Water

El Niño is doing its job nicely — up north

total-accum-moisturejan172016.jpgA lot of moisture is expected in Northern California in the coming week.

Los Angeles so far is losing out in the delivery of Pacific storms that can be attributed to the El Niño effect, but California as a whole is doing pretty well on the water side. A new assessment of the season's rain and snowfall by the Daniel Swain and the California Weather Blog, which examined a plethora of stats and forecasts, says all in all the state is looking a lot like it did in previous milestone El Niño years.

Several major North Coast rivers are currently approaching or exceeding flood stage, and sections of the Sacramento River may exceed flood stage as well over the next few days (in any case, the weirs will certainly be overflowing). Meanwhile, snow accumulations in the Sierra Nevada have been equally impressive, and are the best in years (110% of average to date, a number I expect to rise further this week). Right now, all of this water in Northern California is very good news. Enough precipitation is falling to generate significant runoff without greatly exceeding the capacity of river systems to manage the influx (most flooding to date has been relatively minor in NorCal). Conditions have been cool enough for this precipitation to fall as snow at the higher elevations, meaning that California’s critically important “snow reservoir” is starting to fill. There’s still a lot of empty space in Northern California’s man-made reservoirs and natural groundwater aquifers, but at the moment things are moving in the right direction from a drought relief perspective.

Los Angeles County, says the blog, has been among the driest spots in the state so far this rain season. But there is still time, historically speaking, to catch up and some positive indicators for us.

Nearly all of California has fared better than Los Angeles County so far this season–and prospects for significant Southern California precipitation remain high in the coming weeks. It’s important to remember than California’s rainy season is relatively short and sharp (particularly in the south), and that the vast majority of precipitation falls over the course of a handful of heavy precipitation events (like, for example, the early January storms). Even during California’s wettest winters, most winter days are dry in Southern California–and multi-week dry spells occur virtually every winter. It’s also true that many of Southern California’s wettest seasons were primarily the result of only 1-2 very wet months. Thus, there is plenty of time for Southern California to catch up–and exceed–average precipitation this winter.

And, helpfully, the blog also reminds readers how it is that El Niño works anyway.

I’ve discussed on numerous occasions that the main mechanism by which El Niño affects California winter precipitation is by enhancing the subtropical jet stream over the far Eastern Pacific. As the jet strengthens and shifts southward, it effectively “straightens” out in the vicinity of California–reducing the amplitude of its typical northward bend that can often cause storms to veer northward into the Pacific Northwest or weaken before reaching California. This past autumn, an enhanced Pacific jet stream did indeed develop, but it was located well to the north of the Golden State (and brought record-breaking rains & floods to Seattle and Portland). Since the fall, though, this El Niño-strengthened jet has shifted gradually southward. The enhanced Pacific storm track is now consistently soaking Northern California, and bringing more transient soakings to Southern California. For reasons relating to the intrinsic seasonal cycle of the Pacific jet stream, I would expect unusually wet conditions to continue to expand southward along the California coast in the coming weeks.

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