THURSDAY'S THUNDERSTORM RISK & THE CAP!

Here's the map for Thursday evening.  There's a Slight Risk for severe thunderstorms over portions of eastern Oklahoma.  It's a very conditional risk.  In other words the atmosphere will be very unstable in eastern and southeastern Oklahoma not too far south and east from OKC but there are some limiting factors.  In short, any thunderstorms that manage to form will likely go severe but it's an all or nothing situation!  Low chance for thunderstorms but if they form high potential to go severe.  Large hail, damaging winds, tornadoes possible if storms develop.  Stay weather aware if you live east and southeast of OKC across eastern Oklahoma!

The maps earlier today and now new data coming in tonight continues the trend for lower storm chances Thursday afternoon and evening......but not for the reasons you are hearing on television or on other blogs / websites.  You will hear lots about the CAP on Thursday.  It's easy to throw the CAP out as an excuse or an easy way to explain why we won't get thunderstorms but most folks don't fully understand what it is.  Even the "weathercasters" don't have a good understanding as it's way over used.  Look every time it gets hot and we don't get thunderstorms it's not always the CAP.  Here's a good tip, the CAP doesn't show up until mid to late May!

What the heck is a CAP?  A CAP is a warm layer in the mid levels of the atmosphere that prevents thunderstorms from developing acting like a lid or a cap EVEN IN THE FACE OF OTHER FACTORS THAT NORMALLY ENHANCE THUNDERSTORM DEVELOPMENT!  That last part in capitals is what everyone is skipping over. 

I learned from Howie "CB" Bluestein back at OU meteorology that you really don't have a CAP unless the 700 mb temp is +10c or higher. Tomorrow ( Thursday) the 700 mb temp is predicted to be around +8c.  It's close to a inhibiting CAP but this is breakable if we have other atmospheric factors in place to break it!  Back in the day when I was young and storm chasing I lived by that rule and it worked.  If the 700 mb temp was +10 or higher I stayed home, if it was lower I hit the road because I had a chance to see something. 

Well lets talk about what's REALLY going on Thursday.  A cold front / trough / dry line will stall out across eastern and southeastern Oklahoma by Thursday afternoon not too far southeast of OKC and the air along and ahead of these boundaries will become very unstable!  Ripe for thunderstorms.  However, tonights data is continuing the trend from earlier computer runs of a WEAKER and SLOWER upper level wave.  In fact, if you look closely at the weather data you'll see the latest data is actually forecasting SINKING motion, not rising motion, Thursday afternoon or what we call SUBSIDENCE!  Yes, it gets warm to hot Thursday but the real limiting factor for thunderstorms is not the CAP but that the upper wave arrives way LATE and goes more NORTH so we have sinking air plus wind convergence is WEAK along the surface boundaries.  

So because of these reasons listed above there is only a low conditional severe threat across eastern and southeastern Oklahoma Thursday evening. Hello Mcfly, it's not a "CAP", there's no upper wave!  If we had a wave there would be a huge risk but the lack of a wave and little forcing along boundaries means only a low conditional severe threat.  So there, I said it.  Enough of this CAP stuff!  Lets talk about the Cap when it's +12c at 700 mb and a strong cold front comes in and still no clouds.  That's a CAP!  BTW you need some kind of CAP on severe weather days, don't you?  I mean on big severe weather days the atmosphere heats up and nothing happens until boom!  On big severe weather days you build up all that potential energy and then there's a trigger and boom thunderstorms explode.  It's the loaded gun sounding, it's may 31st 2013, it's storm chasers flying and people dying.  Remember that, how can we forget.  Hope that never happens again!