713 
fxus63 kilx 180130 
afdilx 


Area forecast discussion 
National Weather Service Lincoln Illinois 
830 PM CDT Wednesday Mar 17 2010 


Discussion... 
issued 830 PM CDT Wednesday Mar 17 2010 


High pressure is located over Kansas this evening resulting in 
mainly clear skies and light north to northwest winds. Main forecast 
issue will be fog formation overnight. All areas eventually cleared 
out this afternoon and this allowed some of the boundary layer 
moisture to be mixed out. This is reflected in the current dewpoint 
readings which range from the lower 40s in the west to around 30 in 
the east. The westerly component to the boundary layer flow suggests 
that some of the moisture in the west should get advected east and 
result in some fog formation again tonight. This is also indicated 
in the boundary layer humidity forecasts from the ruc13 model which show 
near saturation over at least the western half of the area by 09z. 
Therefore will keep the mention of fog after midnight. Overall...the 
forecast looks on target and do not anticipate any changes. 


04 
&& 


Aviation... 
issued 615 PM CDT Wednesday Mar 17 2010 


Surface hi moving southward through cntl plains will keep cntl Illinois in light northwesterly low level 
flow through Thursday morng. Toward 18z Thursday...winds will begin to firm up from the west 
ahead of a cold fnt that will move to cntl Iowa by 00z Friday. Only weather issue will 
be fog potential tonight. Surface winds are expctd to back to light northwesterly as 
the boundary layer decouples...with a corresponding rise in dewpoint and relative humidity. 
Boundary layer relative humidity forecasts and forecast soundings suggest fog will develop over at least 
the western taf sites as the westerly component to the low level flow tugs higher 
dewpoints over western Illinois eastward lt tonight. Do not expct widespread 1/4sm as was the 
case this morng...but we should see visibilities at least down to a cpl of 
miles after 06z Thursday extndg until about 15z Thursday. 


04 
&& 


Previous discussion... 
issued 322 PM CDT Wednesday Mar 17 2010 


Forecast problem of day is weekend storm and fog tonight. 


Short term...tonight through Friday night. 
Though fog and low cloudiness cuaght beneath strong inversion this 
morning eroded by dry air aloft by midday...expect to see some fog 
again tonight. Moisture will still be caught beneath 
inversion...however mixing today of drier air into lower levels will 
make extent much less tonight and so do not expect as much fog. 


Long term...Saturday through Wednesday. 
Forecast problem of possible system for weekend is interesting. 
Models still do not have a good handle on system. GFS is still very 
fast on front and upper trough moving through area with NAM 
slower...but with a weaker surface low development. Eur 12z is 
closer to NAM in movement but stronger on a surface wave. Tried to 
update the grids for cooler temperatures on Saturday in the northwest area 
due to colder air behind front and possible snow and mixture of snow 
and rain. But due to temperatures and mixture...kept snow amounts lower 
than quantitative precipitation forecast. Little unsure on south extent of snow extent on Sat 
night...due to the strength of the surface low depicted in each 
model. Lowered temperatures and added snow rain mixture. Sunday will again 
have mixture due to precipitation behind system...but may have to make later 
modifications on temperatures. 


After system moves through Monday...warming to occur into midweek. 


Goetsch 
&& 


Ilx watches/warnings/advisories... 
none. 
&& 


$$