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Area forecast discussion 
National Weather Service Lincoln Illinois 
526 am CST Monday Nov 23 2009 


Discussion... 
issued 217 am CST Monday Nov 23 2009 


High pressure ridging still extending west across Illinois early 
this morning...but its effects are lessening with time. Regional 
radar mosaics showing some echoes in far west central 
Illinois...but surface observation are showing the clouds in this area 
mainly at the 10000 foot level or higher...and laps soundings in 
this area are fairly dry...so little precipitation is actually 
reaching the ground. 


The effects of a developing storm system in the Tuesday-Wednesday 
time frame remain the primary forecast concern...along with any 
lingering precipitation issues for Thanksgiving day. 


Short term...today through Thanksgiving day... 


Shortwave of interest is currently digging across The Rockies per 
latest water vapor imagery. Most of the short-range models are 
fairly uniform in the evolution of an associated surface cyclone 
over the Central Plains...lifting it northeast to southern 
Wisconsin by Wednesday morning. The NAM model is the exception... 
with a slower and more southern track into central Illinois...so 
this solution will be discounted at this time. Timing of the 
frontal passage has not changed much from the last forecast 
package...likely to be Tuesday evening. Likely probability of precipitation will spread 
from west to east across the County Warning Area on Tuesday...holding off across 
the east until afternoon. Will linger healthy probability of precipitation into the 
evening...then taper them off after midnight as the dry slot works 
its way across the area. 


A separate shortwave will quickly dive south out of Manitoba 
Wednesday morning...becoming the dominant closed upper low as the 
original low shears out as it lifts northeast. All models have 
this feature across northern Illinois by Thursday morning. 
Lingering light rain is expected on Wednesday primarily over the 
northern County Warning Area...in the circulation of the departing surface low. 
However...another trough/cold front will quickly approach from the 
northwest late Wednesday night...and swing through the area on 
Thanksgiving day. Thickness levels in the 1000-500mb layer plunge 
low enough to support a transition to snow Wednesday night...with 
BUFKIT soundings also showing this trend. Precipitation likely to 
continue into Thursday as either rain or snow...although the longer 
range models are a bit divided on the speed of this front...thus 
have differing opinions on the amount of precipitation involved. 
Currently will go with 30 probability of precipitation for this period and let the later 
model runs try to resolve this issue. 


Long term...Friday through Sunday... 


GFS and European model (ecmwf) both agree on a surface ridge pushing east across 
the central Continental U.S. On Friday...with the upper level flow becoming 
more southwesterly by Sunday morning as a pair of shortwaves move 
through the plains and southern Rocky Mountains. Any associated 
precipitation would likely hold off until at least Sunday...but 
the timing is a bit in question with differing solutions on the 
speed of the surface front. 


Geelhart 




&& 


Aviation... 
issued 515 am CST Monday Nov 23 2009 


Two areas of concern this forecast period. The first being with 
the patchy areas of fog this morning...with the second challenge 
being with the MVFR ceilings tracking northward out of southeast MO and western 
Kentucky this morning. Light wind flow coupled with abundant low level 
moisture from wet ground has led to some patchy MVFR/IFR fog...especially 
across the east. Will carry tempo groups for the fog through at least 15z 
and that may not be long enough as the extensive middle and high 
level cloud cover may limit the amount of mixing this morning and keep 
the lower visibilities in for a bit longer compared with the past few 
mrngs. 


Once the fog dissipates late this morning into early this afternoon...will 
see a brief period of VFR conditions before our attention turns to 
the MVFR and local IFR ceilings to our south for this evening and 
overnight. Based on the present movement of the lower ceilings to our 
south...it appears they should begin to affect spi and Dec early 
this evening...shortly aftr 00z...and points further north an hour 
or two later. Seeing a lot of ceilings at or below 1000 feet across Arkansas and 
SW Kentucky early this morning with the low level flow expctd to bring that 
northward during the day. May even see some patchy drizzle move in overnight 
as models do indicate some weak lift pushing across the area during 
the early morning hours...which may actually bring ceilings and visibilities 
down further than currently indicated. Will continue to monitor for 
those trends during the day. 


Smith 
&& 


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


$$