403 
fxus63 kgrb 212058 
afdgrb 


Area forecast discussion 
National Weather Service Green Bay WI 
258 PM CST Sat Nov 21 2009 


Short term...tonight and tomorrow. 


Latest RUC analysis and visible/IR/WV imagery show high pressure moving 
to the east over the central Great Lakes and return flow advecting a 
1.5-2kft stratus deck into central Wisconsin. The fog was slow to 
burn off today...which combined with the incoming stratus has 
prevented temperatures from climbing out of the middle 40s over 
central Wisconsin. Still seeing alot of reduced visibilities this 
afternoon...mostly in the MVFR range...but there are still some IFR 
stragglers at arv and auw. Forecast concerns are plentiful for this 
forecast package...and include cloud and fog potential tonight...and 
precipitation chances tomorrow. 


Tonight...high pressure shifts farther to the east...allowing the 
pressure gradient to tighten across the forecast area. 
Southwesterly winds will strengthen to 25-35kts at 925mb...which 
should help keep the boundary layer more churned up compared to last 
night...and limit fog potential somewhat. These southwesterly winds 
will also continue to advect low level moisture and a stratus deck 
from the Gulf Coast into the region. It looks to be most widespread 
across the western forecast area...but not out of the question that 
it could expand across the Fox Valley as well given the already 
moist boundary layer and nocturnal cooling. Checking out the ups 
fog forecasting technique...boundary layer wind speeds between 
15-20kts suggest an elevated stratus deck but with build down 
potential (absent of clouds aloft). Given the IFR/MVFR visibilities still out 
there early this afternoon...wouldnt take much cooling for visibilities 
and/or ceilings to crash again...especially if there are breaks in the 
clouds like there are in Iowa and northern Missouri. Not confident 
that many breaks will occur...as nocturnal cooling should help any 
breaks fill back in. As a result of the increase uncertainty with 
the cloud cover and winds...will keep in patchy fog for the entire 
area and let the evening crew upgrade if conditions warrant tonite. 


Tomorrow...southwesterly winds will continue to advect moisture 
below 10kft into the area as a cold front moves into Minnesota and 
far Western Lake Superior by the afternoon. Best forcing will reside out in 
that direction...with convergence along the front...and upper 
divergence in the right entrance region of a jet streak. Moisture 
above 10kft will be sorely lacking in our area...and do not see 
enough lift or saturation depth for anything but some 
sprinkles...mostly in the afternoon. So...we are looking at a 
mostly cloudy day west and partly sunny east with highs in the upper 
40s to middle 50s. 


Long term...Sunday night through next Saturday. Fog potential 
and precipitation chances the main challenge through Thanksgiving as return 
flow on the backside of the departing ridge begins to affect the 
area. Models continue to wash out the initial frontal boundary 
Sunday night into Monday over Minnesota in response to the digging 
western trough and persistent ridge over the Great Lakes region. 
Will maintain light non measurable precipitation mention for Sunday night 
as rrq region lifts north...but depending on cloud cover...fog 
development will likely be the main focus through Monday night for 
the long nights. 


Medium range models forecast the developing upper low with surface 
low to begin to lift from the Central Plains into the Great Lakes 
region later Tuesday through Thursday. Latest runs indicate main 
upper support to pass over the south half of WI. Waffled between 
high end chance or likely probability of precipitation but timing and location of best upper 
support in question with this system. In addition...precipitation type will 
be a problem. Temperature profile suggests mainly rain initially but 
changing to a mix first over the northern and western areas as the 
column begins to cool later Wednesday into Wednesday night. But cold 
core with the upper low may also become cold enough to produce a 
band of snow as well. 


Upper heights continue to fall through Wednesday night as an upper 
jet with reinforcing shortwave drops into into the plains to 
deepen the system. Latest European model (ecmwf) continues to deepen the 800 mb low 
over northern lower Michigan through Thursday which indicates some 
accumulate snow potential over NE WI. The GFS is faster with this 
secondary 800 mb low and further south. Would like to get a few more 
model runs with this system before giving a heads up for the 
Thanksgiving travel. 


System prognosticated to depart late in the week to return to a drier 
forecast with near normal temperatures for late November. 
&& 


Aviation...IFR/MVFR visibilities in mist will continue to plague the taf 
sites tonight in a rather stagnant airmass. Satellite imagery and 
model data indicate a steady supply of low level moisture advecting 
from the Gulf Coast into the region tonight...creating a MVFR cloud 
deck at auw/cwa/rhi and possibly even grb. Not out of the question 
the stratus will build down into an IFR deck either. MVFR cloud 
cover and visibilities will persist into tomorrow morning. Some indication 
that ceilings will lift to VFR status around midday. 
&& 


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


$$ 
Mpc/tdh