677 
fxus63 klsx 172213 
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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
513 PM CDT Wednesday Mar 17 2010 


Discussion... 
/347 PM CDT Wednesday Mar 17 2010/ 


Short-term (tonight - Friday aftn)... 


Upper level shortwave along the MO-Kansas border will continue southward into the 
txarla region into midnight. Ridge of high pressure and associated subsidence 
should allow for continued clearing conditions into tomorrow as 
evidenced per visible satellite trends. With sufficient boundary layer 
moisture and calm winds this evening expect nocturnal cooling to 
result in areas of fog into the morning hours. 


Winds will weaken tomorrow and southerly flow will return for Friday as ridge 
progresses into the southeastern Continental U.S. And a trough begins to develop through the 
interior-mountain west. Have gone a few degrees above NAM guidance for both 
the nighttime lows and daytime highs into Friday. In collaboration 
with the pah weather forecast office have surmised the potential for drier conditions 
developing during the day both Thursday and Friday as mixing levels 
roughly 5kft will mix down drier air from aloft. Although soil 
moisture is abundant and river levels high...still felt lower dewpoints 
were possible across the bi-state region. 


Middle-Range (fri night - sun ngt)... 


Complex forecast situation for this weekend with potential winter 
storm. Model solutions still varied in terms of exhibiting run-to-run 
consistency with both surface and upper level features. Nevertheless with 
12z runs better model agreement has emerged between Gem/NAM/GFS 
solutions with European model (ecmwf) as the acting outlier in progressing the system faster 
through the region. 


Have compromised with a blend of the aforementioned forecast package. 
Per WV analysis weak 500 mb trough and attendant upper level energy 
presently over British Columbia will dig through the interior-mountain west 
region into the end of the week thus providing the aforementioned southerly 
flow and warmer temperatures as mentioned in the short-term. 
Although timing and placement of surface and upper level features is still 
uncertain...model blend accentuates Lee cyclogenesis and surface low 
pressure development through western Texas by 0z Sat lifting through the bi-state 
region into sun in concert with positive tilted trough with the main 
shortwave rounding the base. GFS is the Leader with the system...as 
the NAM is the laggard. 


Strong low-middle level southerly flow will exist along and ahead of the surface 
frontal feature allowing for a tongue of higher theate air from Gulf 
coastal regions to isentropically advect aloft across the middle-MS valley 
region. Model solutions succinct in keeping most of the precipitation Post- 
frontal as there will still be sufficient dry air and subsidence 
associated with eastward drifting high pressure into the southeastern Continental U.S.. within the 
Post-frontal region strong middle-layer frontogenesis is evident along 
with sufficient lift within the dendritic growth zone. 


For the going forecast will keep situations along and ahead of the front 
fairly dry and warm...a few degrees warmer than a model blend of 2m 
temperatures and NAM guidance. Temperatures should fall in wake of the front. 
There exists the slight chance for thunder on Sat in proximity of the surface 
cold front as NAM bufr soundings indicate a little bit of elevated 
cape. With northwesterly flow in wake of the front anticipate the onset of a 
rain-snow mix by midnight for northwestern MO progressing into the metropolitan by 
morning. Although snowfall totals are yet to be deciphered as the system 
is still too complex...interestingly enough sref probability output 
highlights a fairly good chance of seeing over an inch across the 
bi-state region...yet it will be interesting to see how well any 
potential snow will stick with soil temperatures above the freezing mark 
and surface temperatures forecast to hover near freezing through the event. 


Extended (mon - Thursday morning)... 


Incorporating a European model (ecmwf)/GFS blend with more weight towards the European model (ecmwf) 
anticipate high pressure to settle through the bi-state region in wake of the 
weekend disturbance. Northwesterly flow should weaken and return southerly as a 
ridge of high pressure shunts eastward by Tuesday morning. Model solutions succinct 
upon developing an area of low pressure through the interior-mountain west that will 
lift northeastward through the western Great Lakes region into Thursday. As such with expected 
Lee cyclogenesis strong southerly flow at middle-levels will advect warmer 
temperatures in the low-middle 60s...along with Gulf moisture and higher dewpoints 
northward through the Central Plains and provide the chance for wet weather Wednesday 
night into Thursday as system and trailing cold front press eastward into the 
MS-valley region by 12z Thursday. Have gone low probability of precipitation for now and will 
monitor trends accordingly as we get closer in time. 




Sipprell 


&& 


Aviation... 
/445 PM CDT Wednesday Mar 17 2010/ 


Drier air continues to push southwest into the County Warning Area with scattered 
diurnal cumulus clouds set to dissipate after sunset and remaining 
stratus across central Missouri to exit to the southwest. Appears 
that MVFR/IFR visibilities are on tap overnight as high pressure 
allows for ideal radiational cooling. Kcou will be the last to 
clear out this evening...so they have not benefited from the 
mixing that KUIN and ksus have had this afternoon. Will forecast 
LIFR conditions at kcou due to late clearing/better low level 
moisture...and at ksus due to its valley location. Will watch 
KUIN this evening...but lower dewpoints upstream prevent ME from 
taking their visibilites to low tonight. 


Specifics for kstl... 


Few to scattered diurnal cumulus will dissipate this evening...then the 
forecast challenge becomes how low to take visibilities as 
radiational cooling sets in. Looking upstream in central 
illiinois...dewpoints have dropped into the 30s. Will keep going 
trend in the forecast of MVFR fog developing by midnight. Am not 
confident enough of IFR conditions to place a tempo into the taf 
at this time. Will reevaluate this evening and update as necessary 
based on upstream observations. 


Cvking 


&& 


Lsx watches/warnings/advisories... 
MO...none. 
Illinois...none. 
&& 


$$ 


Weather forecast office lsx