Weather

613 
fxus63 klot 061134 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
634 am CDT Monday Oct 6 2008 


Discussion... 
318 am CDT 


While models initialize well...diverging system handling begins 
within 24 hours out despite a recent history of consistency. The 
variations focus primarily west of the MS river with the deepening upper 
trough in the plains. NAM is probably the slowest and deepest with the 
southern branch of this system forming a closed off upper low over the Southern 
Plains by Tuesday before weakening in the middle MS valley by Wednesday. 
The GFS is the quickest and much deeper by Wednesday while the European model (ecmwf) 
blends between both. The northern stream across the upper Midwest and Great 
Lakes is handled quite differently from the southern stream mainly in 
timing. We may be able to see by the end of today which model of 
choice to pursue...but for now the blend is probably the wisest 
Avenue. 


The warm front at the surface never made it through northern Illinois today...yet 850 
mb level indicating SW flow with radiosonde observations showing about 4 to 6 degrees 
warmer than 24 hours ago. Models progging surface inversion over northern Illinois 
through Tuesday with drier influx of easterly surface winds under a juicier 
south to SW flow aloft. Thus being in warm sector at least 
aloft...will keep a dry forecast going through early Tuesday. As upper 
divergence northeast of upper trough crosses MS river on Tuesday...can 
expect enough forcing to generate showers and isolated thunder from 
moisture carried north by 20-30kt low level jet. Moistening aloft working down 
through the lower layers by the afternoon. Should continue in this 
pattern through Wednesday night if using model blend...then drying on Friday in 
wake of upper trough passage. 


Post frontal cooling is slight and brief as we quickly enter a 
pattern of SW flow aloft by late Thursday. Models again diverge in 
their solution for the weekend. European model (ecmwf) builds an intense ridge over 
the upper Midwest Friday while the GFS shows another active polar jet 
once again digging southeast into the Southern Plains and increasing the 
amplitude of an oncoming Pacific trough. GFS quickly swings the 
asociated surface cold front through Illinois on Saturday with a -2c 850 
temperature overhead by Sunday evening while the European model (ecmwf) still has the 
Midwest in the warm sector with a +13c temperature. GFS was indicating this 
setup in Friday ngts 00z run and so is maintaining run to run 
consistency. This bears watching but for now...with such model 
variations...am avoiding the extremes and leaning more to a 
climatologically correct scenario Akin to a subdued European model (ecmwf) run. 


This method brings in warm air advection showers to the upper Midwest for the weekend 
but leaves Illinois and Indiana in a dry and mild pattern. 


Rlb 


&& 


Aviation... 
630 am CDT 


12z tafs...the upper level ridge over Illinois and Wisconsin 
continues to amplify...keeping the band of overrunning precipitation well 
north of the terminals...in central Wisconsin/Lake Michigan and 
Southern Lower Michigan. With the surface trough over the Central Plains 
making only very slow eastward progression due to the strong high 
remaining parked to the east...the next 24 hours should be quiet for 
aviation weather concerns. Latest satellite imagery and metars 
indicate broken-overcast high cloud over the region and this should thin 
and scattered later today with the building ridge...but middle and lower 
cloud should begin to overspread northern Illinois/in late in the preiod in 
advance of the approaching trough. Latest model relative humidity timesection 
indicate that cloud bases should remain well into VFR ranges 
through the period. With the center of high pressure to the 
northeast and the trough to the west...winds will remain Ely-east-southeasterly 
through the period. This will leave the region cut off from any 
moisture influx...so dewpoints will remain relatively dry and not 
visby restrictions are expected. 


Krein 


&& 


Marine... 
315 am CDT 


High pressure centered just south of James Bay south extends 
south from the eastern Seaboard to the Mississippi River. This 
high will very slowly move east as a trough of low pressure 
over the Central Plains also very slowly moves eastward. The 
trough axis will cross the Mississippi River Wednesday as a low 
pressure circulation center within the trough moves from Kansas to 
southern Missouri. This low is forecast to track northeast through 
southern Illinois Wednesday night to northern Indiana Thursday 
morning. 


Krein 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...Illinois nearshore waters until 4 am Wednesday. 


&& 


$$