The Sacramento Bee –
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.