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On the days where models are maxing out for severe thunderstorm potential, all around the world. There is something that all of chasers look at closely as its the do or die for a great storm day, and its call the CAP.
So what is the CAP?
A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However if the cap is removed or weakened, then explosive thunderstorm development can occur.
So how is it removed?
Well to begin with.. blue skies in the morning are ideal as this will allow as much heating as possible to occur. This is partial reason as to why severe thunderstorms are so much more common in Summer – because there is far more heat around. You also need a dew point which can create enough moisture or humidity in the atmosphere simultaneously with the surface temperature. This is why when its very hot and quite muggy or humid, Thunderstorms in general appear to be on the forecast.
For Example: On a particular thunderstorm day in South- East QLD during the 2016-17 storm season, the magic numbers to break the cap were quite high. We were looking in general for about 35ºc as the temperature and a dew point of around 22ºc. Typically, when temperatures are reaching the mid 30’s, its hard to get dew points above the teens and that was the case on this particular day, where temperatures were around 34ºc and DP’s around 19-20ºc. For those that did get the temperature, unfortunately (or fortunately) the dew point was around 12-14ºc (i.e. far too dry).
Why didnt the magic numbers occur on that particular day?
Well it appears that the key was a midnight Supercell near Warwick. This Supercell produced large hail, frequent lightning, damaging winds and heavy rain between Mulgowie and Warwick. It then tracked North-East towards Ipswich and Brisbane, before becoming a large rain band. When morning came, clouds lingered from this rain which resulted in a slower build up with the temperature. Drier winds needed to kick in for the temperature to rise, but that means the dew point is going to drop. Now you can see it was always going to be a struggle.
• Everyone has a CAP – just its more noticeable in some areas (like SEQ/NENSW + Coastal areas)
• A CAP is always present above the atmosphere
• This CAP is usually the difference between severe storms and nothing when the atmosphere is extremely unstable.
• Its very hard to forecast whether the CAP will break or not, until the actual day when the actual observations are coming in live
• We use a skew-t plot which generates a vertical sounding of the atmosphere, to access how weak or strong the CAP is, and plug in alternative temperatures and dew points to see what is needed to break it.
Hopefully this gives an insight for everyone as to one of the challenges both forecasters and chasers face on storm days, especially when it could become explosive. You can get the exact same models in back to back days, and with a tiny tiny tweak, completely different results can occur.
The image below is demonstrating where the CAP lies, and how it interacts with the rest of the Atmosphere.