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Area forecast discussion 
National Weather Service northern Indiana 
130 PM EDT Thursday Aug 21 2008 


Aviation... 


18z tafs... 


Moisture will continue to increase over the region. Clouds will 
thicken and lower through the forecast period...but expect 
prevailing VFR. 


Showers forming along the western portion of the forecast 
area...west of a line from ktyq to klwa. These shower will be 
isolated in nature with associated MVFR conditions in/near showers. 
&& 


Update...(1220 PM EDT Thursday Aug 21 2008) 
have updated zone forecast to address isolated showers across the 
far western portion of the forecast area this afternoon. Moisture is 
streaming north across the Mississippi River valley. Convection is 
triggering as a result of a weak shortwave riding along the flow. 


&& 


Short term... 
..today through Friday night... 
Moisture will surge north rapidly today as a surface high pressure 
area retreats east. Precipitable water values should rise to near 
1.75 inches. At the same time...an upper level system will move 
north into Missouri...and then Illinois. This system should help 
trigger some storms over west areas tonight...so have left ongoing 
chance for storms over the western third of the forecast area. 
Given the ongoing dry pattern...hesitant to go too high with storm 
chances...especially considering the initial upper level system 
should pass over the fringe of the area. Kept a chance for storms 
Friday and Friday night as deep moisture remains over the area. 
Hard to pinpoint a specific forcing mechanism...but this pattern 
should remain somewhat active as a large upper trough approaches 
from the west. 


&& 


Long term... 
..Saturday through Wednesday... 
Once again very few changes needed to long term portion of 
forecast. Increased moisture from remnant closed low in plains 
will be over the area this weekend with weak short waves on 
Saturday which could trigger some convection in unstable 
atmosphere. Upper low moving across southern Canada will help drag 
a weak cold front into the area later Saturday into early Sunday. 
This front coupled with lingering moisture will once again provide 
a focus for some much needed rain via thunderstorms and rain. Dynamics not looking 
that great so probability of precipitation will remain in chance categories given dry 
pattern and less than optimal dynamics and timing for our forecast 
area. 


High pressure builds in behind this frontal passage for much of next week with 
return back to comfortable temperatures and humidity levels for late 
August. What is left of tropical system Fay will continue to be a 
forecast concern in the later periods as models continue to struggle 
with its evolution and movement. This could possibly begin to affect 
our area by late next week but very low confidence on timing and its 
movement. 


&& 


Iwx watches/warnings/advisories... 
in...none. 
Michigan...none. 
Ohio...none. 
Lm...none. 
&& 


$$ 


Short term...Lewis 
long term...Lashley 
aviation...Lewis