You are now in California and the U.S. category.

The weather has quieted down across California after last week’s soaking storm across the state. The setup involved several different factors that led to the significant rainfall event across the state, especially along the coast of northern and central California. For starters, a plume of moisture was observed extending from the tropics near Hawaii to northern California. This transport of high moisture content is referred to as a atmospheric river as I noted in the last blog and can be seen below. The persistent plume of moisture provided the fuel for the heavy rain.

This fall has seen a ridiculously persistent ridge of high pressure — which has driven the majority of the Gulf of Alaska storms north of the Central Coast over the past six years — give way to a trough of low pressure along the West Coast. This condition has allowed a plume of subtropical moisture, or an atmospheric river, that stretched past the Hawaiian Islands to bring abundant rainfall to San Luis Obispo County during the first half of December. In fact, above-average rainfall has fallen throughout the Central Coast since October.

The first winter storm of 2017 to drop welcome rain over the rivers, pumps, pipes and canals that move California’s water north to south likely will open a new era of tension over how much water goes to fish or farms under a new U.S law. Legislation signed Friday by President Barack Obama dictates that the federal portion of California’s heavily engineered water systems gives agricultural districts and other human users the biggest possible share of the most fought-over resource in a state with a six-year drought.