Weather

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afdilx 


Area forecast discussion 
National Weather Service Lincoln Illinois 
1128 PM CDT Wednesday Aug 27 2008 


Discussion... 
issued 855 PM CDT Wednesday Aug 27 2008 


Forecast in pretty good shape for the remainder of the night. 
Temperatures...clouds...winds appear OK. High cirrus will pass across the 
County Warning Area through morning. A convective complex is expected to develop 
in eastern Iowa after midnight...and move toward northwest Illinois by 12z. Our 
County Warning Area should remain dry until at least middle morning...with the better 
chances of storms coming later in the afternoon and into the 
evening. 


Only minor adjustments were needed to the hourly temperatures...so no 
updated zones will be necessary this evening. 


Shimon 


&& 


Aviation... 
issued 1128 PM CDT Wednesday Aug 27 2008...for the 06z tafs 


The convective complex that was indicated by GFS/NAM has developed 
in west Iowa...and corfidi vectors and mean storm level flow point 
toward an east-northeast progression the rest of the night. The NE trend 
should keep our area out of the path of the stronger storms by 
morning. Plus...expect a bit of weakening with the system during 
the morning. Storms should redevelop along the front later in the 
afternoon and into evening on Thursday. The 00z NAM is keeping our 
area dry in the evening...while the GFS keeps the storms active 
through 06z. Will go with a window of storms from late afternoon 
through middle evening for now. Will use thunderstorms in the vicinity to add a mention 
outside that period of time. 


Some light fog has developed in cmi with 5sm br...so will keep 
some fog in the eastern tafs...where cloud cover has not reached 
yet. MVFR should be as low and visible levels get at any one 
point...especially with cirrus clouds moving eastward. 


Shimon 
&& 


Previous discussion... 
issued 302 PM CDT Wednesday Aug 27 2008 


Short term...tonight through Saturday... 
primary forecast focus this period will be on the upper trough now 
moving across the northern rockies. This system and its accompanying 
cold front will enter the northwestern portions of the forecast area 
late Thursday afternoon and make very slow progress into the 
southeast Friday morning. The GFS model continues to produce 
convective feedback and appears to move the front too fast...and 
prefer the slower and weaker NAM/NGM models. Moisture on this model 
run looks a bit better than previously...so instability is a little 
stronger Thursday afternoon as the front approaches. Best deep layer 
shear however is confined to Iowa and northern Illinois. Therefore 
think that best prospects for any severe storms will be isolated and 
over the extreme west before sunset. Will have chance probability of precipitation for the 
northwestern half of the area on Thursday...but will keep the east 
and southeast dry as weaker instability and stronger convective 
inhibition will probably preclude anything too far ahead of the 
front. 


Will have chance probability of precipitation everywhere Thursday night and over the east 
and southeast on Friday. By Friday...the main upper trough will be 
exiting to the northeast...and the trailing cold front should slow 
down and lose its identity. So will keep the mention of 
precipitation into Friday afternoon despite the dry GFS. 


At this time...Friday night through Saturday night look dry as high 
pressure builds in for the end of the week. 


Long term...Sunday through Wednesday... 
deep ridge surface and aloft will hold across the Midwest through at 
least Tuesday...with temperatures near or a little above normal. 
Precipitation chances well below mentionable levels. 


By next Wednesday...the European model (ecmwf) seems to have the better handle on the 
progression of Gustav and has it around Louisiana or East Texas. 
Given that the GFS fails to recognize Gustav...its synoptic pattern 
ahead of the hurricane over the midsection of the Continental U.S. Is highly 
suspect. While the GFS pushes a cold front through Illinois on 
Wednesday...the European model (ecmwf) stalls it out from Minnesota into Kansas. At 
this time...will side with the European model (ecmwf) and keep mentionable probability of precipitation out 
of the forecast until the situation with Gustav becomes better 
defined. 


04 




&& 


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


$$