What is “elevated” convection?
Meteorologists across southern California are talking about Wednesday’s thunderstorms. For many in an area from Encinitas to Palm Springs, frequent rumbles of thunder gave many a sleepless night late Tuesday night into Wednesday morning. Thunderstorms are already uncommon in southern California, but the amount of lightning in these storms is rare. The last time many locals remember such frequent, widespread lightning in coming in waves hour after hour dates back to October 2005. (See the lightning map from the National Weather Service, below.)
So why did this happen?
For any thunderstorm to occur, a sufficiently deep warm moist bubble of air needs to condense as it rises past the freezing level in the cloud, building positive and negative charges. In southern California, especially near the coast in the summer, the air is generally cool and stable… so the threat of a thunderstorm is minimal.
But in this case a storm system approached from the southwest and rode up over the top of the dome of cool air. The thunderstorms began above, or were elevated, above the level where storms normally develop (called the boundary layer). Meteorologist call storms that develop this way elevated convection.
Computer forecast models are getting better at helping Meteorologists predict these storms, but they are still challenging. In a dry southern California summer, they also carry considerable wildfire risk, since the storms often do not produce sufficient, widespread rainfall to put out any lightning induced wildfires.
James K. Purpura
Certified Consulting Meteorologist
Director, Weather Forecasting