873 
fxus63 kilx 240645 
afdilx 


Area forecast discussion 
National Weather Service Lincoln Illinois 
145 am CDT Thursday Jul 24 2008 


Discussion... 
issued 145 am CDT Thursday Jul 24 2008 


Main concern with this forecast package will be probability of precipitation over the next 
couple of days...as active weather pattern once again sets up 
across the Midwest. 


Latest surface analysis shows ridge axis stretching from the Great 
Lakes into the Ohio Valley...providing mostly clear and cool 
conditions across central Illinois. Cold front that passed through 
the area a few days ago has now settled into the northern Gulf 
states...with the tail end of the boundary arcing up into the 
Central Plains. Convection has developed along and just north of 
the boundary across Nebraska...thanks in large part to a 35 to 
40kt 850mb jet streak and a weak middle-level impulse tracking over 
the top of the Central Plains upper ridge. 


Short term...today through Saturday 


Big question today will be if any of the precipitation currently across 
Nebraska and Iowa will make it into central Illinois. With deep 
upper low spinning over the eastern Great Lakes and ridge across 
the plains...general trajectory will take convection on a 
southeasterly track...which would keep the bulk of it west of the 
kilx County Warning Area for the balance of the day. In addition...a very dry 
airmass is currently in place across Illinois and eastern 
Iowa...as evidenced by the 00z soundings from both kilx and kdvn. 
Despite mitigating factors...00z 24 Jul models try to bring precipitation 
into the Illinois River valley as the day GOES on. Think the 4km 
nmm-WRF and the 24km WRF-Eta model from the u of I depict the current 
quantitative precipitation forecast field best...albeit about 3 to 4 hours too slow. Both models 
suggest precipitation will give the kilx County Warning Area a glancing blow to the 
W/SW...with most areas across central Illinois remaining dry. 
Based on preferred model output...as well as latest radar 
trends...will sharpen up probability of precipitation somewhat for today. Will keep all 
areas along and east of I-57 dry...with low chance probability of precipitation along 
I-55...and likely probability of precipitation west of the Illinois River. 


Next issue will be potential mesoscale convective system development tonight...and how 
that will affect central Illinois. Latest suite of models continues 
to indicate convective development tonight...although timing and 
track of system varies. GFS appears to be the odd model 
out...taking the mesoscale convective system from southern Iowa/northern Missouri 
southeastward into southern Illinois...essentially staying just 
south of the County Warning Area. This solution is further south than its previous 
runs. Meanwhile...the NAM-WRF and European model (ecmwf) maintain their more 
northern track...with mesoscale convective system coming into central Illinois from the 
northwest after midnight. Based on what is currently happening in 
the plains...as well as model consistency...will follow the 
NAM-WRF/European model (ecmwf) most closely here. As a result...will continue with 
likely probability of precipitation across the central and western zones...tapering down 
to chance probability of precipitation along and east of I-55. System moves eastward 
Friday morning...continuing the rain chances across the board. 
Due to uncertainty of mesoscale convective system track...will only carry chance probability of precipitation for 
now...but these may need to be bumped up once exact evolution 
becomes more clear. Weak upper wave still prognosticated to move east of 
the region by Friday night...essentially taking best upper forcing 
out of the area. Will therefore only mention slight chance probability of precipitation then. 


Upper wave drops southward out of Canada on Saturday...pushing a 
cold front toward central Illinois. None of the models seem to be 
too excited about developing much convection along this 
feature...although GFS does hint at it. Meanwhile...NAM-WRF 
focuses its attention a bit further south across Missouri and 
southern Illinois along old boundary left behind by convection on 
Friday. Quite a bit of forecast uncertainty exists on 
Saturday...with several boundaries potentially coming into play. 
Will therefore only carry slight chance probability of precipitation at this time. 


Long term...Sunday through Wednesday 


Northwesterly upper flow pattern will continue throughout the 
extended...with numerous weak waves potentially setting off 
convection across parts of the Midwest. Does not appear to be any 
strong feature moving through the area during this time frame...so 
best bet is to keep forecast dry through Wednesday...with slightly 
below normal temperatures for this time of year. 


Barnes 
&& 


Aviation... 
issued 1151 PM CDT Wednesday Jul 23 2008...for the 06z tafs 


The radar trends continue to look much worse than the models are 
indicating. Will go with vcsh for the pia and spi tafs after 
09z...but do not expect measurable precipitation...mainly sprinkles 
reaching the ground. Ceilings are quite high...with mostly 10k feet and 
some pockets of 7k feet. Overall the consensus of guidance shows 
very light quantitative precipitation forecast brushing our far west zones through 12z. The new 00z GFS 
is now mostly dry for Thursday...except for the far SW corner of 
our County Warning Area. The 00z NAM is trending that way as well. I pulled back 
on mention of thunderstorms Thursday afternoon. The GFS now keeps Thursday night 
dry as well. That is a major change from the 12z and 18z GFS. The 
00z NAM still shows a strong storm system moving out of Iowa into 
c Illinois Thursday night. Will keep a mention of thunderstorms in the vicinity for all sites after 
00z Thursday evening...for now. Still enough uncertainty that 
IFR/LIFR visible and ceilings could occur in strong storms overnight. 


Shimon 
&& 


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


$$