An atmospheric river has caused enormous amounts of precipitation in California over the past few hours and days. The greater Los Angeles region has been particularly affected, with record-breaking rainfall totals in places.
Over the past three days, it has rained heavily in parts of California. So far, the greater Los Angeles region has been particularly affected. A total of 100 to 200 mm fell here, and locally significantly more. In the city center of Los Angeles, it has not rained this much for over 56 years (a good 175 mm in 72 hours). According to media reports, the measuring station at the University of California (UCLA) even recorded almost 300 mm within a day! This corresponds to a "1 in 1000 year event". The following map gives a rough overview of the distribution of precipitation to date. As English units of measurement are used in the USA, the figures shown must be multiplied by a factor of 25 to give us our usual metric millimetres (1 inch = 25 mm).
Fig. 1: Precipitation totals for the last 72 hours in inches (1 inch = 25 mm); Source: NWS
In addition to widespread flooding and flash floods, this intense rainfall also caused landslides and widespread power outages. At one point, more than 850,000 households were without power, and around 220,000 households are still affected.
In the higher elevations of the Sierra Nevada, a lot of fresh snow fell. In Soda Springs, for example, a good 50 cm fell in two days. On Donner Pass, the fresh snowfall over the past seven days even amounted to 120 cm. Despite this increase in fresh snow, the winter of 23/24 is lagging behind the climate norm in terms of snow. This is primarily due to a very dry November and December.
The reason for these enormous amounts of water is easy to find. A so-called atmospheric river brought moisture-saturated tropical and subtropical air to the US west coast. Due to its origin in the region around Hawaii, it is colloquially referred to as the "Pineapple Express". This blue "moisture band" can be clearly seen in the image below.
Fig. 2: Atmospheric river over the Pacific; Source: Cooperative Institute for Meteorological Satellite Studies (CIMSS)
A current look at the precipitation radar shows continued heavy downpours in southern California (e.g. San Diego). According to high-resolution regional models, a further 50 to 100 mm of rain can be expected over the next three days. The situation is therefore unlikely to ease any time soon.
Fig. 3: Current precipitation radar for California; Source: College of DuPage
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