224 
fxus63 kilx 062000 
afdilx 


Area forecast discussion 
National Weather Service Lincoln Illinois 
300 PM CDT Sat Sep 6 2008 


Discussion... 
issued 300 PM CDT Sat Sep 6 2008 


Short term...tonight through Tuesday... 
broad upper trough centered across the plains will move little 
through early next week. A fairly swift upper jet associated with 
the trough extends from the Pacific northwest into the Central 
Plains then up into the Great Lakes. A couple of shortwaves 
traveling along this jet will be the primary weather-makers for 
Illinois through Monday. 


The first wave is now moving across Nebraska/Kansas rapidly toward 
the Midwest. Precipitation has already been reaching the ground per 
surface observation as far east as Columbia Missouri...with radar echoes 
increasing as well. Low level moisture is not the best in the wake 
of the remains of Gustav...but certainly enough deep-layer moisture 
to produce a decent coverage of light to moderate rain especially 
toward midnight. This wave will exit the area as rapidly as it 
enters...and see little prospects for any further measurable rain 
after 12z Sunday. Will go with likely probability of precipitation in the west with healthy 
chance probability of precipitation in the east tonight...then only some slight chance probability of precipitation 
in the north Sunday morning closer to the upper jet axis. 


The next shortwave is now moving southeast toward the northern 
rockies and will sharpen up as it enters the Central Plains Sunday 
night and Monday. Deeper surface cyclogenesis will be likely with 
this second wave than with the one tonight. This will induce better 
quality moisture advection ahead of the developing surface low and 
eastward-extending baroclinic zone late Sunday night and Monday. 
More convection should accompany this system...although the threat 
for severe is uncertain. The optimum instability from the Southern 
Plains into the Ozarks of Missouri may be far enough removed from 
the better upper dynamics from Illinois northward to mitigate any 
organized severe threat. Irregardless...will begin Sunday night with 
chance probability of precipitation after midnight and increase to likely Monday into Monday 
night. Precipitation amounts should be greater than with the first 
system tonight...but excessive rainfall is not anticipated at this 
time. 


Long term...Wednesday through Saturday... 
persistent low amplitude cyclonic upper flow over the plains/Great 
Lakes will become more zonal toward midweek ahead of the next upper 
trough that will be coming into the Pacific northwest on Wednesday. 
This feature will progress into the plains on Thursday and drive the 
next cold front through the Midwest Thursday night and Friday. 


This timing toward the end of the week will become increasingly 
contingent on where Hurricane Ike ends up going. NHC/HPC forecasts have 
Ike in the northern Gulf next Friday and Saturday...which suggests 
that the Friday front will be progressive and slow up only after it 
clears Illinois. So for now will have mentionable probability of precipitation Thursday 
through early Friday and keep Saturday dry. 


04 
&& 


Aviation... 
issued 1246 PM CDT Sat Sep 6 2008 


Diurnal cumulus field beginning to Blossom across central Illinois early 
this afternoon...with ceilings generally between 3000 and 4000ft. In 
addition...plenty of high clouds are streaming in from the southwest 
in advance of a middle-level wave tracking across the plains. Cumulus should 
rapidly dissipate toward sunset...replaced with an increasing and 
thickening middle-level cloud deck around 10000ft. Forecast soundings 
are initially rather dry...but moisten sufficiently to support light 
rain later in the evening into the overnight hours. Based on current 
satellite/radar loops...will introduce -ra at both kspi and kpia at 
05z...then further east to kcmi by around 07z. Instability and lapse 
rates are meager through the period...so do not think thunder will 
be an issue. Once the rain tapers off an comes to an end toward 
dawn...have included a period of ceilings at 3500ft and restricted 
visbys down to 3sm. Drier air will slowly advect into the area after 
14z...allowing the visbys to rise and ceilings to scatter out. 


Barnes 
&& 


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


$$