Bone dry in Southern California, and below average precip throughout CA
Why has California been so dry? (Regular blog visitors already know where I’m going with this.) Well, a remarkably persistent zone of atmospheric pressure has been present more often than not across the region for the past few months. ….More recently, the bigger & stronger West Coast ridge has pushed the Pacific storm track even further north. Remarkably, this powerful ridge has forced several very moist atmospheric river storms over the mid-Pacific to make a hard “left turn” over the open ocean–veering directly northward and bringing almost inconceivably heavy snowfall to the coastal mountains of southern Alaska.
Thomas Fire becomes largest wildfire in modern California history–in December
….While December wildfires are not unheard of in this part of the world, the extent and severity of the December 2017 fires in SoCal really is unprecedented in California history. The Thomas Fire–which has now burned nearly 275,000 acres and over a thousand structures in Ventura and Santa Barbara counties–yesterday became the single largest wildfire in modern California history. That this dubious milestone was reached in December, which is typically the midst of the California rainy season, is truly extraordinary. Indeed, recent months have brought not only near-record low precipitation, but also record-high temperatures across a wide swath of SoCal….
…..While long-term humidity records are hard to come by in most spots, all signs suggest that these were at or near record-low humidity values for many of these recording stations (and certainly for the time of year).
Is this (another) return of the Ridiculously Resilient Ridge?
…The not-so-good news: parts of Southern California that depend exclusively on local water supplies (such as much of Santa Barbara and Ventura counties) never really recovered from the last drought, and these regions remain quite susceptible to the impacts of drought re-intensification. And even further north, forested regions remain quite stressed as a result from the previous multi-year drought, and tree mortality in the Sierra Nevada remains far above historically observed levels.
Unfortunately, 2+ weeks of unusually dry conditions still probable….The “Warm West/Cool East” pattern discussed in the previous post is still quite prominent over North America. The present West Coast dryness is largely consistent with seasonal model predictions for this winter, and those same models presently suggest that the present pattern is likely to persist for much of the California rainy season.All of this is to say: it’s still too early to say whether we’re headed into a new drought, though there are some compelling signs that we may be (especially in Southern California). And even in a dry year, California can still experience big storms and very wet months. But at this point, it probably makes sense to start thinking about the possibility of yet another big swing in California–from drought, to flood, and then (perhaps) back again.