16 Dec 2014

California rain nibbling away at exceptional drought

California, a state hit hard by exceptional drought, is finally receiving some welcome rainfall, bringing hope that this winter will see its fortunes change.

As of 11 December, the National Drought Mitigation Center’s weekly drought monitor showed that 94 per cent of California was in the midst of a severe drought.

More than half (55 per cent) of the state is considered to be in the most extreme category of exceptional drought.

california_clouds_g_wp

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 past year – especially during autumn and early winter 2013/14 – 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.

Lack of snowfall

Late autumn and winter are when California would expect to receive the greatest portion of its annual rainfall.

Snowfall across the mountains also plays a huge role in California’s water resources because it builds up in the winter, then provides a steady supply of water for the reservoirs as it melts during the spring.

The persistent dry weather has meant that there has been little snowfall across the mountainous parts of the state so far this winter.

The NOAA satellite image below comparing 13 January 2013 to the same day in 2014 shows a huge difference in snow cover.california_snowdiff_NASA_wpSnow cover for the state as of the 16 January 2014 was just 17 per cent of the average. With little snowfall during the rest of the winter, there was little snowmelt to top up the reservoirs in spring.

From drought to flood

During the past week, one of the stormiest spells of weather in six years struck California, bringing heavy rain, mountain snow and gales.

With the ground so dry and compact after a prolonged period of drought, it was unable to soak up the intense rain.

This lead to flash floods and mudslides as the rainfall rapidly flowed down the sides of mountains, running into rivers very quickly.

Despite the disruption, the rain has nibbled away at the drought, with only 55 per cent of the state in exceptional drought, compared to 58.4 per cent at the end of October.

In the next seven days, a further 1-5 inches of rain is expected to fall across California, with further spells of rain expected in the coming weeks.

qpf_california2_NOAA_wpWhile there is still a long way to go before things return anywhere near normal, the trend to wetter weather will help improve the situation.

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