Daniel Swain: Series of atmospheric river storms to bring risk of significant CA flooding

January 4, 2017 – A series of extremely moist Pacific storm systems will take aim at California this week. The first of these is already bringing increasingly heavy rain (and mountain snow) along with gusty winds to much of northern California. This initial system probably won’t cause too many problems outside of some notable travel headaches over mountain passes and perhaps some minor local flooding. But this first round of precipitation will set the stage for bigger problems later this week into next week by saturating the soils north of about Santa Barbara and by depositing multiple feet of fresh snow to the Sierra Nevada, even at more moderate elevations.

The second storm is (by far) the one of greatest concern, as it will take the form a moisture-laden and slow-moving atmospheric river. While the details with this second system are still somewhat uncertain, virtually all numerical forecast models are painting a very broad area of extremely high precipitation totals over the next 6-7 days across the entire Sierra Nevada mountain chain and also in the coastal mountains from the Oregon border south to Monterey County. It’s still too early to say exactly how much precipitation will fall, but the potential is there for some very impressive numbers–perhaps greater than 20-25 inches along favored western slope regions of the Sierra Nevada and greater than 15 inches in the coastal mountains. Even in low elevation urban areas near the Bay Area and Sacramento regions, 7-day totals exceeding 5-7 inches are entirely possible.

Since this system is expected to be slow moving, the associated atmospheric river may stall over some portion of northern or central California on Sunday or Monday–or even waver back northward temporarily. If and when this occurs (as has been suggested by recent runs of both the ECMWF and GFS), there may be a 100-200 mile wide band of even higher precipitation totals. It’s impossible to say at this time where any stalling or frontal waves might occur, but that has the potential to be a serious situation locally.

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