Jan 21 2014Jan 22 2013
Weather conditions over the past two months have varied wildly throughout California. Extremely heavy precipitation and unusually warm conditions in early December in the northern/central coastal regions gave way to very cold and mostly dry conditions around the new year (though not dry enough to preclude some remarkable low elevation snowfall in the lower hills of Southern California).
Photograph taken along Tioga Pass Road on January 12, 2015 illustrating the remarkable lack of snow in the High Sierra this winter. Photo courtesy of Bartshé Miller.
Early January continued to bring a mixed bag of conditions–Central California has remained completely dry since the subtropical tap in early December shut off, while parts of SoCal saw some unexpectedly heavy precipitation courtesy of a rather unusual cut-off low pressure area that moved in from the southwest. The Sierra Nevada, for the most part, has been dry over the past month–and in many places, quite warm (with daytime highs and even some overnight lows remaining well above freezing). Meanwhile, far northern California has experienced some significant precipitation over the past couple of days. When taken together, all of these varied conditions paint a complex meteorological picture–but on a statewide basis, conditions have trended once again toward much drier and much warmer than average over the past 30+ days.
A highly-amplified atmospheric pattern over North America–once again
What has been the cause of all this California weather volatility and the recent trend towards warmer and drier conditions (despite the fact that January is historically California’s wettest month?). A high-amplitude atmospheric flow pattern has once again developed over the Eastern Pacific and North America, deflecting the Pacific storm track north of its typical cool-season position along the West Coast and allowing repeated intrusions of extremely cold Arctic air to invade the American Midwest and Eastern Seaboard. This unusual atmospheric configuration has occurred with remarkable frequency and intensity over the past several winters, and has been a major contributor to California’s ongoing extreme drought. While December’s heavy coastal precipitation–associated with a strong zonal Pacific jet–brought a substantial reprieve from this recurring high amplitude flow pattern for a brief period of time, recent observations (and, unfortunately, forecasts for the next couple of weeks) suggest that this persistent pattern has returned in the new year.
Is this the Ridiculously Resilient Ridge, Redux?
I’ve received a lot of questions lately regarding the possible re-appearance of California’s now-infamous persistent atmospheric pattern. As many have pointed out, California is no stranger to mid-winter dry spells lasting multiple consecutive weeks. In fact, these break periods–which often follow a particularly active period of storminess (like the one we received back in early December)–are a characteristic feature of cool-season climate in our part of the world. Dry spells lasting longer than 4 weeks, though, are very unusual, and the mid-winter break across much of California appears destined to last at least that long (and in the Bay Area and Sacramento regions, has already exceeded that duration). The Ridiculously Resilient Ridge (as I defined it back in 2013) is a persistent region of unusually high atmospheric pressure in the middle levels of the atmosphere centered over the far northeastern Pacific Ocean. It’s a feature that existed in the long-term average geopotential height field (over many consecutive months), and does not refer specifically to the extraordinarily intense high pressure system that was in place for a 6-week period during Dec-Jan 2013-2014….. As a result, there’s an excellent chance that January 2015 could go down in the record books as the driest on record across a wide swath of California–especially near the Bay Area, where a number of stations have a respectable shot at recording 0.00 inches for the entire calendar month….