Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 132306
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
506 PM CST Tue Feb 13 2018

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Wednesday
Issued at 226 PM CST Tue Feb 13 2018

Warm air advection through the afternoon will bring highs into
the the upper 20s and lower 30s today. Did bump the overall highs
down a degree across central Wisconsin from this morning however,
due to the thicker and longer lasting sky cover. Clearing skies
are still expected by this evening, as upper to mid level moisture
pulls out of the region.

The low level jet will subside somewhat by the overnight hours,
which coupled with the building low level inversion, will remove a
lot of the gust potential overnight. However, southwesterly winds
will continue with sufficient strength to keep mechanical mixing
of the lower levels going overnight. Moisture will also increase
in the lower levels, sufficient to bring back lower level clouds
to the region starting from midnight to the early morning hours.
Together, this will keep temperatures from falling as sharply
overnight as the previous couple nights. Bumped temperatures up a
degree from the initial guidance as well, but there is a chance to
be even warmer if the clouds come in thicker or earlier
overnight.

Warm air advection continues on Wednesday as persistent
southwesterly flow pushes across the region. Stayed close to the
initial guidance for high temperatures with only a small
adjustment for the typical warm and cool spots for the day. Cloud
cover should become more sparse once more as sufficient winds
aloft, coupled with diurnal heating, help mix in the drier layers
aloft.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 226 PM CST Tue Feb 13 2018

A gradual change in the mean flow in expected over the extended
period with an eastern Pacific upper ridge slowly weakening, the
subtropical upper high moving from the Gulf of Mexico to the
western Atlantic and the mean flow into WI shifting from west-
northwest to southwest early next week. Precipitation chances look
to be low for the rest of this week, but tend to increase once the
flow turns southwest and a more active pattern develops with gulf
moisture becoming more available. Temperatures will continue to
yo-yo over the next week going from above normal (Thursday), to
below normal (Friday), above normal (this weekend) and closer to
normal (next Tuesday).

Models are in general agreement with sending a clipper system
east-southeast into northern sections of the Great Lakes Wednesday
night. A weak surface trough will move east across WI during the
night, however all the precipitation associated with this clipper
will be located north of a cold front that is forecast to stretch
west-southwest from the surface low to the northern Plains. For
northeast WI, we should see clouds on the increase, especially
over northern WI closer to the clipper/cold front. However, do not
see the need to mention any pops of consequence in the forecast.
Min temperatures to range from the lower to middle 20s north-
central WI, upper 20s to around 30 degrees east-central WI. The
cold front moves through WI on Thursday, however precipitation
chances remain spotty as incoming gulf moisture is interrupted by
a system moving just north of the Ohio Valley. Therefore, the only
pops mentioned will be over northern WI (as CAA develops and
northwest winds may produce some lake effect) and over east-
central WI (as this area may be clipped by the southern system).
Max temperatures on Thursday to range from around 30 degrees
north-central, middle to upper 30s eastern WI.

Chance of light snow will be needed across most of the region
Thursday night as a shortwave trough swings into the western Great
Lakes. In addition, lake effect snows will continue over north-
central WI, although trajectories gradually become unfavorable as
winds go from north-northwest to west-northwest. Overall, this is
a minor event with any accumulations limited to one-half inch or
less (except for Vilas County where lake effect may push amounts
toward one inch). The main story for Thursday night will be the
arrival of arctic air with min temperatures around zero north-
central, 10 to 15 above east-central WI. Wind chills could dip
into the middle teens below zero over north-central WI. An area of
high pressure is forecast to move from the central Plains into the
Midwest on Friday. Look for skies to be mostly sunny, however it
will be cold with max temperatures only in the middle teens
north-central/parts of central WI, upper teens to around 20
degrees over eastern WI.

The surface high to move east-northeast toward the eastern Great
Lakes Friday night, with a return flow into WI, especially after
midnight. We could see a gradual increase in high or middle clouds
during the overnight hours, but prefer to hold off on any
precipitation as it will take a little time for the atmosphere to
saturate. Models disagree on how much saturation will occur headed
into Saturday and this is key to the forecast as a cold front
pushes across WI. May need to add a small pop to our forecast,
especially with the GFS and GEM indicating measurable
precipitation. Max temperatures to rebound with readings in the
upper 20s to lower half of the 30s.

After a quiet Saturday night with a ridge of high pressure sliding
through the region, attention turns to an area of low pressure
that is progged to move from the northern/central High Plains into
the Upper MS Valley on Sunday. The onset of stronger WAA, coupled
with a quasi-stationary front stretched from the Upper MS Valley
through Lake Superior, will bring an increasing chance of
precipitation to northeast WI. Exactly when the precipitation
develops will determine how warm we can get which in turn, would
impact precipitation type. Right now, currently have max
temperatures in the lower to middle 30s north-central, middle to
upper 30s elsewhere. This would keep precipitation type mainly as
snow north-central, a mix or all rain for the rest of the forecast
area.

As the surface low moves northeast through northern sections of
the Great Lakes Sunday night, an accompanying cold front is
expected to push into central WI toward daybreak. Models show bulk
of precipitation to be post-frontal, thus higher pops for Sunday
night should be over the northwest half of WI. Temperatures
should be cold enough such that precipitation type would be all
snow for north-central WI where an accumulation appears likely.
This cold front drives through the rest of WI on Monday with
temperatures cold enough to support snow over the entire forecast
area. Better frontogenetical forcing signal over central/east-
central WI, thus higher pops placed there with accumulations
possible. Max temperatures Monday to be cooler with readings in
the lower to middle 20s north-central, lower 30s east-central WI.

An unsettled weather pattern continues into Tuesday as the
southwest flow aloft helps to pull gulf moisture northward and the
cold front stalls to our south, allowing for an overrunning
situation to develop across WI. Exactly how of this plays out is
still in doubt, however temperatures look to remain cold enough to
keep any precipitation as all snow. Max temperatures on Tuesday to
range from the lower 20s north-central, to the upper 20s across
eastern WI.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 506 PM CST Tue Feb 13 2018

Low level wind shear will be in place for most of the TAF sites
through the early morning hours of Wednesday, before diminishing.
Some MVFR conditions will be possible after midnight as low level
moisture gets trapped underneath a subsidence inversion. Skies
will clear again towards the end of the TAF period as a chance of
low level wind shear returns.
&&

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

$$

SHORT TERM.....Uhlmann
LONG TERM......Kallas
AVIATION.......Kurimski



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