The figure above (and again a bit below to avoid you scrolling constantly) demonstrates how it all happened. In short, multiple landfalling atmospheric rivers (ARs) brought substantial precipitation to the Sierra Nevada throughout much of January. There were some scary times for the snowpack and regarding flooding on either side of the Sierra (notably January 7-8 and the time leading up to that event when we knew we were in for a big one but the outcome was uncertain). The one-two punch of a colder storm (AR #1) followed by a warmer AR #2 (note high snow levels around 8500-9500 ft) is a 3 Michelin Star recipe for rain-on-snow induced flooding. Following these storms, we got extremely lucky with the consistently cooler temperatures and lower snow levels that produced copious snowfall and low elevations during ARs #3-5.
Let's break this down a bit. An excellent metric that has been developed to identify atmospheric rivers, which are long (2000 km), narrow (typically less than 1000 km), corridors of enhanced precipitable water (blue dots; pwater or pdub for you hip hop fans) and water vapor transport (wind times precipitable water). They (ARs) exist in the warm sector of cyclones, hence their often high snow levels. Note how the black dots below tend to be higher during AR conditions (green boxes, when observed pwater exceeds 20 mm along the California Coast). ARs are a form of a low level jet (strong winds), which when these strong winds slam into terrain, produce incredible precipitation totals over multi-day periods. In fact, California sees some of the largest 3-day total precipitation magnitudes in the United States, on par with hurricane precipitation totals in the southeastern U.S.
Next, note how significant precipitation (blue bars) was observed during these events along the western slope of the Sierra (Blue Canyon, one of the wetter spots in the northern central Sierra Nevada). The peaks and valleys of the precipitation (bars) line up beautifully with the peaks in pwater (blue dots).

The slow decline in snow depth during non-precipitating conditions is likely due to settling of snow.
Had I plotted snow water equivalent, or the amount of water stored in the snowpack, the line would just launch upwards in all cases. This concept is where we got lucky again (at least I think so). AR #1, though somewhat of an upside down storm (snow levels rose through the first part, leading to more dense snow being deposited atop less dense snow), gave us a nice snowy sponge to absorb much of the rainfall that came during the flood event (AR #2). This massively boosted the water stored in the snowpack (very good for water resources and the destruction of certain sneaky persistent weak layers). Had we not had this pre-storm boost, who knows what the outcome may have been!
A discussion on these flood/mega snow events can be found here: http://www.powdiction.com/2017/01/throwback-to-1997-or-just-2005.html
The subsequent storms entrained much more cold air and had lower snow levels, especially during ARs #4-5, which was the icing on the already triple layer cake as we went from 'some snow down low' to 'oh yea, the 5000-9000(!) foot crest-to-valley lines go!'
To make things even more awesome, when it wasn't snowing, it was cold and/or cloudy (sorry, I could have plotted solar radiation but I got lazy at the end), and we avoided the wicked wind of the east, with a six hour exception. This favored extremely good snow conditions from summits to the lowest elevations, and created much rejoicing and probably got more than a few people (myself included) a bit behind in work.
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Dr. Hatchett researching snow transport. |
Rad post Dr. Ratchet.
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