Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
649 PM CDT Friday may 24 2013 


Short term...(through late tonight) 
issued at 353 PM CDT Friday may 24 2013 


Cool Canadian surface ridge holding tough late this afternoon from 
Lake Michigan to southeast MO. Ridge will loosen its grip over the region 
overnight as it continues to drift southeast...allowing southeast flow that has 
developed over middle MO today to develop/strengthen across the 
remainder of the County Warning Area. Primary question for tonight is if/when 
convection will begin to threaten area. Consensus of 12z guidance 
is a bit slower with intensification of low level jet into the 
Mississippi Valley...and a bit further north with the 850mb Theta-E 
bulls-eye...than indicated by earlier runs. This would suggest probability of precipitation 
bulls-eye would primarily be to our north across Iowa...with some of 
this elevated shower and isolated thunderstorm activity possibly 
dropping south during the predawn hours. I've delayed the onset of 
the precipitation in our north counties by an hour or so...but do allow 
chance probability of precipitation to drop as far S as a cou-ppq line by 6 am. This may 
be too far S...but trying to leave some leeway as 850mb warm air advection does 
show a subtle increase from southeast Iowa into north Ozarks during the predawn 
hours. 


Temperature-wise...have attempted to depict impact of retreating 
surface ridge over southeast part of the County Warning Area with mins in the 40s...while 
increasing winds and clouds should mean lows in the l-m50s over northwest 
part of the County Warning Area. 


Truett 


Long term...(saturday through next friday) 
issued at 353 PM CDT Friday may 24 2013 


Surface high pressure center over the Great Lakes will shift eastward on 
Saturday while a persistent Lee side low over the plains pushes a 
baroclinic zone northeastward as a warm front. This will cause winds to become 
southeasterly to southerly across the lsx County Warning Area with the warm front stretching along the 
MO/Illinois border through Sunday before the boundary lifts farther northward. A 
general warming trend is expected through next week however there 
will probably be a couple of cooler days mixed in due to the 
effects of nocturnal mesoscale convective systems /either upstream 
or overhead/. These effects may include lingering rain showers...thick 
cloud cover...an mesoscale convective vortex which focuses scattered afternoon thunderstorms and rain...and/or outflow 
bdrys which push the effective warm front farther south than was 
previously forecast. Temperature trends in the previous forecast look 
reasonable and few changes were required. 


As for precipitation chances...models continue to show a couple of vorticity maxes 
which may support scattered afternoon rain showers/thunderstorms and rain as they move across the area 
over the next several days. A persistent and redeveloping low level jet 
interacting with a baroclinic zone will also support nocturnal 
thunderstorm development across parts of the central Continental U.S. Over the next 
couple of nights. Some of the thunderstorms and rain clusters will probably congeal 
into mesoscale convective systems and could move into the lsx County Warning Area 
/based on corfidi vector forecasts/ depending on where they initially 
form...and where they initially form each night will depend on the 
position of the effective warm front which may end up being pushed 
farther south by outflow bdrys. These mesoscale details cannot be 
accurately forecast very far in advance. In other words...we expect to 
see unsettled weather through early next week before the warm front 
lifts far enough north to limit the mesoscale convective system potential. 


By the middle of next week...the upper air pattn over the central 
Continental U.S. Will shift from ridging to SW flow after a trough becomes 
established over the western half of the country. GFS/European model (ecmwf) disagree on 
the amplitude of the trough and on the position of a strong vorticity maximum 
within it. The vorticity maximum may bring a chance of precipitation to MO/Illinois when it 
lifts through the plains late next week however there is 
considerable uncertainty at this time due to model disagreement. 


Kanofsky 


&& 


Aviation...(for the 00z tafs through 00z Saturday evening) 
issued at 649 PM CDT Friday may 24 2013 


While there are chances for rain showers/thunderstorms and rain over the area beginning 
late tonight and continuing through the taf period... confidence 
is not high enough to include mention of rain at this point. One 
exception is that I have included a prob30 group at KUIN tomorrow 
morning when I expect an area of -shra to be dropping southward 
out of Iowa. This area could drop further into the St. Louis metropolitan 
taf sites middle- late morning...but there is too much uncertainty to 
include at this point. There is also the potential for storms to 
generate as a warm front moves northeast through the region during 
the late afternoon/early evening hours. 


Expect mainly VFR conditions the next 24-30 hours. The exception will 
be MVFR/possibly IFR ceilings/visibilities associated with the heavier 
-shra/-tsra. 


Specifics for kstl...expect dry and VFR conditions the next 30 hours. 
There is a small chance of -shra/-tsra on Saturday afternoon/ 
evening as a warm front approaches the terminal from the 
southwest. However...chances are too low to include in the 
forecast at this time. 


Britt 


&& 


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


$$ 


Weather forecast office lsx