What is a high pressure system?
Think of it as a dense blob of air. When it parks itself over the Pacific Ocean, it can act like a wall – and prevent rain and snow from reaching Northern California.
When meteorologists say a “high pressure system” or a “ridiculously resilient ridge” is keeping the West unseasonably dry, this is what they mean.
A series of persistent high pressure systems over the Pacific factored heavily in the five-year drought that ended last spring. The latest high pressure system produced one of California’s driest Decembers on record, left the Sierra Nevada snowpack woefully thin and raised fears of a drier-than-average winter. Although the system appears to be fizzling out, allowing some precipitation to hit California this week and next, the outlook for the rest of winter remains uncertain.
Climatologists say California is essentially at the mercy of high pressure systems. Ideally, this time of year they pass through the state every few days, allowing rain and snow to roll in. But sometimes a complicated pattern of air movements in the upper atmosphere can cause the high pressure system to stall out near California’s coast, preventing rain and snow from getting through.
A high pressure system is, at its simplest, a bloc of compressed air. As schoolchildren learn, hot air rises and cold air falls. In a high pressure system, cold air is pushing down and becoming dense. Humans don’t feel it, but the effect on weather can be profound.
“Picture a big bubble in the atmosphere that has air that’s dense enough that other air flows around it, that other air being (wet) weather systems,” said Tim Brown of the Western Regional Climate Center, a Reno think tank. “That’s why we can have these very extended periods of clear sky.”
At its worst, a stubborn high pressure system in California can turn deadly.
Instead of moist winds blowing in from the ocean, the state receives dry winds pushing in from the Nevada and Utah. In 2017, that phenomenon produced the Diablo winds that fueled the devastating wine country fires in October and the Santa Ana winds that fed Southern California’s December firestorm.
“What really caused the fires was the duration of the Santa Anas and the Diablos and the persistence of such dry weather over not just days, but weeks,” said Bill Patzert, a climatologist at NASA’s Jet Propulsion Laboratory in Pasadena. “The high pressure system didn’t normally move through California from West to East, it just sat there. It was long-lasting. It was dry.”
So how do high pressure systems form?
Northern hemisphere weather patterns are a function of multiple factors, including the earth’s rotation and the tendency of warm air to move north from the equator and cold air to push south from the Arctic. Variations in the jet stream, the meandering river of high winds in the upper atmosphere that drive storm patterns, also play a role, said UCLA climatologist Daniel Swain.
These swirling air masses interact with each other, causing cold air to collide with warm air and wet air to collide with dry. The resulting pressure patterns are those wavy lines that TV meteorologists present on their weather maps, said private consultant Jan Null of Golden Gate Weather Services.
Ordinarily, for Northern California, those pressure patterns move from west to east, and the weather changes every few days. But sometimes an air mass in one part of the world stalls out.
When that happens, “instead of the weather changing every few days, it can stay stationary for weeks,” Patzert said. That’s what happened in December as a high pressure ridge stalled over the northern Pacific Ocean.
Scientists acknowledge they don’t completely understand what causes the ridges to form. Swain, who coined the phrase “ridiculously resilient ridge” to describe the weather patterns during the drought, said storms in the tropical Pacific as far away as Indonesia can contribute to the formation of the ridges. He also said climate change might play a role, as recent studies suggest that melting Arctic Sea ice can be a factor in ridge formation.
The origins of these ridges are “definitely something we continue to research,” the UCLA scientist said.
Making the ridge hang around even longer this time was a blast of intense cold air moving out of Canada into the eastern United States. The cold air in the East and warmer air in the West formed a feedback loop resulting in warm, dry conditions cycling nonstop over California – and the frigid cold snap that’s gripped much of the rest of the United States, said Michael Anderson, the state’s climatologist with the Department of Water Resources.
The dry pattern that gripped the West during the drought may have become known as the “ridiculously resilient ridge,” but Null said that was something of a misnomer. There wasn’t a single high pressure system over the Pacific, he said. Rather, there was an unusual series of long-lasting ridges, periodically interrupted by rain and snow.
The good news is that it appears that the high pressure system off the coast has relaxed its grip – at least for the time being. Although the storm that rolled through Northern California last week wasn’t major, it delivered more rain in Sacramento (about 0.4 inches) than the city received in all of December (0.13 inches). The National Weather Service expects rain and showers through at least the first half of next week.
Null said that’s what eventually happens with high pressure systems – sooner or later, they fade away and stormy weather moves in.
“It’s like a clog in a drain,” Null said. “Eventually ... things will start moving again.”