We've covered enough of the ridiculously resilient ridge of high pressure that nudged storms away from California, so we'll also cover this. "For the first time in 14 months, some truly substantial precipitation is headed for parts of NorCal," reports the California Weather Blog. "Current satellite imagery depicts the early stages of what promises to be a significant precipitation event over the weekend."
Atmospheric rivers” are long and narrow bands of highly-concentrated water vapor that occur in association with extratropical cylones (i.e. winter storms) throughout the Earth’s middle latitudes, especially over and near ocean basins. These features are often associated with extreme precipitation and flooding, especially along the mountainous western coast of North America (including California). To put the magnitude of these events in context: atmospheric rivers are (in aggregate) responsible for over 90% of global atmospheric water transport between subtropical and mid-latitude regions, and a single strong event is capable of moving an amount of water equivalent to 15 times the flow rate of the Mississippi River!
Those familiar with California weather and climate can attest to the critical role played by atmospheric rivers in driving wintertime precipitation extremes. I’ve noted in the past that California is dependent on just a handful of significant storm events for the majority of its annual precipitation, and this handful is usually associated with atmospheric rivers. California’s great floods in the historical past were almost all associated with atmospheric rivers, including the spectacular event of 1861-1862–which filled much of the Central Valley with floodwaters and inundated the fledgling state capital in Sacramento. Interestingly, scientists recently highlighted the important role atmospheric rivers have played in breaking many of California’s historical droughts.
One subset of atmospheric rivers affecting California are colloquially referred to as “Pineapple Express” storms, so named for their propensity to draw moisture northward from the Pacific Ocean in the vicinity of the Hawaiian Islands. Not all atmospheric rivers have such a trajectory, though most have some degree of connection to the subtropical atmospheric moisture reservoir. This weekend’s storm does actually appear to have a deep subtropical moisture tap clearly extending southward to Hawaii, so the upcoming atmospheric river does indeed meet the common definition of a “Pineapple Express.”
Some Northern California coastal mountains and the foothills of the Northern Sierra could get 6-7 inches of rain, the blog says. "Southern California, unfortunately, will probably receive only very light rainfall from this event. As is often the case with subtropical moisture plumes/atmospheric rivers, snow levels will be very high for most of this event–well above 8000 feet in most cases."
By the way, the ridiculously resilient ridge began to fade earlier this week. Also from the California Weather Blog: "The RRR has weakened considerably over the past week and is not currently the formidable obstacle it was in January, seemingly impervious to the effects of Central Pacific storm systems and incursions of the Pacific jet."