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fxus63 klsx 152019 
afdlsx 


Area forecast discussion 
National Weather Service St Louis MO 
319 PM CDT Monday Mar 15 2010 


Discussion... 
/311 PM CDT Monday Mar 15 2010/ 


Main challenge deals with how clouds will affect temperatures the 
next few days. Clouds have thinned over the western half of the County Warning Area 
this afternoon...right on the western edge of the low level cyclonic 
flow. Amount of low level moisture...curvature and presence of 
inversion would favor scattered-broken clouds through tomorrow...but then 
middle-level clouds will increase ahead of a storm system diving southward 
Tuesday night into Wednesday. Models are in good agreement that 
this system will only produce weak ascent as it moves through...so 
have dropped rain chances into the slight category. 


NAM/GFS MOS guidance is in reasonably good agreement. Did cut temperatures 
a bit over central MO for highs the next two days where there should 
be more clouds and possibly some rain. 


Main question over the weekend into early next week deals with 
coverage of thunderstorms and rain and possibly snow. A trough will enter western noam at 
middle week and become deeply amplified over the the MS River Valley by 
late Saturday. The 00z GFS ensemble mean/Canadian Gem/European model (ecmwf) all 
bring a cold front across the County Warning Area on Friday night...while the 
operational GFS is much slower. This is because a decent surface low 
develops along the cold front over western MO in response to the GFS 
being the farthest south with its 500mb closed low. General 
thinking is the same as yesterday that most of the large scale 
ascent will be Post frontal because the middle level forcing will 
lag behind the surface cold front. Have kept just slight chance of 
thunderstorms and rain as there will be little forcing/not much instability ahead of 
the cold front. Model soundings are also showing that thermal 
profiles will be cooling quickly late Friday night/early Saturday 
that means that there could be some changeover to snow before the 
precipitation comes to an end. Will go with a chance for a mix of 
rain/snow over northestern MO/west central Illinois on Saturday to cover this. The 
12z run of the European model (ecmwf) indicates some chance of accumulating snow 
over the southern County Warning Area on Saturday night...but this New Run establishes poor 
continuity and is the southern outlier compared to the 12z runs 
of the Gem/GFS. 


Britt 


&& 


Aviation... 
/1200 PM CDT Monday Mar 15 2010/ 


Cumulus is expected to continue to fill in across eastern MO and west cntl Illinois 
this afternoon leaving uin/sus with broken VFR ceilings. This cloud cover 
is expected to remain overnight...with uin having the best chances 
of becoming scattered. If uin does scattered this evening...fog will probably 
develop as cooling occurs. MVFR ceilings over cou is expected remain 
in place...but may lift to above 2 kft late this afternoon. If 
they do lift...ceilings should lower again this evening with fog 
developing again tonight. 


Specifics for kstl...expect ceilings to remain VFR through tonight and 
Tuesday morning. Clouds may break up Tuesday afternoon. However...have 
low confidence and will not mention in this taf. Winds will remain 
northerly and gusty this afternoon...but should diminish this evening but 
remain northerly. 


Tilly 


&& 


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


$$ 


Weather forecast office lsx