Weather

045 
fxus63 keax 070434 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
1134 PM CDT Sat Sep 6 2008 




.Updated aviation discussion... 


Discussion... 


With strong upper-level trough remaining established across the central 
Continental U.S....fcsting deja vu continues today...as a repeat performance of 
morning fog...and afternoon stratus/light rain is once again taking place 
today. Todays elev rain showers activity is acting nearly identical to rain showers 
activity from Friday...produced by yet another short wave lifting through 
northern Kansas. As was the case yesterday...atmos is still relatively 
dry...however the diff today is that isentropic lift is a bit 
stronger and atmos is just a bit more saturated. The end result is 
another bout with scattered light ra/sprinkles. 


Fcsting persistence is taking precedent this afternoon...as a nearly 
identical solution is shaping up for the overnight and daytime hours 
of sun...with only slght diff. Main concerns overnight will once 
again be focused on fog development by morning. With relatively saturated 
grounds and light winds...potential for fog once again becomes likely 
towards midnight. As upper short wave departs to the NE tonight 
middle-level subsidence should set into the region...trapping large 
quantity of low-level moisture near the surface. Cross sections...time 
series plots...and soundings show this very well. I'm also hard 
pressed to find a piece of numerical guidance that suggests anything 
other than fog and perhaps even dense fog. However...I have two main 
concerns regarding dense fog development tonight. Primary concern is 
the amount of middle-level cloud cover overhead which could be substantial 
due to weak isentropic lift. While in most cases this would limit 
radiational cooling...dew points will remain elevated in the middle-upper 
50s and temperatures have struggled to climb...so we won't have to go 
far to reach low-level saturation. My other concern would be the fact 
low-level winds will be a bit stronger tonight...leading to an 
imbalanced circulation and resulting and a LIFR stratus deck/heavy 
dew result. Obviously...evening shift will need to monitor conditions 
and see just how things unfold. Needless to say...the potential for 
another dense fog advisory is there. 


Otherwise...appears there will a bit of a lull in the rapid secession 
of middle-level short wave overnight. However...still expected 
isentropic lift to repeat itself over Oklahoma/Kansas. Without a 
stronger shortwave...lift should be focused further south of the 
area...thus have left northern Missouri dry tonight with higher probability of precipitation 
in the SW corner towards daybreak. 


For Sunday and Monday...a slight transition in this pattern will 
begin as strong shortwave is expected to dig southward as it crosses 
the northern rockies. Feel any elevated rain showers that forms to the south 
of US will remain focused far enough south it should leave northern 
portions of the area dry. By the overnight hours...strong return flow 
is expected to develop with models surging h850 Theta-E axis/surface warm 
front northward. GFS continues to be too fast with this 
movement...and am following a solution closer to the 15z sref/12z 
European model (ecmwf)/12z NAM which stalls this front near northern MO Monday morning. 
This being said...have steadily increased probability of precipitation for Sunday night into 
Monday morning into the likely range...categorical probability of precipitation may be 
needed. 


Will continue the above trend into Monday...increasing probability of precipitation across the 
entire County Warning Area. Guidance continues to trend stronger with this 
approaching trough...holding surface boundary nearly stationary in the 
morning from SW-NE near the Iowa border. Main influence from the upper 
trough arrives by Monday afternoon which should send front quickly southeast through the 
afternoon and evening. Severe chances will remain low...as it appears 
most of the forcing is tied to the middle-level trough and activity would 
remain Post-frontal. However...guidance does indicate about 1500-2000 
j/kg instability southeast of the front. It would be up to the erosion of a 
weak eml/cloud cover that may preclude pre-frontal thunderstorms. 
Temperatures will also be problematic on Tuesday. Depending on where 
the front bisects the area...readings north may be in the low 60s 
with readings south in the 80s if the sun can break through. 


Dux 


Forecast for days 4-7 remains on track. Cold front moving across the 
region by Monday night will push effective moisture south of the 
area. Cool high pressure will dominate on Tuesday and have held 
forecast highs in the lower and middle 70s. Return flow sets up across 
western and northern sections Tuesday night...spreading across the 
rest of the forecast area Wednesday morning. Have slight/low chance 
probability of precipitation in for Wednesday with region of isentropic lift developing and 
overspreading the region...as well as increased temperatures in the 
southwesterly flow. Continue to see next shortwave...with associated 
isentropic ascent and subsequent cold front passage to affect the 
region Wednesday night through Thursday night. Due to multiple 
consistent GFS and European model (ecmwf) model runs showing this timing...have 
increased probability of precipitation a little but for now still within the chance category. 


Nrr 


&& 


Aviation... 


For the 06z tafs...very challenging forecast for tonight. Upper disturbance 
moving into northwestern MO will continue to trigger a few patches of light 
rain through around 07z. Little if any clearing was noted in the wake 
of the disturbance as a LIFR to IFR low cloud deck quickly forms 
behind the shortwave in southeastern Nebraska and northestern Kansas. This low deck will progress 
southeast into the terminals after 08z. To further complicate matters. 
Very slow radiational cooling in a very moist environment just south 
of mkc was beginning to develop LIFR ceilings. This development will also 
spread slowly north...gradually merging with the lower ceilings 
developing from the northwest. 


The low cloud cover will limit the development of fog through the 
early Sunday morning with mainly MVFR visibilities. The only possible exception 
will be at stj toward 12z as the ceiling may become thin enough for 
development of visibilities to around 1 mile. 


The confidence in the timing and the scenario is relatively low at 
this time. Will continue to forecast trends and amend as necessary. 


The ceilings and visibilities will slowly improve after 15z Sunday morning. 


Db 


&& 


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

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)