Why California faces a dangerous wildfire season this year

Earlier this week, 20,000 homes in and around San Diego, California, were evacuated as a fast-moving wildfire spread through the area.

Driven by high temperatures and strong winds, fire and country officials said that more than a square mile of land had been blackened.

No homes have so far been damaged or destroyed by Tuesday’s fire, upon which hundreds of firefighter, assisted by water-dropping aircraft are battling the flames. 

California is about to enter the peak of its fire season, and state officials are concerned that it could be particularly intense and dangerous this year, due to much of the state being in extreme drought.

California – the drought-ravaged state

Back in January, I wrote a blog about the how California was suffering from extreme drought, following the driest year on record.

During 2013, California had just 32.8 per cent of the rainfall that it would normally expect to see, leaving fresh water reservoirs with a fraction of their normal reserves.

The situation become so bad that California Governor, Jerry Brown, declared a drought emergency across the state.

As of 14 January, 62.7 per cent of the state was in extreme drought and 89.9 percent in severe drought, according to the US Drought Monitor.

In recent months, the intensity of the drought has increased. As of 6 May, 76.7 per cent of the state is in extreme drought and 96 per cent in severe drought.

What has caused it to be so dry?

California relies on rain and mountain snow moving in off the Pacific Ocean, driven by the jet stream – the fast-moving ribbon of air five miles up in the sky that forms and steers areas of low pressure.

However, during the last year – especially during autumn and early winter – the jet stream has been persistently stuck much further north than usual, resulting in a blocking area of high pressure forming over the state.

This has kept the weather largely dry, as the usual train of low pressure systems travelled further northwards into western Canada, taking the rain and snow there.

Why so much concern for this year?

With California suffering so badly from drought, much of the land and vegetation is tinder-dry and is ripe for burning if a wildfire breaks out.

The state receives nearly all of its annual rainfall in the autumn and winter months, so typically, little rain falls during late-spring and early summer.

Soaring summer temperatures combined with the dryness of the landscape, will only elevate the risk of wildfires further.

Summer heat will also bring the risk of dry thunderstorms. These are thunderstorms that produce thunder and lightning, but no rain that reaches the ground.

As the air in the lower part of the atmosphere is so dry, the rain that falls from the clouds high in the sky evaporates long before reaching the surface.

The lightning strikes from these dry thunderstorms can set fire to dry vegetation, which can then spread quickly when fanned by brisk winds.

One possible factor that may help California’s drought situation later in the summer is the formation of El Nino – a warming of surface water temperatures in the central and eastern Pacific Ocean.

When such events take place, California tends to get above average rainfall, with a train of moisture flowing in off the ocean.

However, until then, there’ll be a heightened sense of awareness about the dangers that wildfires may bring.