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fxus63 keax 211740 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
1240 PM CDT Thursday Aug 21 2008 


..updated aviation discussion... 


Discussion... 


Long lived cutoff trough that has plagued the central Continental U.S. For 
seemingly the past week is finally shearing north...with the best 
estimate of the central circulation along the Missouri/Arkansas 
border. WV and local radar imagery indicate several vorticity maxima 
embedded in the parent circulation...one of which is lifting into 
northern Missouri...and has been responsible for a large swath of rain 
and drizzle. Most extensive coverage and intensity has been 
displaced just east of this feature...where better isentropic lift 
on the 305-310k layer has occurred with more ready access to 
subtropical moisture feed. Model forecasts continue to stretch and 
shear this feature for the next 36 hours...before absorbing the bulk 
of the energy in a seasonably strong and progressive wave along the 
US/Canadian border. Until that time...elongating trough axis will 
reside from northestern Oklahoma through the upper Midwest...and while 
coverage and probabilities of precipitation do not appear to be extreme 
(with the exception of this morning with more defined vorticity 
structure)...cannot discount showers for any area east of the shear 
axis through Friday afternoon. In general...have trimmed probability of precipitation for 
the western half of the County Warning Area...while maintaining similar numbers from 
previous forecasts farther east. 


Should see some brief breaks in cloud cover later this 
evening...however with residual wet ground and boundary layer 
moisture advection from the south...potential for overnight fog 
arises. One limiting factor against widespread dense fog appears to 
be fairly strong winds at the top of the inversion layer...which may 
act to provide enough mixing to keep fog light and more of a low 
cloud deck. However...given appearance of forecast bufr 
soundings...felt it prudent to introduce fog mention in grids. 
Assuming fog/clouds burn off in a timely fashion Friday 
morning...temperatures should rebound to near climatology with 
increasing temperatures aloft...and have taken a blend of met/mav 
guidance to reflect this. 


Models remain fairly consistent in driving a frontal boundary into 
far northwestern Missouri by Saturday morning...though dprog/dt over the 
past several model runs have slowed the forward progression. Will 
need to watch for mesoscale effects from any convective development 
along the front...which could push the effective boundary further 
south quicker...though overall model depiction does not show a 
typical convective response...and thunderstorm coverage and intensity may 
not be all that great. Frontal boundary may essentially wash out 
over the area by Sunday morning...as 590+dm 500 mb heights build east 
from the central rockies and likely bring increasing subsidence and 
stronger capping aloft into the region. Only minor changes were made 
to previous pop grids...with thinking that some storms will fire 
along and ahead of the front...though not extensive enough for a 
total washout and more than a chance mention. 


Little to no changes made past Sunday...as current forecast 
accurately captures more of a dry Ely Post frontal boundary layer 
component...and reduced precipitation chances. Also...model appearance 
gives little impetus to stray far from climatology during this time 
frame. 


21 


&& 


Aviation... 


For the 18z tafs...the shortwave shearing out across Missouri today 
will keep the cloud cover prevailing across the region for much of 
the next 24 hours. Some scattering of the cloud deck should develop 
this evening...but with the abundant moisture in the region ceilings 
are expected to redevelop overnight. Went with marginal VFR ceilings at 
kmci and kstj based on current observation trends...and visible 
satellite images that show that a scattered to broken cloud deck 
will dominate those terminals through the rest of the afternoon 
hours. For kmkc...left the MVFR ceilings in based on upstream 
observations across southeast Kansas. 


For the overnight hours...continued to run with a 2sm visibility 
during the early morning hours. Cloud cover should inhibit dense fog 
formation...but still be enough to dump the terminals into IFR 
conditions through 15z Friday morning. 


Cutter 


&& 


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