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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1137 PM CDT Tuesday Aug 19 2008 


Discussion... 
/426 PM CDT Tuesday Aug 19 2008/ 
short range guidance is in xclnt agreement with the progression of weather 
ftrs over the central U.S. During the next few days and night. Surface ridge 
that has been anchored over our area will gradual weaken as upper low over S 
plains finally begins its northward drift...and this will eventually spread precipitation 
over much of our County Warning Area during the last part of the work week. 


While hi clouds associated with the low will begin to spread into area tonight...east 
flow associated with lingering surface ridge will keep the lower level moisture and precipitation 
threat S of our area tonight. As west periphery of ridge begins to weaken...moisture 
will deepen and dynamics of upper system will allow precipitation to spread into central 
MO during the day on Wednesday...with showers and isolated thunderstorms and rain advection north and slowly 
pushing east Wednesday night and Thursday. Kudos to the middle shift as pop timing 
and trend as depicted in current forecast lakes very good...and only change 
was another upward nudge of probability of precipitation in the Wednesday night and Thursday time 
frame...primarily along and west of msrvr. 


Air mass advection into area will be very moist...with precipitable waters  forecast around 2 inches by 
Thursday. This deep Gulf moisture and slow mvmt of upper system will certainly pose 
a heavy rain threat in some areas...and have mentioned this in updated severe weather potential statement. 
However...greatest areal cvg of heavy rain should be in the vicinity of the upper 
low...so through Thursday this threat should remain S of our County Warning Area. 


By Thursday night and Friday the upper system begins to open up as upper trough begins to 
advection into North Plains and upper msvly. For now have stayed with going 
forecast for these time periods which backs off probability of precipitation to chance Cat. Think 
there should be less cvg in the precipitation by then...but its certainly possible 
that this may be a touch too low. At this time think its best to watch model 
trends and alter probability of precipitation if and/or when there is gd consensus to do 
so. 


Clouds and precipitation with upcoming system will upset persistent temperature trend 
that area has been experiencing since last weekend...and main impact 
early on will be in S areas where clouds will begin to encroach. In 
genl...have gone with the idea of increasing clouds limiting diurnal temperature 
swing in the S areas through Wednesday...with temperature trends in the north continuing 
their trends of the past few days. Temperature forecasts for Thursday reflect lots 
of clouds and rain...which should really put a Dent in maximum temperatures in 
areas along and west of MS river. 


Going forecast trends also holding as we head into the medium 
range...with precipitation threat in the Friday-sun time frame transitioning 
from that associated with upper low to that associated with aprch of upper trough 
and cold front. 


Models continue to have trouble in their handling of Fay. Given the 
difficultly of the short range guidance in pinning down this tropical 
system have very little Faith in the location and intensity depicted by 
any guidance by this time next week. For now...have gone with nceps 
idea that system could be impacting Ohio Valley...with west fringes 
bringing clouds and a chance of rain to southeast parts of the forecast area. 


Truett 


&& 


Aviation... 
/1135 PM CDT Tuesday Aug 19 2008/ 
for the 06z tafs... 
upper low will continue to slowly push east over the next 24 hours 
spreading a shield of cirrus northeast across the forecast area. 
Believe easterly wind direction and more cloud cover will likely 
keep fog in MVFR range tonight at ksus. Otherwise...diurnal 
cumulus around 6kft will develop tomorrow morning...with gradual 
increase in middle level clouds tomorrow evening. Have included a 
vcsh at kcou after 00z...with better chances coming just after the 
end of this current taf. 


Cvking 


&& 


Lsx watches/warnings/advisories... 
MO...none. 
Illinois...none. 
&& 


$$ 


Weather forecast office lsx