797 
fxus63 kilx 212105 cca 
afdilx 


Area forecast discussion...corrected 
National Weather Service Lincoln Illinois 
305 PM CST Sat Nov 21 2009 


Discussion... 
issued 305 PM CST Sat Nov 21 2009 


A large area of high pressure continues to dominate the eastern 
1/3 of North America...including central and southeast Illinois. 
This will continue to be the case into early next week providing 
quiet and mild weather for late November. Main attention this 
forecast package revolves around a storm system that is expected to 
develop over the plains on Monday. This system will push through the 
forecast area Tuesday with an associated risk of rain. Much cooler 
temperatures...and possibly some wrap around light rain/snow...are 
expected in the wake of the system. 


12z forecast guidance is starting to come to a consensus on the 
timing of Tuesday/S system after quite a bit of flip-flop and 
spread over the past several runs. Hopefully...this trend 
continues and is not a one model run fluke. Have tried to follow a 
solution that is in the middle of this closer model spread...which 
probably most closely resembles the NAM or sref solution. 
However...none of the solutions would have a tremendously different 
outcome at this time. 


Short term...tonight through Tuesday...quiet and mild weather is 
expected to persist through Monday as the surface high drifts off 
to the east and return flow increases. Aloft...a split flow...with 
relatively weak flow overhead is expected into Monday. Do not 
expect fog to be much of a concern the next two nights despite 
light winds and partly cloudy skies. There was not much fog this 
morning...and the low levels/surface soils have likely dried a bit 
more with a full day of insolation. 


A vigorous upper wave is expected to dig into the Central Plains 
on Monday...with associated surface cyclogenesis. A lead short 
wave will eject across the Midwest as the main wave digs in...but 
moisture profiles appear too shallow to produce much more than 
clouds with the lead wave. The main surface low will lift into the 
Great Lakes by Tuesday night...with the trailing cold front 
expected to clear the forecast area by around 00z Wednesday. This 
system should be pretty dynamic...though already occluded by the 
time it gets here...with decent jet forcing prognosticated over the 
forecast area on Tuesday coupled with the surface frontal passage. Have 
bumped up western half of the forecast area to likely rain chances 
for Tuesday...with the eastern half probably needing a bump as 
well once the timing is more firm. 


Long term...Tuesday night through Saturday...a secondary short 
wave is prognosticated to dig into the mean upper trough centered over the 
upper Midwest/Great Lakes on Wednesday. This wave may spread some 
wrap around precipitation into the forecast area...though it 
should be more widespread further to our north. The cold air mass 
surging in behind the system may have some snow mixing in with any 
wrap around rain...especially Tuesday night and Wednesday night 
when surface temperatures are coolest. The system will finally 
pull away Thanksgiving day into Friday as high pressure/upper 
ridging build in...bringing a return to quieter and milder weather. 


Bak 
&& 


Aviation... 
issued 1143 am CST Sat Nov 21 2009 


A storm system over Texas continues to push middle-level moisture 
northward across western Illinois. Latest satellite imagery 
indicates scattered-broken clouds with bases around 7000ft along and west 
of I-55...with clear skies further east. Clouds will tend to 
dissolve as they try to move northeastward into a drier air mass 
beneath surface high pressure centered over Indiana. As a 
result...will only carry scattered clouds at kcmi and kdec. Further 
west...will maintain broken middle-deck at kpia and kspi through the 
afternoon into the evening. 12z NAM-WRF and GFS relative humidity profiles 
indicate a drying trend overnight...so will scatter out the clouds 
across the western terminals between 05z and 06z. Winds through 
the entire 18z taf period will remain light out of the E/se. 


Barnes 
&& 


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


$$