Saturday, January 7, 2017

Liveblogging the Deluge: Perspectives on the Biggest Flood in a Decade, Part One

See updates at end of post... BTW, if you are on the scene in places like Yosemite Valley, please send photos or updates! I will gladly post them here.
Printable PDF of current conditions can be found at this link 
It's clear by now that I am fascinated by the current atmospheric river storm that is blanketing the state of California with precipitation from one end of the state to the other. I'm right in the thick of it, in the Great Valley, sandwiched by mountain ranges that will be getting rain amounts measured not in inches, but in feet. On this blog, I've already talked about the last record-breaking storm, the 1997 event that caused unprecedented damage across California, but especially in Yosemite and along the Tuolumne River. I also blogged about the preparations being made in anticipation of the storm: the operators of Don Pedro Reservoir have ramped up the flow of the Tuolumne River to near flood stage to make room for storm runoff. Then, last evening, we had a spectacularly clear view of the Sierra Nevada from the valley floor, covered with snow, and seeming to wait for the coming storm event. From here on, I'll be liveblogging, sort of, the storm (it will be separate posts at times).
Radar of precipitation in Central California as of 8:00 PM Saturday. Source: Intellicast

And now the storm has arrived. Atmospheric river storms are linear streams of moisture-rich air coming northward out of the tropics (they have been called Pineapple Express storms at times). In "normal" front-related storms, the period of precipitation is fairly limited as the front sweeps through the state. Atmospheric river storms are different; they spray the state with storms that can last for days, and in some extreme examples, weeks. They are also warm storms. The snow level can reach very high elevations so that rain falls on the snowpack, melting vast amounts of ice. It all adds up to a potential for the worst kinds of floods in California.

I don't know what all is going to happen in the next few days, but I want to establish some baselines so it can be understood just what the storm means for California and our water situation. We have been in the grip of a crippling drought since 2011, and reservoirs across the state have been at historical, even catastrophic, low levels. The first diagram in this post provides a benchmark to track the changes in the next few days. It's already been a fairly good precipitation year, as some reservoirs already sit at 100% or more of their desired levels. A few are still very low, including especially New Melones Reservoir on the Stanislaus River just north of me. It is only at 27% of capacity. Lake McClure on the Merced River below Yosemite Valley is about where it is supposed to be at 47% of capacity. My special river, the Tuolumne which also flows out of Yosemite National Park, is a bit too close to full at 76% of normal. If the storm dumps too much rain in the park, it has a small chance of overflowing the floodgates in an uncontrolled manner as it did in 1997. That's the reason the dam operators ramped up flows in the last two days.
Source: California Nevada River Forecast Center

It's going to be especially interesting to see how the storm predictions pan out. The flood hydrograph above is the expected outcome in Yosemite Valley. The red dotted line is flood stage (at 10 feet, or around 7,000 cubic feet per second). Earlier predictions for the flood peak have wavered between 15,000 cfs and 24,000 cfs. The high end prediction would be of 1997 magnitude, but even the low prediction would be enough to flood valley roads and possibly close the park. The high flow history at Pohono Bridge in the valley is shown below:

(1) 23.43 ft on 01/03/1997
(2) 23.43 ft on 01/02/1997
(3) 21.52 ft on 12/23/1955
(4) 20.98 ft on 11/19/1950
(5) 20.10 ft on 12/11/1937
(6) 16.96 ft on 12/23/1964
(7) 13.11 ft on 04/11/1982
(8) 13.01 ft on 01/13/1980

The current outlook is for a peak of 21,590 cfs (19.7 feet).  If it happens, it would be the 6th highest flow ever recorded. As you might expect, I'll be watching this one closely.
Flood hydrograph for Dry Creek. Source:

The other local river that has my undivided attention is Dry Creek. It's not a well-known waterway outside our region, but it is capable of causing some havoc at times. Although it doesn't have a vast drainage basin (it arises in the Sierra Nevada foothills in the Mother Lode), it has no dams or flood control structures. It is therefore very responsive to variations in storm intensity. I've seen it flowing at 3,000 or 4,000 cfs, but the prediction as of this evening is a bit ominous: it may peak at 8,400 cubic feet per second. Let's be clear what that means: yesterday, the Tuolumne River was flowing at 8,000 cfs, and that's considered very close to flood stage. For the Tuolumne River. A minor tributary to the Tuolumne River may be as big as the Tuolumne, but in a channel that is many times smaller. I will be out and about on Monday to get pictures, but to set a baseline of sorts, here's how the creek normally looks at the Oakdale-Waterford Highway bridge:
Dry Creek on Dec. 17, 2016 just prior to a previous high water event late last year.

So, here is where we stand with the atmospheric river storm in my backyard. We've received about a half inch of rain today, adding towards an expected 4 inches by the end of the week. If things transpire that way, it will represent about a third of the precipitation expected in my town over the course of an entire normal year. We of course are in the driest part of Central California (aside from the desert east of the Sierra Nevada), so literally everywhere else will be getting a lot more. As I've been saying in my previous blogs, be safe out there! Don't travel if you can avoid it, don't try crossing flooded bridges, and don't try walking either. If you are headed into the mountains, imagine being stranded by yourself for a few days and pack accordingly, especially with warm clothing and extra food rations. My distant relatives, the Donners, failed to do that in the Sierra Nevada in 1847, and look how they ended up. If authorities issue a flood warning, take it seriously. They're getting their advice from scientific professionals, and that still means something.

UPDATE: Officials closed Yosemite Valley roads and visitor services as of yesterday, and will reopen after the flooding subsides.
UPDATE: (although more like an addendum) For comparison's sake, here is where the reservoirs of the state were at a year ago:

A huge difference!
UPDATE (1/8, 1:52 AM): Yes, up late, listening to intensified rainfall. Gauge has picked up another .40 inches in two hours.

UPDATE  (1/8, 10:18 AM): Good morning! There was another 0.20" in the backyard gauge this morning, and now we are in a lull between major storms (most of the action is north of here at the moment). Another 2/3 inch is expected later today on valley floor. There is better news in Yosemite; the expected flood is now projected to be less than catastrophic, with a projected high flow of 9,600 cubic feet per second. Yesterday, the projection was twice that. It's still above flood level by 2 feet, but would not cover valley roads. Several more inches of warm rain is expected (~4"), but tomorrow the precipitation will turn to snow, and that is a good development.

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