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fxus63 keax 081741 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
1140 am CST Thursday Jan 8 2009 


..updated aviation discussion... 


Discussion... 
/330 am CST Thursday Jan 8 2009/ 
complicated conglomeration of shortwaves are currently rotating 
about a longwave trough extending from Hudson Bay through the Great 
Lakes...with predominant ridging aligned along The Spine of The 
Rockies. Stronger than anticipated subsidence has cleared skies 
effectively over the lower Missouri River valley overnight...with 
modest cold air advection emanating from eastern Nebraska and Iowa. Narrow surface high 
pressure ridge has become established from central South Dakota 
through central Kansas...with this feature becoming repositioned 
towards central Missouri by later today. The last in a series of 
shortwaves now approaching the Pacific northwest coast...with a 
notable subtropical fetch and associated energy leading to copious 
amounts of rainfall and extensive flooding...high winds...and 
landslides/debris flows. This trough and its energy will essentially 
destroy the ridge over The Rockies...with positive height anomalies 
retrograding into the east Pacific basin...while more extensive 
middle/high clouds spill into the plains over what is left of the 
ridge. 


Early morning satellite imagery suggests increasing clouds 
throughout the day...and similar to the past several days...this 
increased cloud cover will most likely negatively affect potential 
warming during the afternoon hours. As such...have trended at or 
below the coolest met guidance...supported further by limited mixing 
heights and light surface winds. As the western trough crosses The Rockies 
and obtains a positive tilt...Lee surface pressure falls will be 
induced...creating modified return flow across the plains. Feel low 
temperatures tonight will be achieved around midnight...with slowly rising 
temperatures towards daybreak brought on by increasing southerly winds. Model 
forecasts have backed off slightly regarding the magnitude and 
steepness of the elevated warm nose (only forecast to +10c now - 
still impressive for the climatologically coldest part of the 
year)...and although feel numerical guidance is still slightly too 
cool (difficult for statistical guidance to resolve forecast temperatures 
nearly 2 normalized Standard deviations above average)...have 
tempered high temperatures back just a bit. 


Still think operational GFS frontal passage timing is abnormally fast...and 
have continued with a NAM-WRF/European model (ecmwf) blend...which brings the cold 
front into far northwestern Missouri just after noon Friday...and towards 
the eastern counties of the forecast area just after sunset. Analogous 
to several other frontal passages this cold season...two differing 
precipitation mechanisms appear to be splitting the forecast area. 
Better moisture tongue and ample Theta-E advection/isentropic lift 
becomes focused ahead of the front along the I-44 corridor...while 
midlevel frontogenetical forcing remains limited far into a Post 
frontal regime closer to the wave aloft along the I-80 corridor. 
Continued with some version of lower probability of precipitation mentioning rain/rain mixed 
with snow Friday evening...however investigation of forecast 
soundings shows it may be moisture well behind frontal passage in a wrap 
around saturated layer (reaching a -12c to -14c temp) early Saturday 
morning that provides the best chance for snow flurries or rapidly 
moving snow showers. 


Temporary shot of more winterlike temperatures will advect into the 
area on Saturday...though the source region of the Post frontal 
airmass appears to be more from the west Canadian rockies versus 
northwest territories. Regardless...below average temperatures 
should become replaced by more average reading on Sunday with more 
downsloping component. It should be noted...this initial surge of 
somewhat colder air over the weekend will serve as the proverbial 
warning shot for things to come later next week. Made little if any 
changes to the forecast past Sunday...though will need to watch a 
possibly more amplified clipper system sometime on Monday as a 
winter weather maker. Otherwise...operational GFS continues to be an 
outlier early next week in the apparent premature longwave 
amplification of a positive pna type pattern...with European model (ecmwf) and 
majority of GFS ensemble members delaying such movement of positive 
height anomalies poleward along the western North American coast...and 
progression of negative anomalies equatorward over eastern North 
America. Previous grids captured this trend very accurately...with 
the initial Canadian airmass shot arriving Tuesday...but the more 
impressive blast arriving Wed/thurs. More disconcerting is the 
recent European model (ecmwf) forecasts of near cross polar flow and development of a 
1056mb high diving south into the plains for the end of the 
week...bringing 1000-850mb thicknesses down to 120dm over northern 
Missouri. Big wild card will be snow pack in the immediate 
area...which could make a 10f-15f difference in temperatures with 
any high this strong. Regardless...ensemble forecasts and simple 
pattern recognition would support temperatures at least 1-2 
normalized Standard deviation below average for the latter half of 
next week. 


21 


Db 


&& 


Aviation... 


For the 18z tafs...VFR conditions will continue through this 
forecast period. However...sct-bkn stratocu and Alto cumulus will prevail 
through late this afternoon with ceilings no lower than 3500 feet. 
Otherwise...light and variable winds with the surface high across western MO 
early this afternoon should swing to the southeast as the high moves 
east. Winds should increase some in strength overnight as surface 
gradient increases between departing high and the front developing 
in western Kansas. 


Pc 


&& 


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


$$ 

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