Northern California hit repeatedly by intense storms and power outages

UC Davis faculty and alumni explain the weather pattern hitting the Bay Area and Central Valley

By BRANDON NGUYEN— [email protected]

Kicking off the new year, high-impact rainstorms have battered the Northern California coasts, bringing seemingly endless rainfall to the Bay Area and Central Valley. Daniel Swain, climatologist at the Institute of Environment and Sustainability at UCLA and a UC Davis alumnus, highlighted the unforeseen intensity of the storm despite weather forecasts in the media in a recent Blog to post.

“A strong storm on New Year’s Eve brought very heavy 24-hour precipitation accumulations to a relatively narrow but highly populated stretch of NorCal, from around San Francisco in the Central Bay area east to the foothills of the Central Sierra,” said the Blog read. “Here, some places actually came close to (or even exceeded) all-time 24-hour precipitation records. This was quite unexpected.”

Dr. Matthew Igel, assistant adjunct professor in the UC Davis Department of Land, Air and Water Resources, discussed the cause of the forecasted heavy rains and flood warnings issued last week.

The storm of a couple of days ago, scheduled for this past Wednesday [Jan. 4], was what was called a ‘bomb cyclone’ in the popular media,” Igel said. “This was, by itself, a very strong storm, but I think what’s really interesting from a meteorological standpoint right now is the fact that we’ve been getting repeat storms in California; the expectation is that we would get about three weeks of essentially continuous rain here in Northern California. And while we have had some exceptionally strong storms associated with this [rainfall]the story really is just the fact that it’s been storm after storm after storm.”

A “bomb cyclone,” also known as an extratropical storm, is a rapidly intensifying low-pressure system, according to Igel. The center of the storm must meet a specific metric where its atmospheric pressure decreases at a specific rate to be classified as a “bomb cyclone”, and this type of storm typically occurs just outside the tropics during the winter season.

Dr. Ian Faloona, associate professor and biomicrometeorologist in the UC Davis Department of Land, Air, and Water Resources, explained how the bomb cyclone facilitated the heavy rainfall seen in recent days.

“What these cyclones, or these storm systems, are doing is pulling air out of the hot, humid tropics, where most of the water vapor on Earth is,” Faloona said. “And so I think [the bomb cyclone] like a paddle wheel moving forward, and they [the storm systems] they generally move from west to east due to how the winds generally move in our latitudes, and many times they hit the coast, accumulate along it and move towards the pole”.

The movement of water vapor in the atmosphere is also known as an “atmospheric river,” and the circular motion of winds created by recent storm systems has helped drive these atmospheric rivers toward the northern California coasts. According to Igel, California’s unique topography also plays a role in the heavy rainfall observed. Mountain ranges like the Sierra Nevada Mountains cool and condense hot water vapor as rain due to the higher altitude of the landscape.

“Atmospheric rivers are essentially long regions of water vapor in the air,” Igel said. “They tend to be just overhead, in the first mile of the atmosphere, so the term ‘Pineapple Express,’ specifically, is used to describe an atmospheric river that draws its moisture from the area around Hawaii. But in general, an atmospheric river really takes all that moisture in the hot, humid tropics and brings it here to the west coast of California, or really, to the west coast of any continent.”

Both Faloona and Igel said they could not identify a single causal factor for why the observed and forecast severe weather patterns are occurring.

“You see a lot of things you’ve never seen in the weather before, and it’s a bit of luck,” Igel said. “As a scientist, it’s tempting to try to go back and really find the root causes, and I’m sure there will be some evaluation in the next few years on this sequence of storms. The storms are unlikely to be directly due to climate change. At this point it’s hard to describe a proximate cause other than weather just happens and sometimes doesn’t. Winter storms are more intense to begin with, and we happen to be at peak storm intensity this time of year.”

Written by: Brandon Nguyen — [email protected]