First major storm of season on its way
Weak La Nina pattern, below-average rainfall predicted for rainy season
The Central Valley will experience its first major storm of the winter season this weekend, but long range forecast models are currently predicting a weak La Nina weather pattern, which likely means below average rainfall totals for the upcoming rainy season.
Scott Borgioli, chief meteorologist at Weatherag, reports that weather forecast models show an “unseason- ably strong” low pressure system affecting California on Friday, and will continue into Monday. The Central Valley will see an increase in clouds on Friday with the chance for a few isolated light showers.
“While isolated showers are certainly possible Friday, it appears the bulk of the rain won’t fall until sometime between Saturday afternoon and Sunday night,” said Borgioli. “Therefore, our forecast right now will have isolated showers possible on Friday, scattered to numerous showers on Saturday, and numerous showers Sunday. [We] could see some isolated lingering showers Monday morning.”
There is some uncertainty regarding how far south into the Valley the rain line will reach over the weekend, and models are in complete disagreement regarding the amount of rainfall the storm system will bring, with estimates ranging from just over tenth of an inch in spots to about half of an inch.
The mountains and foothills will receive the majority of precipitation from this storm. Snow levels will start out at approximately 8,000 feet on Friday and early Saturday, and will lower to about 5,800 to 6,500 feet by Saturday evening through Sunday.
Although this storm will not be the typical cold winter-like system, Borgioli expresses a concern with a cold air mass following this system due to wind flow. The most current information suggests that some of the colder areas of the
Valley could see freezing or near-freezing lows early Tuesday morning through Thursday, but low temperatures in the citrus producing areas of the Valley will likely stay at or above freezing during that time. Depending upon the amount of rainfall, fog could also develop during the night and early morning following the system early next week.
Although this weekend’s weather system will likely bring the first measurable rainfall of the season to the Valley, the number of such systems to visit the Central Valley this winter will likely be fewer than last year. Borgioli notes that a La Nina is forming and weakly present, and data indicates it will remain in the weak strength category, although it could develop into a moderate strength La Nina. Historically, Central and Southern California generally receive below average precipitation with a weak La Nina, and average to above average during a moderate strength La Nina.
“This is not to say we’ll be 100 percent dry, but the odds are stacked against us,” said Borgioli. “Also, even with the odds not in our favor, it doesn’t mean we’ll be extremely dry, just likely drier than average.”
Borgioli qualified the long-term outlook by noting that certain weather variables can kick into gear and help drive some additional
moisture into California over the winter, even with La Nina conditions. One of these is the Madden-julian Oscillation, which could drive additional moisture into the Pacific Ocean to get absorbed by storm systems trekking into the West Coast, sometimes creating “Pineapple Express” episodes. This oscillation could greatly enhance the La Nina forecast with additional precipitation.
Additionally, Arctic Sea ice remains below average where it should be this time of year, which could also modify La Nina conditions.
“What this translates to is that the Arctic hasn’t recovered, and we can expect, at times, a wavy and erratic jet stream to occur for winter and spring,” said Borgioli. “This throws another variable into the picture in terms of precipitation events and temperature swings. So we’ll have to closely monitor the jet stream and phases of the global weather oscillations for West Coast impacts.”
California is not exempt from cold air outbreaks, and, in fact, a wavy and erratic jet stream can actually increase the likelihood of Central Valley cold air occurrences. Borgioli noted it will be important to identify possible arctic or modified arctic air moving into the state this winter and spring, which could harm agriculture.