662 
fxus63 kpah 282006 
afdpah 


Area forecast discussion 
National Weather Service Paducah Kentucky 
306 PM CDT Thursday Aug 28 2008 


Discussion... 
in the very short term /tonight/...pre-frontal convective 
activity across east central Missouri and central Illinois should 
gradually decrease in coverage for the remainder of the afternoon 
as it moves southward. There is enough local convergence and 
surface insolation...along with elevated weather factors...such as 
instability /lifted index -2 to -4/...moisture/precipitable 
water...1 inch/...and positive laps cape between 1100-1900 j/kg2 near 
the Interstate 64 corridor in Illinois to support isolated 
thunderstorm activity during the late afternoon and early evening 
hours. The main shower and thunderstorm activity will not move 
into the area until after midnight /actually closer to the 4-7 am 
time frame/ as the actual front moves toward the northern edge of 
the weather forecast office pah forecast area. 


For Friday...the GFS/European model (ecmwf) differ on the amounts and extent of 
upper level instability and shear will be in place...but the best 
chance for shower and thunderstorms will occur during this 
time...being maximized during the late morning and early 
afternoon...as some thinning of antecedent cloud cover enhances 
differential heating along the surface frontal 
boundary...especially across southeast Missouri...extreme southern 
Illinois and western Kentucky. Given the position of the middle 
level ridge over MO and the base of the westerlies over northern 
IL/in...there will be some weak differential shear aloft...aiding 
in minor storm organization and enhanced updrafts. This shear...in 
combination with drier and slightly cooler air aloft /as evidenced 
by thermal structure/gradient noted in this mornings upper air 
charts and the GOES water vapor 6.4-7.4 micron imagery...could 
lead to some decent thunderstorm wind gusts and possibly some 
minor hail as well. Since the moisture remnants of Julio in the 
southwestern U.S. Will not be a factor and storm speed/precipitable water 
values will be nominal...heavy rain should not be a problem. 


I still believe that this system will move south of the area by at 
least 10 PM Friday...so kept the area overnight into Saturday 
morning. 


From Saturday through early next Wednesday...dry and seasonable 
weather should continue to be the rule with generally fair 
conditions in place. 


With this forecast package...I finally had to consider the effects 
of Hurricane Gustav and the interaction of the upper level ridge 
departing to the east and a broad trough moving into the northwestern 
2/3rds of the U.S. The interaction of these two features has 
finally placed the weather forecast office pah forecast area...especially southeast 
MO/southwest Illinois/Purchase area of west Kentucky in a sustained 
moist/unstable flow...capable for repeated training of showers and 
thunderstorms...regardless of the time of day. 
Therefore...starting Wednesday afternoon...I introduced a chance 
of thunderstorm activity in the west...slowly spreading the chance 
of rain east through the forecast area. The greatest overall 
coverage and intensity will likely occur next Thursday night and 
Friday. Given the potential for repeat rainfall...there may be 
flooding issues to contend with...especially over southeast 
Missouri. We will have to wait and see how Hurricane Gustav 
eventually impacts the region to determine that potential. 


With this package...I utilized a blend of European model (ecmwf)/GFS 
solutions...with a slightly higher weighting toward GFS 
solution. Some emphasis toward the NAM was used in the short term 
dewpoint forecast over the Labor Day weekend. 


&& 


Aviation... 
/retransmission from 1125 am CDT discussion/ 
VFR conditions will prevail this afternoon. Some cirrus blow off 
from convection across western Illinois will continue 
south...along with continued cumulus development in the heat of the 
day. Southwest winds up to 10 kts will persist. Will likely 
continue at least a mention of fog tonight. However less of an 
inversion is expected overnight. And there may continue to be some 
high clouds. Therefore will not hit as hard. Convective chance 
late in the night into the first half of Friday will be handled 
with cumulonimbus qualifier for now due to timing and placement uncertainty 
and low probabilities in numerical guidance. 




&& 


Pah watches/warnings/advisories... 
Kentucky...none. 
MO...none. 
Illinois...none. 
In...none. 
&& 


$$ 


Short term...cn 
long term....Smith 
aviation...cn