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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Saturday
• 
• High: 75 °
• Low: 59 °
• T-Storms

• Sunday
• 
• High: 81 °
• Low: 64 °
• Chance of T-Storms

• Monday
• 
• High: 77 °
• Low: 66 °
• Chance of T-Storms

• Tuesday
• 
• High: 86 °
• Low: 63 °
• Partly Cloudy

• Wednesday
• 
• High: 86 °
• Low: 64 °
• Mostly Cloudy

Area forecast discussion 
National Weather Service St Louis MO 
325 PM CDT Sat may 25 2013 


Short term...(through late tonight) 
issued at 323 PM CDT Sat may 25 2013 


An mesoscale convective system remnant will continue to weaken this afternoon as it moves across 
the area. Schc-chance probability of precipitation look reasonable for the late afternoon period based 
on radar trends. 


Attention then turns to the overnight period. Late afternoon surface observation revealed a 
sharp warm front stretching across western MO and southwestern Iowa into the northern 
plains. This boundary is expected to provide the focus for additional 
areas of precipitation tonight however it is not yet clear whether precipitation will 
take the form of another mesoscale convective system /possibly more than one/...a broad area 
of precipitation anchored near the 850 mb warm front...or some combination of the 
two. These scenarios seem equally plausible given the current boundary 
location...forecast low level jet strength and orientation...instability 
forecasts...and observed trends over the previous 18 hours. At this time the 
best precipitation chances appear to be over the northern and northestern County Warning Area...but it is 
equally possible that a broad area of precipitation could remain north and 
east of the County Warning Area for most of the night or that an mesoscale convective system could form and 
bring widespread rain to a large portion of the County Warning Area. In 
short...unsettled weather will continue through tonight. 


Kanofsky 


Long term...(sunday through next saturday) 
issued at 323 PM CDT Sat may 25 2013 


(tonight-tuesday) 


Given how difficult determining todays convective trend has 
been...pinning down this threat over the remainder of the Holiday 
weekend will certainly remain very challenging since there is 
essentially no change in the overall synoptic pattern. 
Synoptic and hi-res expicit thunderstorm models are in agreement in 
a very broad sense that an elevated thunderstorm threat will persist 
across the region as weak shortwaves propagate through the middle level 
ridging...interacting with and occaionally intensifying broad zone 
of low level warm advection along 850mb baroclinic zone that is 
prognosticated from the middle MO valley into Illinois. However...there is almost 
no agreement on where and when storm initiation will occur since low 
level jet and forcing seems much less focused than what occurred 
last night and today. 


For now will maintain a broad brush approach to forecast through 
Monday...and fine tuning both probability of precipitation and temperatures will become a 
Nowcast issue once convective trends become better defined. Highest 
probability of precipitation will remain over our north/NE counties near the 850mb thermal 
gradient where lift should be maximized and/or southeast propagation of mesoscale convective system 
clusters will be most likely. However...as was proven today 
certainly cannot rule out thunderstorm chances further S due to weak 
shortwave energy...a more unstable airmass in the warm sector...and 
possible boundary interactions...so will maintain chance probability of precipitation 
elsewhere. 


In the Monday night-Tuesday time frame upper ridge will push east 
and SW flow will become established over area as upper trough deepends 
over the western U.S. With time this should push the 850 mb 
thermal gradient and mesoscale convective system threat further north and east...with a 
corresponding drop in probability of precipitation from S to north across the County Warning Area during this 
time. 


(Wednesday-saturday) 


SW flow aloft will dominate the middle Mississippi Valley for the 
end of the week as upper level low slowly forms over the northern 
rockies. Primary question is how much upper level dynamics will be 
able to work east into our County Warning Area as large blocking ridge will also be 
forming over the southeast Continental U.S.. consensus of medium range 
guidance suggests lead trough initially over the SW U.S. Should lift 
into the plains Wednesday night and clip our area on 
Thursday...followed by several additional shortwaves rotating 
around the upper low. Throughout the period...it appears that main 
dynamics will stretch from plains into upper Mississippi 
Valley...suggesting that main thunderstorm threat will be over west/north 
sections of our forecast area...with the storms struggling to work into southeast MO 
and S Illinois. 


Since region will be well entrenched in the warm sector... 
temperatures should be above average with daytime highs primarily in 
the 80s. 


Truett 


&& 


Aviation...(for the 18z tafs through 18z Sunday afternoon) 
issued at 1233 PM CDT Sat may 25 2013 


Ongoing showers and thunderstorms will continue to percolate 
across the area early this afternoon. There will probably be at least a 
few hours of VFR/dry weather before another thunderstorm complex 
develops later tonight and moves southeastward towards the taf sites. There 
is low confidence at this time regarding the timing of overnight 
convection. Beyond the end of the 24hr taf period...there may be 
additional scattered thunderstorms tomorrow afternoon. 


Specifics for kstl...look for continued unsettled weather through 
the taf period. A broken line of thunderstorms will move through kstl 
within the next few hours. There will probably be at least a few 
hours of VFR/dry weather before another thunderstorm complex 
develops later tonight and moves southeastward towards kstl. There is low 
confidence at this time regarding whether the overnight complex will 
actually reach kstl. There may also be scattered thunderstorms tomorrow 
afternoon. 


Kanofsky 


&& 


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


$$ 


Weather forecast office lsx