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


Discussion... 
/452 am CDT Friday Mar 19 2010/ 


Look for temperatures today to push well into the 60s thanks to southwesterly surface 
winds...mostly sunny skies and deep mixing heights. It is for the 
latter reason that temperatures above MOS were forecast despite h850 temperatures only 
rising to 6c. 


Models have come into much better agreement over the past few days 
with the major winter storm set to impact parts of the Midwest as 
the various components of this storm can now all be sampled over 
land by the noam upper air network. Needless to say...the European model (ecmwf) from 
several days out is the Winner...with all models now merging these 
components into a deep wound up storm over the Southern Plains by 
Saturday night and then heading eastward S of MO on Sunday and Sunday 
night...taking a slight northward turn by Sunday evening. 


This event will have two components to it. The first will be largely 
independent of this strong wound-up storm and be more associated 
with a Stout cold front for tonight through Saturday evening. It is here 
where the majority of snow potential will exist. The cold front will edge 
into northestern MO by early tonight and then push southeastward towards stl metropolitan 
by Saturday morning and then stall just S and east of stl metropolitan over 
parts of southeastern MO and southern Illinois for Saturday night and Sunday. This 
front will create a rather sharp temperature gradient as it slowly pushes 
through the forecast area. Post-frontal precipitation will breakout in response to good 
frontogenetical forcing aided by upper jet dynamics as main system 
approaches. Enough cold air should rush in behind front initially to 
change precipitation from rain to snow over in northestern MO and parts of central MO 
and west-central Illinois. Depending on how cold the air at the surface gets and the 
intensity of the snowfall...could see several inches from this 
episode...especially in parts of northestern MO. Areas from stl metropolitan and S 
and east will just see rain. Areas to the north and west of stl metropolitan may also 
see some sleet due to the further advance of cold air at the surface 
expected than aloft...and this will leave a pocket of warm air aloft 
that will play a much larger role with the main storm component. 


The second component to this event will be the strong wound-up storm 
that will pass to our S later Saturday evening through Sunday night and 
develop a well-defined deformation zone with it. The wound-up nature 
of this system is significant...as it will drive in warm air aloft 
well into its northern periphery to the point that much of the 
deformation zone precipitation will be liquid with only some sleet or snow 
despite the track being very favorable for our area. Boundary layer 
temperatures generally above freezing will also play a factor in further 
limiting any frozen precipitation potential. Depending on the forward speed 
of this storm...may still see precipitation lingering in parts of southwestern Illinois and 
stl metropolitan into Monday morning. 


All in all...see enough doubt with wintry precipitation accumulate to prevent any 
watch issuance. But do see the potential of 2 to 4 inches in 
sections of central/northestern MO and west-central Illinois. Early and late season 
winter storm always are tricky...so stay tuned. 


Tes 


&& 


Aviation... 
/1228 PM CDT Friday Mar 19 2010/ 


Primary concerns this taf period are onset of wind shift and 
gusts...onset of precipitation...and precipitation type associated with an 
approaching storm system. Clouds will increase and lower overnight 
as the system moves out of the Southern Plains. Meanwhile...a cold front 
located across far northwestern MO at 17z will continue moving slowly 
southeastward. Winds will shift from southwesterly to north-northeasterly after frontal passage and gust to 
20-30 kts with the highest gusts expected at KUIN. Precipitation will begin 
as rain when it overspreads the area late tonight and tomorrow. 
Bufr soundings suggest there will be a period of ice pellets across 
far northestern MO and west central Illinois /including KUIN/ as the rain 
transitions to snow. -Sn is possible at kcou beyond the end of the 
24hr taf period. 


Specifics for kstl...VFR conditions will persist through early 
evening before clouds increase ahead of an approaching storm 
system. A southeastward moving cold front is expected to stall in vicinity of kstl/ksus 
leading to a period of light/variable winds. Winds will turn north-northeasterly and may 
gust to 20 kts after frontal passage. -Ra should hold off until tomorrow 
afternoon. 


Kanofsky 


&& 


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


$$ 


Weather forecast office lsx