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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1205 am CDT Sat Mar 20 2010 


Discussion... 
/307 PM CDT Friday Mar 19 2010/ 


Dry and warmer weather to give way to mixed precipitation later 
tonight and through the rest of the weekend. 


Next weather system is already spreading clouds into northwestern MO 
this afternoon. Cold front is sliding into northern portions of 
forecast area as of 18z today and will slide through rest of 
forecast area tonight...before exiting Saturday morning. 12z model 
runs coming into better agreement with this system...so will go with 
a blend of them. Precipitation will be mainly along and behind this 
boundary...so with slower onset...do not expect precipitation to 
move into the forecast area until closer to 06z tonight. Still some 
concern on precipitation type. There may be a mix of rain and snow late 
tonight over the extreme northwestern counties of the County Warning Area as colder 
air tries to filter into the County Warning Area behind the cold front. 


The precipitation will overspread much of the area on Saturday 
as main surface low strengthens and lifts northeast into Arkansas by 
Saturday night. Still some concern about snow with this system since 
the track of the surface low would keep parts of County Warning Area in area of 
heavy snows. However...system gets so wrapped up with warm air being 
pulled northward and wrapped around surface/850 low centers...that 
most of precipitation will be in the form of rain or a rain/sleet/snow 
mix...with best chances of any snow accumulation over northeast MO 
and west central Illinois. This area will see between 2 and 4 inches 
of snow on Saturday and Saturday evening...with higher amounts 
possible over western MO...just outside of forecast area. 
Therefore...have issued a Winter Weather Advisory for areas along 
and north of a Quincy to Shelbina line from 12z Saturday to 06z 
Sunday. Another issue is rainfall amounts. Areas along and south 
of I-70 will see between 1 and 2 inches of rainfall. However...it 
falls over an extended period of time...so do not feel that a 
Flash Flood Watch is needed at this time. 


Surface low to lift out into Ohio Valley Sunday into Sunday night 
with mixed precipitation tapering off late Sunday night. 


Beyond Sunday night...drier and warmer weather to return once again 
with temperatures warming up into the 60s for Tuesday and Wednesday. Models 
indicating another frontal boundary to move through the region by 
late Wednesday through Thursday...so kept low chance category probability of precipitation 
Wednesday night through Thursday. This system would be all rain at 
this time. 


Byrd 


&& 


Aviation... 
/1123 PM CDT Friday Mar 19 2010/ 


Challenges continue for the going forecast...moreso upon the Mode of 
precipitation aside from potential ceilings to which greater confidence lies. 
Latest surface and satellite observation analysis highlights IFR-LIFR conditions 
stretching from southern WI through central Kansas. It is with certainty that 
this Post-frontal region will impact the terminals through the daytime 
hours Saturday for KUIN-kcou as also agreed upon by sref probs with 
1-2 kft ceilings dropping to 300-600 feet above ground level under strong and gusty northerly 
winds. Shifting focus...latest guidance affirms precipitation lagging 
later in time and greater entrainment of warm air around the system. 
Have thus lessened the chances for -sn potential and low visibility 
restrictions. NAM bufr soundings seemingly highlight that through the 
course of Saturday the low-levels will remain near freezing 
providing the opportunity for a wintry mix at the KUIN-kcou 
terminals. Have focused within the forecast the dominant modes of 
precipitation anticipated...but it is with concern and poor confidence 
that prevailing precipitation could easily change at any time and 
potentially visibilities as well. Thermal profiles will need to be 
scrutinized through the course of Saturday. 


Specifics for kstl...with latest guidance am more confident the 
metropolitan will remain liquid through the forecast. With warmer air wrapping 
into the system visibilities believe restrictions concerning visibilities and ceilings 
will be limited remaining at MVFR. Strong and gusty northerly winds will 
also be of concern. 


Sipprell 


&& 


Lsx watches/warnings/advisories... 
MO...Winter Weather Advisory from 7 am Saturday to 1 am CDT Sunday 
for Knox MO-Lewis MO-Marion MO-Monroe MO-Shelby MO. 


Illinois...Winter Weather Advisory from 7 am Saturday to 1 am CDT Sunday 
for Adams Illinois. 


&& 


$$ 


Weather forecast office lsx