Why California’s wettest storms can be the difficult

Last week, as forecasters at the National Weather Service in the San Francisco Bay area, looked at a series of storms that would hammer the region, they predicted that some areas in North Bay would see six to eight inch rain between Saturday and Tuesday.

Instead, around a foot of rain fell in the wettest places. Motorways were flooded, landslides were triggered, and rivers caught. No later than Tuesday afternoon, a house Glide down a saturated hillside and fell into the Russian river in Sonoma County. Evening wavering orders were issuedhours before it swollen river Started throwing over on a handful of roads.

The storms, known as atmospheric rivers, are a common feature of West Coast Winters. They are thin bands of moisture carried by heavy winds, sometimes in thousands of miles, which can make forecasts especially challenging.

“We got a little more rain than expected, and the rivers also reacted more than expected,” said Dylan Flynn, a meteorologist with weather service. “A few days ago, we didn’t expect any of the rivers to flood.”

The rain could have been even more effective if northern California had not seen an abnormally dry January. The first of the recent rain was mostly absorbed by the dry soil starved by rain, saving the area from more severe floods.

Another of these moisturizing storms is intended to sweep the region on Thursday into Friday, which brings more rain and flood problems, but Mr. Flynn said this is expected to be more “run-of-the-mill” than the last.

This prolonged spell of atmospheric-river activity in northern California has brought rain to valleys and coast and snow to the mountains. The effects of the storm are stretched as far north as Washington and into southern California.

To find out where these systems will land, meteorologists use tools far more sophisticated than what was available two decades ago, including higher resolution satellites and weather models running on supercomputers. There is even a program for flying hurricane hunting aircraft from the National Oceanic and Atmospheric Administration in the storms to collect data.

It is still difficult to predict these systems with perfect accuracy.

A miscarriage can result in surprises. Strong atmospheric rivers carry abundant amounts of moisture, and if a storm unexpectedly changes a little somehow, it can bring a lot of rain to one waters against another, which follows a river for flooding while another remains safe In its widths.

Chad Hecht, a meteorologist at the Center for Western Weather and Water Extremer at the University of California, San Diego, said pockets with intensive rainfall can occur within these storms and lead to dangers as urban areas. “These pockets can be formed in such small scales that forecast models struggle to solve them, and it is difficult to identify exactly when and where they will occur,” Mr. Hecht.

As these storms began to take shape last week, there was some concern that they could switch south to Los Angeles, where destructive fires last month left the earth. Instead of the beginning of this week, forecasters said they expected only up to about an empty rain for some parts of that region. And by Wednesday, they said, the storm was “under -priesting” there, with up to half an empty rain measured.

Atmospheric rivers are corridors of moisture transported through the atmosphere of strong winds. They have long existed all over the world, but the expression was first used in scientific literature in the 1990s, and the media quickly locked in on the colorful description.

On the west coast, the systems over the Pacific Ocean are formed in the subtropics. As they travel across the sea, they pull in moisture and move over a huge, empty width where observation data is sparse compared to what the robust network of weather stations on land can collect. There are about 100 buoys administered by NOAA in the Pacific, the world’s largest sea, helping to predict the approach to a storm against 1,000 observation data points that can predict the course of the storm when it reaches the country, said Marty Ralph, the director of the Center for Western Weather- and water extremes.

“Satellites help, but big holes are back,” Mr. Ralph.

Cliff Mass, a professor of atmospheric science at the University of Washington, said today’s sophisticated satellites mostly compensate for the lack of observation data available over the sea. He wants to see more high -powered, long -distance radar created along the west coast to collect data from the sea as a storm approaches. “We can take the data from these radars and take them into our models, and then we could significantly improve our forecasts,” said Mr. Mass.

Even with all tools available, a forecast is an uncertain business, because while the storms can be thousands of miles long, they are also narrow at just a few hundred miles across.

“Due to the narrow moisture, these system can literally penetrate the needle between observations,” said Brian Garcia, a meteorologist with weather service.

When these storms hit land, they are angry like mushrooms. Forecasts are tasked with finding out where the systems will detach that moisture. One thing they know for sure is that these systems are likely to release the most rain when interacting with mountains on the west coast, where there are a lot of large intervals, including Siskiyous and Sierra Nevada.

“Because the mountains do not move, especially in relation to the weather, it makes it easier to predict where the heavy rain or snow will be,” said Mr. Ralph.

While the mountains are not moving, the actual storms can make weak, yet unpredictable shifts due to disturbances in the inmosphere. Mr. Garcia said that atmospheric rivers travel through the air more like a frisbee with a wobble than as an arrow with a straight path.

In case of the system that swept California from Monday night to Tuesday, Mr. Garcia that it “wing” into the Bay area, resulting in over a foot of rain on Mount Tamalpais in Marin County. Just above the Golden Gate Bridge, San Francisco registered 2.8 inch rain on Tuesday and broke his same day’s rainfall record of 2.2 inches set in 1882.

The region is expected to get more rain on Thursday and Friday, although this storm is expected to be less effective than the previous two. By the end of the day on Friday, Mr. Garcia, some of the wettest locations in North Bay may have registered up to 18 inch rain over the past week.