And the rain, rain, rain came down, down, down
On Thanksgiving weekend, the blocking ridge that kept California bone dry throughout the early fall season finally broke down.
The polar and subtropical jet streams worked in tandem to deliver a series of storms that soaked California and the Bay Area.
A polar jet stream pattern change triggered the eastward movement of the blocking ridge. When the ridge was stationary, the polar jet at our longitude arched well into central Canada. After several weeks of little movement, this jet stream pattern finally began to change.
In mid-November, a polar jet stream trough began to develop over the cold waters of the Gulf of Alaska. After several days, the strong winds in the trough began to weaken the West Coast ridge and nudge it eastward toward the Rockies.
The energized polar jet stream dipped southward as it approached the West Coast. The cold moist air carried by the jet formed a strong surface weather front as it approached our state.
By Thanksgiving afternoon, maximum temperatures had plunged 20 to 30 degrees in two days. Many Bay Area weather stations reported more than an inch of rain, and snow levels dropped to below 3,000 feet.
The abnormally cold weather did not last long. Soon after the polar jet stream pattern became progressive, a strong subtropical jet developed in the central Pacific.
The subtropical jet has some interesting characteristics. Unlike its omni-present cousin the polar jet, the subtropical jet develops then fades out. Also, it doesn’t travel in distinct waves like its kin to the north.
When a subtropical jet forms over the central Pacific, it can be quite moisture-laden as water from the warm ocean evaporates into the air mass below the jet. Steering currents for the subtropical can be provided by the southernmost edge of the polar jet.
In the aftermath of the Thanksgiving day storm, a developing subtropical jet in the central Pacific was steered toward the California coast. Our storm pattern was about to change.
Satellite photographs showed a moisture-packed line of clouds associated with the subtropical jet stream that stretched from the Hawaii to California. Historically, meteorologists referred to this alignment of the subtropical jet as a Pineapple Express, but the popular terminology now is atmospheric river.
Regardless of terminology, several wet storms followed the initial cold storm that broke the dry spell. It is somewhat like watching a train pass by; the polar jet storm represents the locomotive pulling several subtropical jet storms behind it.
The wet weather has finally arrived, and the polar and subtropical jet streams have settled back into their normal winter patterns.
Woody Whitlatch is a meteorologist retired from PG&E. Email your questions or comments to
clayton_909@yahoo.com