192 
fxus63 klsx 162035 
afdlsx 


Area forecast discussion 
National Weather Service St Louis MO 
335 PM CDT Tuesday Mar 16 2010 


Discussion... 
/335 PM CDT Tuesday Mar 16 2010/ 


Latest water vapor loop and model guidance suggests that the 
shortwave slowly dropping S through the middle-upper MO valley will continue 
on its southward course...thereby largely missing the forecast area tonight and 
Wednesday morning as it passes through. Have removed probability of precipitation from most 
locations save parts of central and northestern MO where they have been 
reduced. Precipitation that does fall should be light and will struggle to 
measure. 


A more interesting forecast problem tonight will be clouds and fog...and 
what will they do heading into Wednesday. Models continue to show a 
steady northwesterly flow at the surface through Wednesday with a slight veering to 
northerly near cloud top level...which points to plenty of cloud upstream. 
The sun/S power is waning as we head deeper into the afternoon...and 
should see southeastward advance resume as we get closer to sunset. This is 
to cover the forecast area in clouds once again tonight...but with clouds 
dissipating north of the quadrant cities...likely to see breaks develop down 
our way as well until reinforcements arrive later tonight from the 
northwest. Temperature falls to be modest in most locns due to the clouds...with 
the greatest expected in the S and east where some radiational cooling 
will occur before clouds roll back in. If clouds lower again...will 
also see visibility reductions develop and have inserted fog into the 
forecast. Heading into Wednesday...atmosphere to continue to warm 
aloft...but this will merely strengthen the inversion over cloudy 
areas and force the sun to try to dissipate the cloud deck again 
largely on its own. But with added component of downward vertical 
motion in the wake of passage of upper system and approach of center 
of surface hi pressure...this should finally break the clouds heading into 
Wednesday night. Undercut MOS temperatures by several degrees toward 
central/southeastern MO where clouds should hang in the longest...then edged 
towards MOS in the eastern forecast area. 


Daytime temperatures will then recover nicely heading into Thursday and 
Friday into the 60s...perhaps approaching 70f Friday afternoon. 


For the extended period (friday night - tuesday)...main story will 
be which model to choose for what could be a major last gasp of 
winter. European model (ecmwf) continues to be the /snowiest/ model with its farther 
southward track of the upper level system...but is an outlier...with other 
models differing on varying degrees of how Open A wave to bring through 
and where to track it. Used a broad consensus for this package which 
does not differ much from previous thinking...which does incorporate 
the more southerly track of the European model (ecmwf)...but not quite to that extent. As a 
result...there is still much uncertainty as to what we will see out 
of this system...but probs are looking good as to getting some form 
of precipitation throughout the forecast area from Saturday through Sunday. Due to the 
stronger model solutions prevailing...have slowed down the onset by 
6-12hrs. There also does appear to be the good possibility of 
unseasonably cold air coming in behind this system for Sunday and 
Monday and have lowered temperatures...especially during the day... 
accordingly. But how soon this cold air gets tapped to make an 
impact on the precipitation-type is the most uncertain factor of all... 
especially given the fact we will be dealing with this storm so late 
in the season where it simply gets harder to get snow. For now have 
a mixed rain/snow mention on Saturday night and Sunday but way too 
early to even think about specific accumulate numbers until a few other 
hurdles overcome. Temperatures should then begin a rebound by Tuesday. 


Tes 


&& 


Aviation... 
/1224 PM CDT Tuesday Mar 16 2010/ 


Area of LIFR ceilings over cntl MO northeastward into western Illinois will remain locked 
in place through the afternoon. Main question will be if ceilings will 
lift to IFR briefly this afternoon before lowering again this evening. 
Have kept taf dry for now as chances of precipitation are low. Given the 
weak forcing...potential exists for areas of dz. If dz does 
occur...visbys will drop lower than currently forecast. Expect 
visbys and ceilings to slowly lift Wednesday morning into the afternoon hours. 
For sus...please see discussion below for stl. 


Specifics for kstl...terminal remain on the edge of high MVFR/low 
VFR ceilings and and area to the west and north of IFR ceilings. Latest Sat 
images show the area of IFR slowly moving S toward the terminal. 
Believe insolation will be strong enough to help erode this cloud 
deck and it will retreat somewhat northward. Still...as insolation is 
lost this evening...this area of clouds should begin to drift southward and 
impact stl overnight. Some MVFR to poss IFR visbys may also 
accompany these IFR ceilings but have left out for now. Ceilings should lift 
Wednesday late morning or into the early afternoon. 


Tilly 


&& 


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


$$ 


Weather forecast office lsx