523 
fxus63 keax 052008 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
308 PM CDT Sat Jul 5 2008 


Discussion... 
main forecast concerns for the short term...tonight through Monday 
night... 


For this evening...keeping an eye on weak upper wave that is moving 
south through eastern Kansas. This wave should be far enough west 
and south to preclude any thunderstorm development that might impact 
the forecast area after 00z...but something to keep an eye on. 


For overnight tonight...isentropic lift will be increasing across 
northeast portion of the forecast area...as a weak short wave in the 
northern stream moves across Minnesota and northern Iowa...will put 
some slight chance probability of precipitation into that region late tonight and into 
Sunday morning to cover this possibility. 


For later on Sunday...will have the watch for the possibility of 
thunderstorms that may form a complex in Iowa Sunday morning 
possibly dropping south into northern Missouri. NAM suggests this 
may occur...while GFS keeps afternoon convection further north near 
the surface front. 


For Sunday night...main band of convection should remain anchored to 
the frontal zone in Nebraska and Iowa...as the next short wave in 
the northern stream moves into western Nebraska. Convection on this 
front Sunday night will likely organize into one or two mesoscale convective system type 
complexes...which should head east as the low level jet veers overnight. Thus 
will continue to restrict probability of precipitation to northern areas...where outflow 
boundaries from an mesoscale convective system may impact the region. 


For Monday...as northern plains short wave which may be enhanced by 
overnight convection...moves toward northwest Missouri...this should 
help to move the surface front closer to northwest Missouri by 
afternoon. At this point will keep just slight chance probability of precipitation going 
until Monday night...but will have to monitor the timing of this 
front depending on the effects of repeated rounds of thunderstorms. 
We may have to eventually introduce chance probability of precipitation to the northwest by 
the late afternoon hours. 


Pc 




Medium range (tuesday through saturday)... 


Little change in thinking regarding the overall medium range 
pattern...with broad ridging covering the majority of the 
Continental U.S....and a Progressive Flat wave emanating from a mean trough 
position over the eastern Gulf of Alaska...and progressing across the 
Canadian prairies into another mean trough over eastern Canada. Aside 
from some convective feedback issues with the operational 
GFS...suite of medium range models continue mostly in good 
agreement...with very good continuity. However...a few more subtle 
features...both obviously present in the model output and more 
transient and presumed...lead to somewhat larger uncertainties than 
would be expected given the recent model performance. 


First...frontal boundary position Tuesday is ever so slightly slower 
in its southward progression...such that models suggest it may not 
clear the forecast area until Wednesday morning. However...it is 
likely that convection will be more prevalent and robust than along 
this front than models indicate...and cold pool generation will act 
to push the effective boundary faster south out of the County Warning Area. Have 
made some minor tweaks in precipitation chances early in the period to 
account for some of this uncertainty...though confidence is not 
quite high enough yet to advertise more than a chance mention. 
Second...the 00z European model (ecmwf) and previous versions of the operational GFS 
distinctly suggest the influence of a least one tropical feature 
riding into the western periphery of the subtropical high pressure...and 
turning northward into a weakness in heights aloft through the central 
Continental U.S. During the middle to late week period. Given a favorable broad 
area of negative 2h velocity potentials covering much of the 
tropical western hemisphere (which have now manifested into a notable 
mjo signal of 2 Standard deviations removed from average in the 
rmm/Wheeler phase plot western hemisphere/Africa quadrant)...have little 
doubt that deep tropical convection will continue to percolate 
throughout the Gulf of Mexico/western Atlantic...and lead to at least 
one organized/definable tropical reflection in the midlevel flow. 
Problems arise that these types of systems are notoriously difficult 
for models to track...and while confidence that such a system will 
occur is becoming better...the eventual track...whether into the 
lower Missouri River valley or further southwest into the High 
Plains...remains much more uncertain such that only slight chance 
probability of precipitation are warranted at this time. 


Per ensemble mean guidance of a slightly drier and cooler Post 
frontal airmass...and possibility of an infusion of a more 
subtropical/tropical type airmass and midlevel cloudiness 
thereafter...have shaved a few degrees off temperatures for the end 
of the work week...though keeping forecasts only slightly below 
average. Like the idea of rebuilding heights and strengthening 
subtropical high pressure over the region for the end of the medium 
range period...leading to temperatures rebounding back to near (or 
even ever so slightly above) climatological norms. 


21 




&& 


Aviation... 
/1230 PM CDT Sat Jul 5 2008/ 
area of convection that is continuing to the west of the taf sites 
is getting some added impetus from a weak short wave dropping south 
in the the upper flow that was enhanced by the early morning 
convection that occurred in eastern Nebraska. Main area of 
instability will stay west of the taf sites...so will be keeping the 
forecast dry. 


Pc 


&& 


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


$$