Since last week, several cities across the United States’ California have been witnessing heavy rains and flash floods which have led to power outages and at least two fatalities, so far. Additionally, on Friday, after two days of heavy rains and strong winds, thousands of homes across the state were left without power.
These heavy rains and high winds have been attributed to an overlapping weather phenomenon called “atmospheric river” and “bomb cyclone” which have also caused flash floods, landslides, and uprooting affecting road travel. Notably, this comes after the region battled a megadrought in early December prompting strict water restrictions.
What are ‘atmospheric rivers’?
As the name suggests atmospheric rivers are storms which dump massive amounts of rain in one region causing flash floods and landslides. The US National Oceanic and Atmospheric Administration (NOAA) describe them as “rivers in the sky” as they are relatively long, narrow regions in the atmosphere that transport water vapour outside of the tropics.
Notably, the amount of water vapour is at least 15 times the average flow of water at the mouth of the Mississippi River and when they make landfall, this water vapour can be released in the form of rain or snow. Additionally, atmospheric rivers can be at least 350 miles (563 km) wide and up to 1,600 miles (2575 km) long.
According to forecasters, this particular incident is a result of the low-pressure system churning over the eastern Pacific. The NOAA also noted that not all atmospheric rivers cause damage, in fact, they often provide much-needed rain and snow to a region which is often crucial to the water supply and is a key feature in the global water cycle. However, they are closely tied to both flooding and water supply, particularly in Western US.
Another term to describe the phenomenon, particularly in the US West Coast context is called “pineapple express”, which brings a strong atmospheric river to the region from the tropics near Hawaii, which is what California is experiencing at the moment. As the atmospheric rivers which typically originate from the Pacific Ocean, in this case, are beginning from Hawaii which is also the US’ leading producer of pineapples, hence the name.
What are ‘bomb cyclones’?
Meanwhile, a bomb cyclone is a quickly intensifying storm marked by a drop in pressure of 24 millibars in 24 hours. They are also often associated with atmospheric rivers and typically form in winter when cold and warm air masses collide. Due to the nature of the storm, meteorologists used terms like “explosive cyclogenesis” or “bombogenesis” since the drop in pressure is seen as akin to dropping a bomb.
What is happening in California?
As mentioned earlier, the state had been battling a megadrought, however, amid the cycles of wet and dry both intensified by climate change captured a mega storm in order to ease the destructive droughts. The state is essentially forced to battle extremes with high temperatures, heat and wildfires followed by drought and now flash floods. According to a report by Reuters, scientists say warmer air and temperatures which have been attributed to climate change have led to more absorption and dumping of rain over regions, in this case, California.
What has happened till now?
Several cities across California including San Francisco Bay and Sacramento over the New Year’s weekend witnessed massive power outages following heavy rains and high winds. At the time, the National Weather Service (NWS) had also predicted up to three feet of fresh snow in Sierras and widespread rainfall. However, with little to no respite during the week, since Wednesday, at least two people had been killed including a toddler after a tree crushed his mobile home in the northern region of the state.
As of Friday morning, at least 60,000 homes and businesses were left without electricity across several counties in northern California, said a report by Reuters citing the data from Poweroutages.us. Meanwhile, the utility crew in the area scrambled to restore power. The continuous storms led San Francisco to witness at least 26 cm of rain starting from the day after Christmas to January 4 which was the wettest 10-day stretch since 1871, said the NWS.
Will the situation get better?
It might not get better anytime soon. The weather agency said the next wave of the storm will hit the northwestern corner of California late on Friday and move towards the San Francisco Bay Area and central coast over the weekend. Additionally, the NWS has also predicted a “likely stronger” atmospheric river storm for Monday (January 9) while the current weather pattern might continue till at least mid-January.
(With inputs from agencies)
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