Stormy weather across the pond

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  • 24h Rainfall observations taken from the period starting 0600 on the 22nd January 2017 across the Los Angeles region. (Source: Meteogroup).

    Water vapour satellite image from the 22nd January 2017 00UTC (Source: NOAA).

  • Stormy weather across the pond
    28.01.2017 17:33

    While the weather across the UK in January has been quite settled the same cannot be said across the pond. Parts of the U.S.A have been affected by numerous winter storms causing all sorts of problems. One area of that has benefitted from the continual storms is California, which has had a 45% reduction over the last week in areas that were classified to be in extreme drought due to all the rainfall. 

    Storm Leo moved in across the west coast of the USA last weekend (21st-23rd January 2017) bringing gusty winds, high rainfall totals, stormy seas and large amounts of snowfall across the Sierra Nevada Mountains. Observations taken from the region show that in the 24 hour period from 0600 on the 22nd 42.7mm fell at Chino, located to the east of Los Angeles. The high rainfall caused serious disruption with flash flooding, leading to flood warnings being issued across the state. As well as rain up to half a foot of fresh snow fell over the Sierra Nevada Mountains bringing day to day activities to a halt. The fresh snowfall is a top up on record amounts recorded at Mammoth mountain, where they have seen the highest ever snowfall in any calendar month on record, just over twenty feet. These are very impressive statistics for a region that has been blighted by a lack of precipitation over the last few years.

    To understand why there was so much precipitation over California from Storm Leo, it is important to look at the influence of ‘Atmospheric Rivers’. These are, as defined by NOAA (2017), “regions in the atmosphere that are responsible for most of the horizontal transport of water vapour outside of the Tropics”, essentially fuelling precipitation occurring over a given region.  The synoptic set up for Storm Leo caused the winds to come from the south-west, bringing moist laden air from the tropics over the west coast and subsequently high precipitation amounts. This is quite clearly shown in the water vapour satellite image from NOAA, with the white shading showing the ‘Atmospheric River’ stretching from the tropics into California. Whilst it is not uncommon to see this kind of meteorological set up the impact of the last few storms and their associated ‘Atmospheric Rivers’  has meant that some places in California have seen over 300% of the normal precipitation for the whole of January. 

    The drought situation looks set to continue to get better with more rain and snowfall in the forecast courtesy of another storm system moving in later next week.  It could become storm Maya, which would be the 13th named storm of the winter so far bringing disruption across the western seaboard of the USA and California. The storm could give another 10mm of rain across the California region, as well as up to 30cm of snow in the Sierra Nevada Mountains.

    By: Tom Whittaker