance of seeing
precipitation is not forecast to arrive until Tuesday when a
surface cold front/mid-level shortwave trough moves across WI. As
temperatures finally warm to near normal beginning on Sunday,
hydrology concerns increase due to melting snow and runoff into
rivers and streams. More on this below in the Hydrology section.

High pressure is forecast to dominate east-central NOAM Thursday
night, bringing clear skies, light winds and cold conditions to
northeast WI. Guidance temperatures are likely too warm
considering all the snow that is on the ground, so will go below
model guidance for min temperatures. Look for readings to range
from the upper teens to lower 20s central WI, lower to middle 20s
eastern WI. The gradual warm-up continues on Friday as the high
pressure remains in firm control of our weather and the first
upper ridge arrives. Plenty of sunshine and light winds will
prevail with max temperatures ranging from the middle to upper 40s
near Lake MI, to the lower 50s across northern/central WI.

The high pressure is expected to stretch from Hudson Bay, through
the Great Lakes, to the southeast CONUs Friday night. Another
night with mostly clear skies (although some high clouds may
arrive late) and light winds which should again send temperatures
below guidance. Have gone with middle 20s north and middle to
upper 20s south. As the surface high slips farther east on
Saturday, south to southwest winds to return to WI. Temperatures
aloft do not make much in the way of warming and there is still
that snowpack to deal with. Any clouds from a system located over
the southern Plains should dissipate, thus bringing another mostly
sunny day. All in all, another day with below normal/below
guidance temperatures with readings in the middle to upper 40s
near Lake MI, lower to middle 50s elsewhere.

The second upper ridge moves into the Upper Midwest/western Great
Lakes Saturday night and reside over the Great Lakes on Sunday.
Saturday night still looks mostly clear and cool with light
south-southeast winds, while Sunday could see a few more clouds as
warmer air aloft starts to move into the region. These warmer
temperatures, combined with a lessening snowpack should allow max
temperatures to reach the upper 40s near Lake MI, mainly middle
50s elsewhere.

Quiet weather conditions are expected to start the work week as
northeast WI to reside on the western fringe of surface high
pressure located over the eastern Seaboard. Min temperatures by
Sunday night may only drop to around freezing and with relatively
light winds, we may eventually have to add fog to the forecast.
Otherwise, the slow moderating trend to continue with max
temperatures on Monday to range from the lower to middle 50s near
Lake MI, upper 50s to around 60 degrees elsewhere.

The next chance for precipitation potentially arrives Monday night
as a cold front and trailing mid-level shortwave trough push
southeast into the area. The ECMWF is faster with the movement of
the cold front compared to the GFS or GEM. This would impact both
timing of precipitation and how warm we can get on Tuesday. The
slower movement could bring temperatures at or above normal, which
has not occurred since March 28th. Since we are dealing with Day
7 here and models are bound to change several times between now
and then, have followed the consensus solution which does bring a
chance of rain showers to the forecast area.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1235 PM CDT Wed Apr 18 2018

MVFR ceilings covered the region early this afternoon, but partial
clearing was already occurring over the far north. This clearing
trend will gradually work its way south during the afternoon and
evening. Low pressure moving across Illinois will produce light
snow and associated MVFR/IFR conditions south of a line from
Wisconsin Rapids to Oshkosh to Manitowoc during the late afternoon
and early evening. Spotty light showers or drizzle are possible
just north of the snow band. High pressure will build into the
area overnight into Thursday to produce clear skies and VFR
conditions.

Gusty north to northeast winds will continue into the evening
hours. LLWS will develop across parts of the region this
evening, then taper off late tonight. Have added LLWS to the
GRB/ATW/MTW TAFs.
&&

.HYDROLOGY...
Issued at 356 AM CDT Wed Apr 18 2018

Warming temperatures are expected to produce significant melting
of the current snowpack over northeast WI. This will lead to
runoff into area rivers and streams. Most of the rivers in
northeast WI were low before the historic snowstorm hit, thus only
within bank rises are forecast. However, 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.
&&

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

$$

UPDATE.........Kieckbusch
SHORT TERM.....TDH
LONG TERM......Kallas
AVIATION.......Kieckbusch
HYDROLOGY......Kallas



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