809 
fxus63 keax 200829 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
330 am CDT Wednesday Aug 20 2008 


Discussion... 


Cutoff upper low currently spinning near the Dallas-Fort Worth 
metroplex (slightly farther southeast than forecast 24 hours 
ago)...though latest WV imagery seems to show a bit of a northward 
jog just beginning. A few subtle features have been noted regarding 
this system...which could lead to larger forecast ramifications over 
the next 36 hours. First...a substantial amount of downsloping dry 
air has entrained into the southwestern flank of the circulation... 
essentially displacing the subtropical feed to the eastern flank. 
Also...central circulation heights have filled somewhat in the past 
12 hours...slightly reducing the areal extent of shower activity in 
the primary saturated gyre. The end result is the vast majority of 
forecast models now shear the wave out a bit slower than previously 
forecast...and also focus the best jet dynamics...q vector 
convergence...and Theta-E advection more towards central Missouri this 
evening...and northeast Missouri by Thursday morning. Thus...have 
made minor adjustments in time and space regarding highest probability of precipitation 
towards this thinking. 


Otherwise...fog channel satellite image shows cirrus debris has all 
but enshrouded the entire forecast area...with midlevel deck now 
making a run north from southeastern Kansas/southwestern Missouri. Dry boundary 
layer Ely flow will initially counteract top down saturation 
processes...with little more than sprinkles likely over much of the 
area until later in the afternoon. However...as the upper low 
approaches...combination of weak dynamic lift and positive Theta-E 
advection will overcome the dry boundary layer first across the far 
southern County Warning Area...then spreading north this evening and overnight. With far 
more extensive cloud cover than previous days...high temperatures today 
should fall a couple degrees below persistence (sans the northern strip 
of counties where breaks in high clouds will allow for better 
insolation). 


Upper low will continue to lift...shear...and fill across the region 
Thursday afternoon...again with the best precipitation mechanisms 
located immediately under and displaced east of the cold core aloft. 
Regardless...even west of the trough axis...model soundings indicate 
fairly significant saturation in the h9-700 mb layer which should keep 
high temperatures nearly 10f below climatology. In this respect...have 
undercut MOS guidance (probably too much climatological influence in 
this statistical guidance) a few degrees...weighing more towards 
forecast soundings...understanding that any area with more 
persistent showers may struggle to even reach these very low 
forecast numbers. 


Partial clearing and a brief respite to precipitation chances should 
spread into the region Thursday night/Friday morning. By Friday 
afternoon...with forecast models showing increasing boundary layer 
moisture and instability...along with a trailing weak trough axis 
lingering ahead of a much stronger northern stream disturbance...will 
continue with some thunderstorms and rain mention for portions of the forecast 
area...though with general building heights aloft...capping 
inversion should also strengthen as well. 


Cold front associated with the aforementioned northern stream trough 
still forecast to enter the County Warning Area Saturday morning...and progress into 
southern Missouri by Saturday evening. With forecast models fairly 
consistent with this feature...only minor adjustments to timing were 
made. Have continued with low chance probability of precipitation for the time 
being...though upward adjustments may be necessary towards the end 
of the week as confidence increases. 


21 


&& 


Aviation... 


For the 06z tafs...will continue the VFR conditions for this 
forecast period. Will also keep mention of some local MVFR ground 
fog possible at kstj around daybreak. Extensive high level 
cloudiness will overspread the terminals tonight with a middle level 
ceiling arriving Wednesday afternoon. Will slow down arrival of any 
shower activity until after 00z Thursday with possibly a few showers 
near the Kansas City terminals in the afternoon. 


Mj 


&& 


Eax watches/warnings/advisories... 
MO...none. 
Kansas...none. 


$$