822 
fxus63 klsx 192346 
afdlsx 


Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
646 PM CDT Friday Mar 19 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... 
/615 PM CDT Friday Mar 19 2010/ 


Challenging forecast for the next 24 hours. Per 23z surface analysis cold 
front was stretched along a line from kmbq-kmhl-kcnv with southerly flow 
ahead and strong and gusty northerly winds in wake. Have followed 
closely with NAM guidance as it is handling upstream low-level relative humidity 
fields and surface winds fairly well. Agree somewhat with timing and 
Mode of precipitation...but this is valued with scrutiny as there are 
still collaborative differences amongst additional weather 
guidance. Nevertheless it is with confidence all terminals should 
see the bottom fall out over time...with KUIN almost immediate as 
cold front is expect through at the beginning of the period. A Post-cold 
frontal fog-stratus event is anticipated with strong and gusty 
northerly winds advecting a much colder air mass. As such ceilings and 
visibilities should deteriorate towards LIFR by mid-mrng...towards IFR 
for kcou. Mode of precipitation is more challenging as warm air is 
expected to wrap into the system as it lifts through the bi-state region 
into Sunday. All terminals will initially see -ra and slowly 
changeover...KUIN completely to -sn as it should remain 
encompassed within a cooler temperature regime. Kcou is more 
problematic and may encounter hour-by-hour changeovers between 
liquid to frozen precipitation through the later period. Terminals will need to 
be monitored closely and amended as necessary with changing 
weather conditions. 


Specifics for kstl...benign weather is expected for early portion 
of taf period with surface cold frontal passage by 6z. Thereafter expect strong 
and gusty northerly winds. Guidance succinct on keeping going forecast all 
liquid...for now. But with cooler boundary layer temperatures advecting through the 
region MVFR conditions are expected to envelop the terminal by 
morning and remain through evening...thereafter deteriorating to IFR. 
Will need to watch closely for any potential impacts resultant of 
precipitation changeover which is still not out of the question. 


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