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CAPE, LI's and the dreaded MCS: Decoding thunderstorm lingo


A weather lingo breakdown
A weather lingo breakdown

Chris Murphy, Weather Broadcaster, The Weather Network

June 13, 2013 — Around here at The Weather Network, we refer to thunderstorms as the "sexy" weather. Frightening and fascinating, they are a fact of life for so many Canadians from late spring to early autumn - and big thunderstorms make us stop what we are doing and react with either awe ... or action ... as in getting inside!

Being prepared for thunderstorms means starting off the day knowing when and where the conditions are suitable for thunderstorms to develop. 

Sometimes these storms, as you no doubt have experienced first hand, can arrive very quickly, catching us off guard, chasing us off the golf course, abruptly ending our picnics or even scrambling to get off the lake ASAP! 

You will likely start, if you haven't already, to hear meteorologists and weather presenters mention terms that may seem like endless acronyms, so a quick refresher is in order.

We'll begin with CAPE which stands for Convective Available Potential Energy.

Stormchasers often determine where they will go that day based, in part, on CAPE readings. If they are "off the charts" or "through the roof" as they are often overheard saying - this gets their attention.

But what does it mean?

CAPE is essentially the "juice" that makes the potential for thunderstorms become a reality. A warm, humid airmass “cooking” under the warm sun throughout the day creates this energy or CAPE and then the collision with a dry, cool one can produce big thunderstorms.

Once these values exceed a certain number - 2,500 J/kg (Joules per kilogram, this is how they are measured), the potential for big thunderstorms increases.

Sometimes we only require 500 – 1000 J/kg in areas like Alberta. If you look on a weather map, and you have a big blue "H" - high pressure over your entire area, your CAPE values will be low.

Think of CAPE as the precondition for t-storm development. Next we need a trigger. Along with CAPE, meteorologists also look for something called lift. What is forcing that parcel of warm air aloft into the cooler air above creating the clouds and resulting condensation? 

And just as important, at what speed?

The greater or stronger the lift, the quicker the small cumulus clouds turn into towering thunderheads and it is here where the storms get going. 

The lifted index (LI) is a measurement of that lift. Specialized weather models can predict what the lift readings will be and these are measured using negative numbers. For example, let's take that earlier collision of a warm, humid airmass with a dry, cooler one and let's say a cold front is the catalyst for the lift - the trigger if you will, that will help fire off the storms. The model indicates the LI at -6 between 3 and 7pm. This is a good number. Lower than this (eg. -2) represents weaker volatility, and therefore less likelihood for severe weather.

Minus 6 is a good bench mark, but this number can be higher or lower and not always does an ideal LI, nor CAPE for that matter, to guarantee a storm.

MCS - Mesoscale Convective System, and their equally notorious cousin, the Mesoscale Convective Complex (MCC) operate slightly different. First of all, what is an MCS? Mesoscale means big - as in geography. Unlike a microscale event - like a solitary thunderstorm, a microburst or a tornado - we are referring to an event that can span hundreds if not a thousand plus kilometres across.

Convective means the air is bubbly and unstable, and this means thunderstorms. And System, well more often than not these are associated with warm fronts and prime time is June and July in North America.

Sound sinister? It gets worse. These storms don't rely on the heat of the afternoon to fire up their nasty side. They can and often do their worst at night, with long-lasting thunderstorms that can produce anything from flooding rain to the ultimate terror, the night twister.

An MSC or MCC can occur anywhere from Oklahoma northwards. Often in June, Ontario will get at least one of these and it can happen in northwestern Ontario just as easily as in the southwest. 

Derechos, which are huge straight-line wind storms that can wreak much damage over several hundred kilometres, is an example of an MCS-type event. 

Demystifying the language is important. Knowing the potential is what shapes the forecast. But for you the bottom line is and should always be simply this: When thunder roars, head indoors. 

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