Forecast Discussion for GRB NWS Office
FXUS63 KGRB 110344

Area Forecast Discussion
National Weather Service Green Bay WI
1044 PM CDT Sat Apr 10 2021

Updated aviation portion for 06Z TAF issuance

Issued at 312 PM CDT Sat Apr 10 2021

Windy and rainy tonight, especially in east-central Wisconsin.
Scattered showers and seasonal temperatures for the start of the
work week.

The blocking pattern across eastern North America will persist
through most of the period. A transition to a more consolidated
pattern with ridging near the West Coast and a trough over central
or eastern North America will begin late in the period.

Temperatures during the period will vary from day to day, but
probably not stray too far from seasonal normals. Precipitation
will probably end up AOA normal for the period, with the bulk of
it falling early in the period.

.SHORT TERM...Tonight and Sunday
Issued at 312 PM CDT Sat Apr 10 2021

After a few spots saw a brief period of sunshine this morning,
cloudy skies prevailed across the area during the early afternoon
hours. Other than a few sprinkles, it remained dry, as any precip
advancing northward was gobbled up as it encountered the dry in
the mid-levels. Temps across far north central WI and near Lake
Michigan struggled to climb to the 50 degree mark, with most other
locations in the 50s. Main forecast concerns will be timing the
best rain chances, rain amounts and how windy it will be. Despite
the rain and clouds, temps will remain above normal tonight and
near normal on Sunday.

For this afternoon and a vertically stacked low
pressure system slowly tracks into southern Lake Michigan
tonight, increasing moisture and lift will allow the large area of
rain to our south to advance into the area. Heaviest batch of
rain looks to occur later this evening and overnight as an FGEN
band works through the area. A widespread soaking rain is
expected across eastern WI, with the area slowly pushing westward
overnight. PWATS will be hovering around 1" through the night,
which is around 2.5 standard deviations above the climatological
normal. Models still showing 0.40-0.80" of rain for much of
eastern WI, with some totals near/over 1" also possible where the
FGEN band persists the longest due to the impressive deep layer
moisture. Ongoing thunder over MI and eastern Lake Michigan, so
still can`t rule out a rumble of thunder of the SE CWA, as some
steeper lapse rates rotate through the area, but will continue to
keep it out of the forecast. Patchy fog is expected through the
night, but the rain, clouds, and wind should keep the fog in
check. The exception will be on Lake Michigan, where some dense
fog looks to continue into the evening, before the gusty winds and
rain provide enough mixing to allow the fog to lift into a
stratus deck. As the low deepens, the pressure gradient will
tighten up this evening. Northeast winds look to gust between
20-30 mph for much of the area, with a few higher gusts possible
across eastern WI with some funneling help from the Bay of Green
Bay and reduced friction across Lake Michigan. The clouds, rain
and wind will only allow temps to fall into the 40s. A few upper
30s will be possible over far north central WI and near the colder
waters of Lake Michigan.

For Sunday...the FGEN will weaken early in the day, allowing the
more widespread rain to transition to a more showery nature.
Models not in the best agreement where the most concentrated
activity will be, but will focus the highest PoPs over north
central WI where the deepest moisture looks to be. Went with
chance PoPs elsewhere. Additional rainfall will be on the light
side as the better forcing exits the area, but some amounts up to
0.30" will be possible. As the low weakens/fills early in the
day, look for winds to diminish, especially across eastern WI. The
cyclonic flow will keep plenty of clouds across the area. Temps
won`t be able to budge much through the day, only creeping into
the mid- upper 40s or possibly lower 50s.

.LONG TERM...Sunday Night Through Saturday
Issued at 312 PM CDT Sat Apr 10 2021

Upper troughing comprising the southwest leg of the upper block
will linger across the area into the start of the work week.
Forcing won`t be very organized, and likely driven by small scale
shortwaves that aren`t going to be well resolved this far in
advance. So will have pretty widespread slight chance/chance PoPs
for the start of the work week. The blended guidance was
generally dry thereafter. However, it is more likely there will be
some scattered/light precipitation at times during the latter
part of the week as well. But its not really possible to resolve
when and where this far in advance, so stuck with the dry
forecast in later period for now.

.AVIATION...for 06Z TAF Issuance
Issued at 1041 PM CDT Sat Apr 10 2021

Rain continues across the area late this evening as a low
pressure system tracks southeast of the area. MVFR/IFR conditions
are expected to settle in at IFR, with some LIFR conditions
possible with persistent rain, drizzle, and fog. Sunday is a bit
uncertain as some models keep conditions at IFR with drizzle and
rain while others improve conditions to MVFR/VFR. At this point
will lean towards a more pessimistic forecast as only one model
improves conditions at this point.

Winds will be out of the north/northeast through much of the TAF
period, with some east winds likely closer to Lake Michigan. Gusts
in the 15-25 kt range are expected, locally higher at GRB/ATW due
to funneling down the Bay of Green Bay and Fox Valley and at MTW
with the northeast/east flow over Lake Michigan overnight. Winds
are expected to ease on Sunday and turn northerly as the pressure
gradient weakens across the western Great Lakes.



SHORT TERM.....Bersch
LONG TERM......Skowronski

NWS GRB Office Area Forecast Discussion