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Weather watcher explains storm patterns

Simon Smith

Many have been asking why the Lake Tahoe area has seen so much rain so far this winter. Is this the result of the so-called “Pineapple Express” or something else?

While it is true that this has been a rather wet winter so far with 18.55 inches of precipitation from Oct. 1 through Dec. 31, there are a few factors that have contributed to the relatively warm storms. During the winter months the jet stream – the river of air at approximately 25,000 feet – has two branches to it: the northern or polar jet and the southern or subtropical jet.

The polar jet contains the colder and less moist storms, while the southern jet contains the very mild and wet storms. So far this winter, the jet stream has been split, heading into the West Coast with the southern branch delivering Tahoe relatively warm and somewhat wet storms. Once the middle of December hit, the two jet streams combined and thereby intensified the storm systems as they approached the West Coast.

Unfortunately for the Tahoe area, we remained on the southern part of the jet stream and therefore were influenced more by the southern branch which has contained abundant moisture and relatively high snow levels. The moisture source for the recent storms can be traced all the way back to Indonesia and the Philippines.

Since the origin of the moisture is not from the Hawaiian Islands, the recent storms have not technically been the “Pineapple Express” storms but simply strong southern branch jet stream storms. Once the northern branch begins to influence our weather once again Lake Tahoe will enjoy more of the powder precipitation instead of the wet variety. I do not think it is likely that our region will continue in the warm and wet pattern, but instead believe that we will likely see a change to more typical winter storms moving in from the Gulf of Alaska as we get into later January and persisting into the spring months.

Warmer wetter storms will still be possible – just not as likely. For those of you wanting an early spring, it does not look like that will be the case this year. Finally, to sum up the weather so far and looking to the future, take a look back at the winter of 1995-1996 and that is what this winter most closely resembles and will most likely follow a pattern very similar to that year, which was an above average winter.

– Simon Smith lives in South Lake Tahoe and is a co-op observer for the National Weather Service in Reno.


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