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fxus63 keax 071722 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
1122 am CST Wednesday Jan 7 2009 


..updated aviation discussion... 




Discussion... 
/410 am CST Wednesday Jan 7 2009/ 
weak frontal boundary was bisecting the County Warning Area early this morning 
associated with a cold core wave aloft now diving southeast through 
Iowa. Actual cold air advection is lagging the wind shift...and is most evident in 
a slug of shallow SC progressing south along the Missouri River 
valley. NAM-WRF relative humidity plots and bufr soundings appear to have 
initialized this cloud cover the best...bringing the broken-overcast deck 
into the northern half of the forecast area by sunrise. Even though this 
shallow SC deck does not extend more than 1500ft in the 
vertical...the proximity of the cold core over northern Illinois along 
with a shearing wave now into the North Dakota (per WV imagery) and 
saturation up to -12c level...will yield scattered snow flurries/snow 
showers within this cloud mass. In fact...several upstream 
observations across eastern South Dakota and western Iowa already reporting 
very light snow...with trajectories bringing the most favored area 
for very light snow into north central and northeast Missouri this 
morning through very early this evening. Given the amount of drier 
air in the boundary layer and sporadic nature to any precipitation 
echoes...feel this will be non-accumulating trace event. 


Regarding temperatures...much like yesterday clouds may wreak havoc 
on forecast highs...as any sunshine in the western grids would allow 
temperatures to rise versus cloudy areas to the east. As a result...tried 
to tighten thermal SW-NE gradient across the area...trending just 
below the coolest met guidance. Followed a similar trend towards 
coolest guidance on Thursday as well...as surface high pressure centers 
over western Missouri...limiting mechanical mixing. In addition...models 
want to depict at least a scattered midlevel cloud deck...which would 
further limit surface insolation. 


The last in a series of waves barraging the Pacific northwest coast 
Thursday will cross The Rockies into the plains Friday...and obtain 
a positive tilt structure as heights begin to significantly rise 
over the east Pacific. Large temperature bust potential 
Friday...with a very shallow and steep inversion developing by the 
morning hours (h9 temperatures rising as high as +13c!). with its 
typical bias...the 00z operational GFS blasts an associated cold 
front through the area very quickly early Friday afternoon...though 
this timing is about 6 hours faster than just about any other model 
guidance. Therefore...have discounted essentially all guidance from 
this model run...basing forecast primarily on slowest ext-NAM and 
best compromise European model (ecmwf). Numerical guidance (even met numbers) still 
appear too low...and have based highs from forecast soundings and 
mixing to around 925mb (and temperatures up to +10c at that height)...which 
is some 5f or so degrees above the warmest guidance. 


Relatively strong cold front will clear the area Friday 
evening...bringing more typical January temperatures back to the 
area. For the most part...this looks to be another dry frontal passage with 
the best moisture and Theta-E advection limited to southern Missouri and 
points south and east. If frontal passage trends slower towards the ext-NAM 
solution...may need to introduce higher probability of precipitation in central Missouri. 
Adjusted temperatures through the weekend a bit higher...as Post frontal 
high pressure airmass will be primarily of Pacific/southern Canadian 
rockies origin...and clear skies and deeper mixing would promote 
temperatures near climatology. No changes made past this weekend...as 
enough model differences exist regarding the amplitude of a positive 
pna-type North American pattern...that temperatures could range from 
well below average to near average. 00z GFS appears to be the 
highest amplitude outlier (and as such develops the highest surface 
pressures originating in northern canada) versus other medium range 
guidance depicting the classic positive pna pattern (strong positive 
height anomalies across western Canada...and corresponding negative 
anomalies over the central Pacific and southeastern conus). European model (ecmwf) and gefs 
show a far more dampened pattern...with positive anomalies shifted 
further west in the eastern Pacific...and negative anomalies limited to 
the Hudson Bay area (and essentially no height anomaly over the southern 
half of the Continental U.S. - Leading to little numerical reflection of the 
pna measure - at least through the middle of next week). Thus...with 
weaker surface pressure reflection...southern surge of Arctic airmass may be 
limited to mainly the northern plains and Great Lakes...with more typical 
cold middle January temperatures across the lower Missouri River 
valley. Current grids may be a little too warm...though do not 
anticipate readings as cold as the most pessimistic mex numbers 
(though temperatures this cold may be possible just beyond this forecast 
period). 


21 


&& 


Aviation... 


For the 18z tafs...mostly clear skies spread across the terminals 
this morning...but more clouds are poised to the northwest and will 
spread across the region this afternoon. Up stream observations 
provide good confidence that ceilings will should stay in the VFR 
range. However...will need to watch ceilings for the late afternoon 
hours as there are some scatted MVFR ceilings with some light 
snow/flurries tailing back across Nebraska. Overnight and morning 
hours of Thursday...ceilings should stay in the VFR range. 
Otherwise...look for the modest northwest winds to veer a little 
more to the north and stay there through middle morning hours of 
Thursday...but they should go light and variable late Thursday 
morning as a surface high slides across the region. 


Cutter 


&& 


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


$$ 

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