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fxus63 keax 070554 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
1254 am CDT Tuesday Oct 7 2008 


..updated aviation discussion... 


Discussion... 


Upper vortex centered over southwest Kansas appears to be handled best 
by 12z GFS and European model (ecmwf). These two also do a better job with the pseudo 
dry slot showing up on latest water vapor imagery. However...this 
will be partially filled by new scattered convection which has 
formed ahead of a secondary vorticity center now over central Kansas. 
Initial band of convection moving through the western County Warning Area will lift 
northeast but be followed by the second round of developing 
convection. Overall...went categorical probability of precipitation for most of the County Warning Area even 
though there will be an hour or two with no rainfall. Radar has been 
showing some low topped mini-thunderstorms with compact rotation but 
airmass is lacking sufficient instability plus a boundary to enhance 
the low level shear to support severe weather. But will keep an eye 
on the more scattered activity which has formed within the dry slot 
as those cells are more discrete. As for overnight lows favored 
warmer met MOS. 


Like the idea of holding onto probability of precipitation through at least Tuesday morning 
and then slowly shifting the back edge of the rain chances eastward 
with the passage of the primary shortwave trough. Energy via jet 
dynamics from the northern rockies drops southeastward into the 
Central Plains and link up the northern plains shortwave trough with 
Kansas shortwave currently affecting the County Warning Area. This will result in an 
overall strengthening of the primary shortwave and slow down the 
clearing and delay ending the rain chances. Have lowered maximum 
temperatures to more closely mimic the 3 hourly temperatures off the 
mav MOS guidance due to the rain-cooled air and cloudy skies. 


Zonal flow aloft will quickly overspread the region late Tuesday 
night and last through Wednesday. Should see rapid clearing with 
passage of the primary upper trough Tuesday night. Expect good 
radiational cooling for both Wednesday and Thursday mornings with 
possible locally dense fog problems in the river valleys. Will 
address that potential in later forecasts. Temperatures for 
Wednesday should be around seasonal normals. 


Mj 


Medium range (thursday-monday)... 


Much greater than average uncertainty through the majority of the 
medium range period...as models are having difficulty resolving the 
depth and positioning of both a deep trough digging into the western 
Continental U.S....and ridging building over the eastern Continental U.S.. this uncertainty is 
reflected in spread of ensemble 500 mb heights nearing 120dm through the 
mean trough base and ridge axis by this upcoming weekend. Recent 
operational GFS runs were largely ignored for this package...as they 
appear far too deep and progressive as compared to the more reliable 
European model (ecmwf) and a good handful of GFS ensemble solutions (typical bias for 
the GFS in this type of pattern). 


Another key element in determining frontal position through the 
medium range is presence of a northern stream wave...and subsequent 
trailing ridging...and hence surface pressure rises in the Lee of the 
Canadian rockies forcing an inverted trough/cold front into the 
forecast area. Though the European model (ecmwf) keeps the western trough more neutrally 
tilted...and any frontal boundary well northwest of the County Warning Area...cannot 
totally discount some aspects of the operational GFS...and many 
ensemble members do want to depict of more positively tilted western 
trough. Will not bite on the quicker frontal passage at this time...as 
historical trends favor the European model (ecmwf). 


By far...the most confidence comes in the beginning of the medium 
range...as the entire region will enter a dry modified return flow 
pattern...with 500 mb heights well above climatological averages. 
Thereafter...a very large bust potential exists with ensemble 
guidance temperature spreads near 20f and pop spreads around 40 
percent. Given the belief in the less progressive European model (ecmwf)...have 
hedged towards the upper end of ensemble temperature guidance and 
the lower end of pop guidance. Given the very low timing 
confidence...could not justify any more than 20 probability of precipitation for any given 
period...though precipitation sometime towards the end of the medium 
range does appear likely. Will certainly need to increase probability of precipitation in 
future forecasts once more confidence can be achieved. 


21 


&& 


Aviation... 


For the 06z tafs...a meridional oriented line of showers will 
rotate into the Kansas/Missouri border region over the next couple 
of hours bringing periodic showers to all the terminals. Up stream 
ceiling observations indicate that MVFR ceilings should prevail 
along with the showers through sunrise. Ceilings should lift behind 
the band of showers...but only for a little while as wrap around 
showers move in for the later half of the morning hours on the back 
side of the circulation. Ceilings will then improve for the afternoon 
hours as the system moves farther east. A northwest wind will 
prevail through the daylight hours and into the night. 


Cutter 


&& 


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