865 
fxus63 kgrb 230936 
afdgrb 


Area forecast discussion 
National Weather Service Green Bay WI 
336 am CST Monday Nov 23 2009 


Short term...today through Tuesday...the potential for dense fog 
this morning and precipitation trends on Tuesday are the main forecast concerns. 


Aside from a few sprinkles over our far northwest counties...the precipitation has 
shifted west of grb County Warning Area early this morning. With the main moisture 
band expected to stay to our west near a developing frontal boundary 
today into tonight...and weak surface ridging in place over NE WI... 
expect mainly dry conditions to prevail. As the upper low lifts into 
the upper Midwest and the lfq of a jet streak punches into the region 
on Tuesday...expect some patchy light rain to develop during the 
afternoon. The previous forecast had this scenario well covered...so 
only minor changes were made. 


Closely monitoring fog potential early this morning. In areas where 
skies had cleared (ne/c/ec wi)...fog and low stratus had developed. 
A few sites had already dropped to 1/4sm...but widespread dense 
fog had not yet developed. Boundary layer winds are expected to 
diminish to around 10 kts by 12z...which may allow the stratus 
deck to lower to the surface. If this happens...will need to issue a 
dense fog advisory. Will watch trends over the next hour or 
two...and if needed...issue a dense fog advisory well before rush 
hour begins. 


Fog may also be a concern tonight...especially in NE/ec WI...where cloud 
cover not not be as thick...and boundary layer winds will be light. 


Mixing through 925-900 mb supports high temperatures in the lower to 
middle 50s today...close to met guidance. Used a blend of met/mav 
guidance for tonight and Tuesday. 


Long term...Tuesday night through Sunday 
after looking at the models over the last four nights...confidence 
is high to add some beef to the forecast for Tuesday night into 
Thanksgiving day. Models continue to show upper low across the 
middle Mississippi River valley Tuesday afternoon moving into the 
western Great Lakes region by 12z Wednesday. Meanwhile...more energy 
is diving southeast out of Canada on 110 to 120 knot jet streak. 
By 12z Wednesday...models continue to show a 500mb closed low over 
the western Great Lakes and another trough 500mb trough dropping 
into the the eastern Dakotas. This energy will continue to dive 
southeast and these two 500mb features will then combined into one 
closed 500mb low over northern Illinois Wednesday night. This 
system will then gradually pull away from the area on Thanksgiving 
day. This will mean a prolong period of precipitation that will 
begin Tuesday night and continue through much of Thanksgiving day. 


For Tuesday night...low pressure will move from Iowa into southeast 
Wisconsin. Rain will spread northward into the area. The far north 
will be hovering around 0c at 850mb...but 925mb temperatures way too 
warm to support snow. Have increased to high chance probability of precipitation over the 
far north and went with likely over the southern two thirds of the 
forecast area. Did raise minimum temperatures a few degrees based on 
model surface forecast temperatures and numerical guidance. 


On Wednesday...surface low is expected to move from southeast Wisconsin 
into central Upper Michigan. Inverted trough expected to linger across 
northeast into central Wisconsin during the day. This will be one of 
the more unusual meteorological set ups as the models are indicating 
the warmest 925mb temperatures north of the inverted trough. Normally 
do not see that too often. The coldest 850mb/925mb temperatures will 
wrap around to the south of the forecast area. Another problem is 
that air mass ahead of the system and near the inverted trough is 
too mild for snow...so do not expect any snow across the area on 
Wednesday. 


For Wednesday night...the upper level features will consolidate 
into one closed upper low. The surface low will then remain nearly 
stationary through much of Wednesday night with a slow movement 
of the inverted trough. Precipitation type forecasting for this 
period is difficult as the warmest 925mb temperatures will drop 
southeast into northeast Wisconsin. The slow progression of the 
inverted trough impedes the colder air to work into the forecast 
area. Based on the 1000/850mb thickness pattern and boundary 
layer temperatures...will have rain across the east and a rain 
snow mix over the west. 


For Thursday...inverted trough swings slowly southeast into 
southeast Wisconsin. Again boundary layer temperatures will be 
critical for any accumulating snowfall. Thickness patterns 
would suggest more snow in the west and mainly rain/snow mix 
in the east. Boundary layer temperatures off the models would 
suggest more of a rain snow mix in the west with the exception 
over the higher terrain of the far northwest portion of the 
forecast area. Over the east...boundary layer temperatures would 
suggest mainly rain. As a compromise...did add a chance of snow 
over the east except in Door County during the late afternoon. 


As you can see...a very unique and complex situation setting up 
for the busy travel day before Thanksgiving and Thanksgiving day 
itself. Later shifts can fine tune the details. Will highlight 
any snow potential to Wednesday night into Thursday and confine 
to locations northwest of Green Bay and the Fox cities. 


The remainder of the forecast looks to be in good shape with 
little changes made. 
&& 


Aviation...widespread LIFR/vlifr visibilities/ceilings in fog are 
anticipated across much of NE/ec/c WI this morning...due to 
favorable radiational cooling conditions and moist flow off Lake 
Michigan. Lingering cloud cover over NC WI may keep ceilings and 
visibilities from dropping as low there. The foggy conditions should 
improve by around 15z today. More fog is possible tonight...especially in 
NE/ec WI...where partly cloudy skies and light east winds off Lake 
Michigan will once again produce favorable conditions for fog 
development. Generally dry conditions are anticipated until 
Tuesday afternoon...when some patchy light rain may arrive. 
&& 


Grb watches/warnings/advisories... 
none. 
&& 


$$ 
Kieckbusch/eckberg