A Bout of Drought

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  • Map showing drought severity in the western USA. [The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. Map courtesy of NDMC-UNL.]

    Map showing average annual precipitation across the USA. [Natural Resources Conservation Service, United States Average Annual Precipitation, 1961-2009, in National Atlas of the United States, http://nationalatlas.gov]

    Chart showing 500mb pressure level. It is possible to see the strong westerly flow of the jet stream where the isoheights are closest together, heading north and then dipping south again over the eastern USA. [Meteogroup]

  • A Bout of Drought
    18.01.2014 15:21

    While eastern parts of the USA have been experiencing extreme cold and heavy snowfall, as reported in previous articles by John and Seán, western parts have not seen much in the way of rainfall at all.

    According to the United States Drought Monitor, a website set up in 1999 to bring together drought data into a simple map representing drought severity, most of the western USA has been unusually dry. California is one of the worst affected areas, having received less than 20% of its usual precipitation this water-year, while 2013 was California’s driest calendar year on record. A water-year starts on 1st October, and is designed to include the annual cycle of snow accumulation and snow melt in the same one year period.

    The US Department of Agriculture announced on Wednesday that it was designating 252 counties across 11 western states as Primary Natural Disaster Areas. This allows farmers in the areas access to low interest emergency loans from the US Farm Service Agency, among other benefits.

    Much of the south-west has not had any rainfall yet in 2014, and the last significant rainfall in California was at the start of December. The only rainfall it has seen this year was on 11th, and even then only 10-15mm in the far north. The average rainfall for western parts of California in the first half of January is around 60mm.

    Expected effects are restrictions on water usage as reservoir levels drop, and an increased risk of forest fires as vegetation is made crisp for burning.

    Why is it so dry?

    The upper atmosphere circulation around the northern hemisphere is such that the jet stream weaves towards the north pole over western America, allowing high pressure to build and pulling in warm air from the south, before dipping south again over eastern parts, dragging freezing arctic air down into the east. This pressure pattern has persisted for the past several weeks.

    One reason why this dry weather has been so relentless is due to unusually warm sea surface temperatures in the northern Pacific, above the climatic average by up to 3.5°C. This encourages stable conditions and the building of high pressure, which deflects frontal systems from bringing in rainfall, and subdues the formation of showers.

    A strong jet stream across the western Pacific may force the high pressure away at some point, but for the foreseeable future it looks set to stay dry and sunny in the south-west.

    By: Ben Windsor