Forecast Discussion for GRB NWS Office
FXUS63 KGRB 301112

Area Forecast Discussion
National Weather Service Green Bay WI
612 AM CDT Wed Sep 30 2020

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 254 AM CDT Wed Sep 30 2020

The region will stay under the influence of an upper level trough
through Thursday, which will continue to bring periods of showers
to the region as several shortwaves round the trough during this
period. In addition to the showers, some thunder is also possible
this afternoon with steep low level lapse rates of 8 to 10
Celsius/km through 800 mb along with some weak MUCAPE. Wet- bulb
zero heights will be very low, at 4 to 5k ft, bringing the
possibility for some small graupel and/or small hail. In
addition, there could be some gusty winds of 35 mph at times with
the aforementioned steep low level lapse rates bringing gustier
winds to the surface. Highs this afternoon will mainly be in the
50s, with some 60 degree readings possible across eastern

Shower activity is expected to drop off a bit tonight with the
loss of daytime heating, however, winds turning northerly will
bring some lake effect rain showers to far north-central Wisconsin
tonight. Lows tonight are expected to range from the upper 30s
across north-central Wisconsin, to the lower to middle 40s across
central and east-central Wisconsin.

Shower activity will once again pick up on Thursday with daytime
heating as a reinforcing shot of cold air pushes south across the
region. The thunder threat should be fairly low on Thursday as
low level lapse rates will not be as steep as this afternoon with
hardly any CAPE to speak of. Afternoon highs on Thursday are
expected to range from the middle to upper 40s across the north,
with highs in the middle 50s across east-central Wisconsin.

.LONG TERM...Thursday Night Through Tuesday
Issued at 254 AM CDT Wed Sep 30 2020

500mb ridge across the western United States with downstream
troughing across eastern North America is expected at the
beginning of the period. The western ridge will flatten over
the weekend with down stream troughing continuing across the
western Great Lakes region into early next week.

For Thursday night into Friday morning, northerly flow off Lake
Superior along with a weak disturbance moving southeast into
northern Wisconsin will be the catalyst for a small chance of rain
or snow showers during the evening with a chance of snow showers
after midnight. Any lingering rain or snow showers across the
north should end Friday morning with dry conditions expected for
the rest of the day. Across central and portions of northeast
Wisconsin, frosty conditions are expected Friday morning (and
again Saturday morning. We are still issuing frost/freeze
headlines for locations generally south of a Merrill to Antigo to
Wausaukee line. Frost headlines will likely be needed Friday
morning and probably again Saturday morning.

The cool cyclonic flow aloft will bring a small chance of
instability showers Saturday afternoon. The system for Saturday
night into Sunday morning has shifted south and east on the
GFS/ECMWF. If this track holds true, a steadier rain would only
impact northeast Wisconsin with much of north-central and far
northeast Wisconsin remaining dry once the instability showers
dissipate around sunset. The Canadian model remains furthest
north. Did not want to make wholesale change due to the drastic
southern shift in the storm track. It is possible that the track
could trend northward with later model runs. Dry conditions are
expected Monday and Tuesday with a gradual warming trend.

.AVIATION...for 12Z TAF Issuance
Issued at 612 AM CDT Wed Sep 30 2020

A shortwave and frontal boundary will track through the area
today, bringing another round of widespread showers and MVFR cigs
to the TAF sites. A few thunderstorms are also possible, however
coverage will be too isolated to include in the current set of
TAFs. The strongest storms may produce graupel/small hail as well
as gusty winds to 35 mph. Winds will become gusty out of the west
to northwest between 20 to 25 knots as the pressure gradient
tightens. Shower activity will wane this evening with the loss of
daytime heating. The main exception will be north-central
Wisconsin where some lake effect rain showers will continue
through much of the overnight hours.

Issued at 254 AM CDT Wed Sep 30 2020

Cool 850 mb temperatures and fairly warm lake temperatures,
around 15 to 16 degrees Celsius, will bring the chance for
waterspouts at times to the nearshore waters this afternoon into
Thursday. 850 mb temperatures today will cool to 2 to 3 celsius
this afternoon, with convective cloud depths of 20 to 25k ft. By
Thursday 850 mb temperatures fall to -1 to - 3 celsius, with
slowly falling convective cloud depths through the afternoon
hours. Due to the deeper cloud depths, will include waterspouts
starting this afternoon, and continue through Thursday with
better over-water instability. The threat for waterspouts should
dissipate by Thursday evening as convective cloud depths become

Increasing west to northwest winds will gust over 25 knots at
times today into early this evening, and again Thursday into
Thursday evening. A Small Craft Advisory is in effect from 10 am
to 10 pm today with another one likely on Thursday.



SHORT TERM.....Kurimski
LONG TERM......Eckberg

NWS GRB Office Area Forecast Discussion