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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1252 PM CDT Sat Mar 20 2010 


Discussion... 
/446 am CDT Sat Mar 20 2010/ 


Banded precipitation currently moving through western MO...and extends into southeastern 
Iowa and northwestern MO. This is associated with strong frontogenetical 
lift behind a Stout cold front that is now near a khuf-kuno line...and 
is through stl metropolitan. Temperatures rapidly fall off into the 30s behind this 
front but do not actually attain this for nearly 75nm behind the 
front. 


Of most concern is this banded precipitation...of which the higher 
reflectivities have already dropped 3-4 inches of snow in short 
order in northwestern MO. This favorable area will slide eastward into the western 
edge of the forecast area today...including northestern and central MO...and then 
gradually fade late tonight and Sunday morning. Vertical temperature 
profiles will be critical...and it appears that these will be most 
favorable for snow today before the Southern Plains system gets wrapped 
up and begins to push back the cold air to the west with its warmer 
air aloft. This will ultimately change the snow with some sleet 
over to rain with some sleet later tonight and this will continue 
into Sunday morning. Therefore...much of the expected snow accumulate 
will occur today over the advisory area...which has been expanded 
to include more of central MO...where 2-5 inches are anticipated. 
Would not be surprised to see a few spots approach 6 inches if 
bands are persistent enough. Farther to the S and east...including 
stl metropolitan...look for just rain out of this system...with most of 
it occurring later tonight and Sunday. 


By the time the system gets going enough to push its deformation 
zone precipitation into the forecast area by Sunday...and continue through Sunday night 
and into Monday morning...precipitation should be all rain except for some 
sleet and snow possible in northestern/central MO. 


A large temperature gradient will setup across the forecast area today as a result 
of the slowing down cold front with temperatures in the 50s pushing 60f in the 
far southeastern forecast area and remaining steady in the 30s in the northwestern forecast area from 
KUIN to kcou. This will be short-lived...as by Monday...even the 
areas that receive snow today will be rebounding into the 50s. 


Tes 
&& 


Aviation... 
/1252 PM CDT Sat Mar 20 2010/ 


Mixed precipitation began falling this morning over central and 
northeast Missouri...with it changing over to all snow at 
KUIN...while kcou keeps changing back and forth from rain to snow. 
Will see a bit of a break in the precipitation at these sites until next 
round over southwestern MO moves northeast into taf sites. So kept 
low MVFR ceilings/visibilities until 19z at kcou and until 22z at KUIN...then 
lower ceilings/visibilities to IFR. Other issue is precipitation type...will see 
snow/sleet mix until early this evening...then warmer air moves in 
aloft... changing the precipitation to all rain by midnight. Precipitation will then 
taper off towards daybreak. So far have gotten between 2 to 3 inches 
of snow accumulation for cou and uin areas...but is melting pretty 
fast... could see another inch or two before changing to all rain. 
As for winds...will be north to northeast...up to 15 kts with higher 
gusts persisting through end of forecast period. For ksus...please 
see discussion below for stl. 


Specifics for kstl...MVFR deck has moved into metropolitan area with some 
light rain finally falling. Will see MVFR conditions with rain 
through the rest of today and into this evening...before tapering 
off around 09z. Ceilings will then lower to high end IFR as system 
passes to our south...then raise up to low end MVFR by 15z. As for 
winds...will be north to northeast...picking up to 15 kts with 
higher gusts later this afternoon and persisting through end of 
forecast period. 


Byrd 
&& 


Lsx watches/warnings/advisories... 
MO...Winter Weather Advisory until 1 PM CDT Sunday for Audrain MO- 
Boone MO-Callaway MO-Cole MO-Knox MO-Lewis MO-Marion MO- 
Moniteau MO-Monroe MO-Ralls MO-Shelby MO. 


Illinois...Winter Weather Advisory until 1 PM CDT Sunday for Adams Illinois. 
&& 


$$ 
Weather forecast office lsx