About That Weather

Why is it that storms back up over the Pacific every few years like so many 747 jumbo jets waiting to land--or to pounce--on California? Why is it they seem to schedule themselves so that rain-drenched Californians get just a little respite, just a slice of blue sky and hope, before the next one hits the saturated earth? It does not take much imagination, after being pounded by rain cell after rain cell with seeming regularity, to believe that Nature has some perverse design in all this.

Natural phenomena like earthquakes and volcanoes used to be inexplicable events or acts of God. But now science can explain these things with logic and data even if it cannot always predict when they will happen. The weather still has its mysteries, however. And it has been a relatively rare combination of events that has visited days of pounding rains on the coast.

Most of California’s winter storms follow a northwesterly track, coming down from the general direction of the Gulf of Alaska. They carry relatively normal amounts of moisture along with cool air. This time, as in 1969, 1979 and 1982-83, the storm track was forced to the south, in part because of the blocking action of a high pressure area that formed over northeastern Canada and, untypically, moved toward the west. Or, as Weather Service meteorologist Jim Henderson of San Francisco puts it, the high went into “retrogression.” With the storm track pushed down into the Pacific tropics, it became especially powerful and laden with moisture. When such air hits the California coast ranges and is forced upward, it dumps its massive cargo of moisture with a bang, Henderson explained.

The tropical air is notably troublesome this time of the year because the moisture falls as rain rather than snow high up in the Sierra. This compounds the runoff problem by melting the existing snowpack and, in the process, creating super moisture-laden levels of snow that are prone to avalanche.

The normal storm now has been forced so far south, it has dipped into the zone of “intertropical convergence.” This is a region near the equator where the opposing trade winds come together, creating a caldron of weather turbulence. It is this area that spits out Pacific hurricanes in the summer.

So what does it take to break up this pattern? Gradually, that erratic Canadian system will pass on westward, allowing a new pressure ridge to build in the eastern Pacific. The storm track will gravitate back north, where it should be, picking up colder, drier air. Signs of that occurring are visible on the satellite pictures now.

For what it’s worth--which is not much to those who have lost their homes and possessions--the wretched weather is the result of “very unusual conditions,” Henderson said. Still, he said, the pattern is not unlike those that almost routinely bring torrential rains to the Gulf states.

Except when it happens in California, the rest of the nation seems to view it as a sort of a devilish payment due for all our sunshine, bucolic outdoor living and taunting of Nature with construction of houses where everyone knows they should not be built. If so, our dues are paid for some time to come.