Weather

704 
fxus63 keax 282005 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
305 PM CDT Thursday Aug 28 2008 


Discussion... 
short range (tonight through Saturday night)... 


See previous discussion below for mesoscale discussion about severe 
potential tonight. One additional concern will be with the Post 
frontal development of storms...already occurring in southeast 
Nebraska. This convection is tied more to the 700 mb layer...which 
should gradually slide to the south this evening with the 700 mb 
short wave. This may become marginally severe with large hail as 
elevated convective available potential energy of 1500 to 2000 j/kg exist at this point in the area. 


Due to the Post frontal potential...will pull chance probability of precipitation a little 
further north for the evening hours. Will raise probability of precipitation to likely 
categories in the south to cover development ahead of the cold 
front. Even after midnight will keep slight chance probability of precipitation going in 
southern half of the forecast area. NAM showing some weak isentropic 
lift continuing after midnight on the 305kt surface until after 09z. 


For Friday...will keep dry forecast going...although NAM showing 
potential of convection in southeast Kansas and southwest 
Missouri...that could build as far north as the extreme southwest 
portions of the forecast area. GFS keeps convection further to the 
south...so will not change the going forecast...but something to be 
watched. 


Much drier dewpoints that are showing up over Nebraska at this 
time...should be able to push into northwest Missouri late 
tonight...but big question on how far south and east this dry air 
can make it. Will follow the NAM lead on bringing this drier air 
into northwest Missouri late this evening...spreading it further to 
the east overnight...but doubt we will see much push to the south of 
the Missouri River...as core of surface high moves across southern 
Iowa and northern Missouri on Friday...then east into Illinois by 
Saturday morning. 


For temperatures...given readings that are occurring behind the cold 
front today...will go slightly on the cool side of guidance for 
highs on Friday. Then with an expected cool start Saturday morning 
and still east to southeast winds...will also go slightly below 
guidance for Saturday. 


Pc 






Medium range (sun-thu)... 


Continuity rules the first half of the medium range portion of the 
forecast as surface ridging is expected to continue to have a strong 
influence on the region for Sunday and Monday. Have made only 
subtle adjustments to temperatures both days...as h850 temperatures 
around 20c with coinciding mixing levels should produce readings in 
the middle to upper 80s in most locations. 


In the second half of the medium range /Tue-Thu/ some uncertainty 
still remains with regards to the track and evolution of Hurricane 
Gustav. The European model (ecmwf)...while maybe not the perfect solution...still 
has a better grip on Gustav and the possible effects on the long 
wave pattern over the Continental U.S.. feel that next upper trough crossing The 
Rockies Monday night/Tuesday will slow somewhat...pushing a surface cold 
front towards the County Warning Area Tuesday night. Then the real questions begin 
with how Gustav will track once it arrives onto the Mainland. The 
European model (ecmwf) is driving this system too far west...while the GFS appears to 
shear out the disturbance which is a solution that is completely 
disregarded at this point. Regardless...highest chance for precipitation 
appears to arrive in the Wednesday through Thursday timeframe...thus 
have increased probability of precipitation in this time period. Depending on the track of 
the tropical system...this front may stall out near the County Warning Area for the 
end of the week providing cooler temperatures and increased precipitation 
chances...especially over the eastern half of the County Warning Area. 


Dux 


&& 


Previous discussion... 


/1152 am CDT Thursday Aug 28 2008/ 
shortwave in eastern Iowa continued to progress east with 2-3mb pressure 
rises in eastern Nebraska/western Iowa driving the cold front in extreme northwestern MO southeast at 
10-20 miles per hour. Morning upper air depicts moderately steep lapse rates 
between 700-500mb with the SC/altocumulus castellanus in west-central and central giving a hint 
of the potential instability for this afternoon. Profilers as well as 
morning upper air also depicts that the strongest 700 mb-300 mb wind fields 
will not be juxtaposition along the front this afternoon...remaining 
well north into Nebraska...Iowa and extreme northwestern MO late this afternoon. 


Have generally followed the latest 12z NAM in the evolution of the 
convective event for this afternoon and early tonight. Model soundings 
suggest that sbcapes will rise to around 3500-4000 j/kg by late this 
afternoon...along and ahead of the front which should move southeast 
to a Unionville to kci line by 21z. As per model 
soundings...sufficient heating just ahead of the boundary should 
break the strong 800mb capping inversion...first across the northestern County Warning Area 
around 20z...then west-central MO between 20-22z. 


Wind profiles will be generally unidirectional ahead of the boundary 
with surface-3km shear values near 20kts with surface-6km shear around 25-30 
knots. The strong instability along with marginal surface-3km shear will 
likely be sufficient for a modest squall line development by 00z to 
02z with a southeast movement around 15-25 knots. Initially the convection 
will be scattered to broken during the late afternoon hours. 


Given the drier air upstream from 800 mb through 500 mb...dcape 
values may exceed 1000 j/kg...increasing the threat of strong 
straight line winds. Unidirectional wind profiles do not favor 
tornadic development. However...given the strong instability...large 
hail is possible. 


Given the projected movement of the cold front...the line of convection 
should move out of the County Warning Area between 03-06z. 


Db 


&& 


Aviation... 
/1152 am CDT Thursday Aug 28 2008/ 
for the 18z tafs...concern is the timing and convective threats along 
a cold front moving through extreme northwestern MO. The front should move through kstj 
around 19z to 20z...likely too soon to develop any convection along 
the boundary. Will hold a mention of a cumulonimbus at this time frame due to 
proximity of convection likely to be to the south. Further south...at 
kci and kmkc...the convective threat is higher as the front moves 
through these terminals between 21-23z. The later timing will allow 
for a much higher threat of convection as convective temperatures 
are reached. Once convective temperature is reach near the 
boundary...the convection will develop quickly and become quite 
strong...possibly containing hail and strong gusty winds. The threat of 
convection will be limited to 1 to 2 hours as the front moves 
through the terminals. Will fine tune timing as the afternoon progresses. 


Winds will become light north to northeast behind the front with a 
small chance of VFR showers after midnight. 


Db 


&& 


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

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