Weather

195 
fxus63 kilx 070443 
afdilx 


Area forecast discussion 
National Weather Service Lincoln Illinois 
1143 PM CDT Sat Sep 6 2008 


Discussion... 
issued 858 PM CDT Sat Sep 6 2008 


The latest infrared Sat pictures indicate an increase in lift is developing 
in the Lee of the 500 mb trough...as a shortwave ejects to the east 
and enhanced DPVA develops on the leading edge of the wave. Lift 
is being aided by divergence aloft as the nose of an 300 mb jet is 
progressing toward Illinois. Cloud tops are cooling in west Illinois/north MO/ and S 
Iowa over the last 30 mins...and that trend will most likely 
continue overnight. It appears our northwest County Warning Area will be in the path of 
the best forcing for precipitation...and our grids have that covered well 
with likely probability of precipitation in that area. 


Temperatures will not have much of a diurnal swing tonight...due to 
clouds and precipitation. Our middle-upper 50s look on track. 


Only minor adjustments were done to the grids...so no zone update 
will be needed this evening. 


Shimon 


&& 


Aviation... 
issued 1143 PM CDT Sat Sep 6 2008...for the 06z tafs 


Showers are continuing to progress across the County Warning Area. The lowest 
visibility upstream is at Macomb down to 3sm in rain. Will keep 
lowest visible only in the MVFR range for any taf site...mainly in a 
tempo. Will keep prevailing visible VFR...with tempos for any MVFR visible 
and ceilings. 


Any lightning strikes have been confined to the base of the upper 
trough...back in SW Iowa and northwest MO. It appears that secondary wave 
will shear out prior to reaching Illinois. Showers should diminish 
by sunrise...and no thunder is expected in our area. 


Was originally a bit too optimistic with clearing...so I am 
bumping it more toward midday...and will keep 3k-4k ceilings around 
longer. 


Shimon 
&& 


Previous 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 




&& 


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


$$