Forecast Discussion for GRB NWS Office
FXUS63 KGRB 261112

Area Forecast Discussion
National Weather Service Green Bay WI
612 AM CDT Wed Sep 26 2018

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 313 AM CDT Wed Sep 26 2018

Gusty west winds and cooler temperatures will prevail across the
western Great Lakes today in the wake of a departing cold front.
Steep low level lapse rates and mid level troughing will cause
abundant afternoon cumulus clouds to form across the region,
especially across the north. Highs today will only range from the
middle 50s across the north, to the lower 60s across the east.

An approaching cold front will bring the chance for showers to the
north late tonight, then across the rest of the region on
Thursday. The best chances for rain on Thursday will be across the
north where the best moisture and dynamics will coincide.
Southwest winds will increase ahead of the cold front on Thursday,
with gusts to 25 mph. Lows tonight will generally be in the lower
to middle 40s. Highs on Thursday will range from the middle 50s
across the north, with lower to middle 60s across the southeast.

.LONG TERM...Thursday Night Through Tuesday
Issued at 313 AM CDT Wed Sep 26 2018

Models remain consistent in advertising upper ridging from the
West Coast toward Alaska, a broad longwave trough from central
Canada into the central Plains and a subtropical upper ridge from
the Gulf to the western Atlantic. This pattern would bring a cool,
somewhat unsettled period of weather through Friday. As the upper
ridge breaks down this weekend and is replaced by an upper trough
over the western CONUS, the mean flow into WI will turn from
northwest to west-southwest early next week. This pattern shift
would bring a warmer, more moist air mass into WI with an
increased chance of seeing precipitation. Trying to time these
precipitation chances to be the main focus.

Chance for showers to linger over eastern WI into Thursday evening
as the cold front to still be in the process of exiting the
region. Behind this front, a good shot of colder air is expected
to overspread northeast WI with 8H temperatures dipping to the
zero to -3C range by 12Z Friday. A northwest wind above the
surface could actually bring a few lake enhanced rain showers into
north-central WI later Thursday night. Otherwise, some partial
clearing is expected for the overnight hours with min temperatures
in the upper 30s north-central WI, around 40 degrees far
northeast/central WI and lower to middle 40s east-central WI.

Models now indicate the passage of a shortwave trough (with an
embedded shortwave) into the western Great Lakes Friday afternoon.
The forcing from this mid-level trough, combined with modest
instability from the cold air aloft interacting with daytime
heating, may be enough to allow for a small chance of showers to
develop. The rest of Friday should see more clouds than sun with
cool conditions to persist. Max temperatures to only reach around
50 degrees north-central WI, to the middle 50s east-central WI.

The exit of the shortwave trough, the loss of daytime heating and
the approach of high pressure will end any precipitation threat by
early Friday evening. As skies become partly cloudy and winds
diminish through the night, this cold lead to frost formation
especially across northern and central WI as temperatures fall
into the lower 30s. Even a few colder spots of east-central WI
could see some patchy frost with temperatures in the middle to
upper 30s. If skies can clear enough, a frost advisory may become
necessary for parts of the forecast area.

This area of high pressure moves across the western Great Lakes
region Saturday morning and already reach the eastern Great Lakes
by late Saturday afternoon. Some mixed morning sunshine is
forecast to give way to more clouds through the afternoon as
isentropic lift begins to develop on the backside of the surface
high. The cold start to the day, coupled with the increase in
clouds, will bring another cold day to the area. Look for max
temperatures to range from the upper 40s north-central, to the
lower 50s east-central WI.

Models begin to differ for the latter half of the upcoming weekend
with regards to the location of a warm front and how fast to lift
this warm front northward. The GFS is fastest with this frontal
boundary, along with a slug of WAA Saturday night, while the ECMWF
prefers to wait until later Sunday afternoon. There is also the
possibility of more frost Saturday night if the ECMWF is correct.
Conversely, there could be some wet snow mixed with the rain over
northern WI Saturday night should the GFS be correct. Confidence
is rather low on how this will actually play out and have kept
any pops on the low end for now until this can get sorted out.
Temperatures should uptick a bit by Sunday with lower 50s north-
central, mainly middle 50s elsewhere.

Models take this warm front into southern WI Sunday night and with
an increasing south-southwest low-level jet aimed at the Midwest/
western Great Lakes, expect to see showers and elevated
thunderstorms move across most of northeast WI. Some of this rain
may be locally heavy across central and east-central WI where
lapse rates are steeper and moisture is deeper. Showers/chance of
thunderstorms will continue on Monday as the warm front lifts into
central WI. Once again, locally heavy rainfall is possible, but
the location is still in doubt depending upon the location of the
warm front. There are questions headed into Tuesday as to whether
this frontal boundary will still be in our vicinity or shifted
farther east. For now, have kept chance pops in the forecast.
Temperatures should warm early next week with max temperatures by
Tuesday able to reach around 60 degrees north, middle 60s south.

.AVIATION...for 12Z TAF Issuance
Issued at 612 AM CDT Wed Sep 26 2018

A ridge of high pressure will move through the western Great Lakes
today and this evening. Winds could gust again to the 15 to 20
knot range this afternoon. Despite breaks in the clouds this
morning, afternoon cumulus clouds could bring a broken MVFR
ceiling at times. An approaching cold front could bring some
showers to north-central Wisconsin late tonight, mainly impacting
the RHI TAF site.



SHORT TERM.....Kurimski
LONG TERM......Kallas

NWS GRB Office Area Forecast Discussion