Time for some weather geeking

science-of-atmos-rivers.jpgClick to enlarge. More on atmospheric rivers.

The Pacific has lined up some serious weather for California to start off 2017. It's no more than we've experienced in previous wet years, and below the really historic years, but it feels like it has been a long time since the weather aficionados were geeking out about atmospheric rivers and the like.

What's going on is a line of storms that began by dropping very large amounts of snow on the upper Sierra Nevada. The next storm to roll over Central and Northern California is expected to bring a lot of rainfall and be relatively warm, which could melt a lot of the snow and lead to early winter flooding. Yosemite National Park, for instance, is making preparations to close this weekend in anticipation of the Merced River flooding across Yosemite Valley.

The meteorologists are watching closely to see how much of the storms drift over Southern California. But the main events, for impact on the drought and California water storage, will be in the north.

From UCLA climate scientist Daniel Swain at his California Weather Blog:

Much of the Sierra Nevada above about 6000 feet in elevation is in the process of being buried by many inches of snow–and heavy snow rates will likely continue periodically for the next several days. By late Thursday, many places will likely be reporting totals in feet rather than inches. Due to the cold airmass still in place across NorCal, some spots as low as 2500 feet will experience unusually large accumulations before warmer air moves in. These recent, large snow accumulations have already led to a high risk of avalanches in steeper terrain–and may add to the already considerable flood risk developing for the coming weekend.

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.

This system will also be drastically warmer than its predecessor, and while snow levels could actually start out unusually low (locally in the 2,000-2,500 foot range), they will skyrocket as very strong warm advection occurs Saturday into Sunday.... Much of this snowpack could be erased by 24 hours of warm rain later this weekend.

Swain makes updates on Twitter as the storms arrive and do their thing, sometimes defying the forecasts.

Some other sources:

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