Monday, December 21, 2015

The Force Awakens: King Tides Projected for Christmas

Back in July, I presented projections of King Tide flooding in the San Francisco Bay area as a result of this year's El Niño. El Niño conditions in the Pacific have developed into one of the strongest events on record - rivaling the 1997-98 and 1982-83 winters. The figure below shows the evolution of this year's El Niño as measured by the sea surface temperature (SST) anomaly in the Niño 3.4 region - an area of the equatorial Pacific - compared to past events. Anything above 1.5 degrees is considered "strong" El Niño conditions.


Figure 1. Sea surface temperature anomaly in equatorial Pacific - December 16, 2015.


The changes in the ocean and atmosphere as a result of El Niño produce more southerly storm tracks, which bring increased precipitation and coastal storm events (storm surge and waves) to the California coast. The warm water associated with El Niño also exacerbates storm surge conditions through thermal expansion of ocean waters, which can further elevate high tides by 0.5 to 1.0 feet above normal. These factors combine to produce higher than normal tide levels along the California coast.

Here's a summary of my projections from July at the San Francisco tide station compared to historical strong El Niño events and observations so far this year. We have seen 13 days of King Tides so far this year in San Francisco (through December 20). This means we are nearly halfway to the projected total of approximately 30 - quite impressive!

Table 1. Projected number of King Tide flooding days at San Francisco, CA - Winter 2015/16

Strong El Niño
Projections
1982-83
Winter
1997-98
Winter
2015-16
Winter

Low
Mid
High
October
0
1
3
0
1
5
November
2
5
7
4
7
5
December
1
4
7
6
8
3*
January
1
5
8
9
11
-
February
4
8
13
6
6
-
March
3
8
13
6
0
-
Total
-
31
-
31
33
13*
Notes: Based on July projections. *Through December 20, 2015

With King Tides predicted for the week of Christmas, I wanted to revisit my projections and adjust them for the observed conditions so far this December. I also wanted to add a southern California location - La Jolla, in San Diego because this area is seeing significant coastal flooding impacts so far this year. The video below is from the Marine Room restaurant in La Jolla on November 25, 2015.



The average difference between observed and predicted tides ("residual") has been 0.59 ft at La Jolla and 0.44 ft at San Francisco during the month of December. I used this information to tune my projections for the coming week. The figures below provide the projections for La Jolla and San Francisco, CA for the month of December.

La Jolla King Tide Projections

The projected number of King Tide days for the month of December at La Jolla is 12, with a range of 9 to 15. The week of Christmas is projected to see 7 consecutive days with high tides exceeding 6.1 ft NAVD88 (approximate King Tide threshold). The red dates indicate days where greater than 50% of the simulations exceeded the King Tide threshold.

Figure 2. Timing and likelihood of King Tide Flooding at La Jolla tide station, December 2015.

The figure below shows the projected range of tide elevations. The red error bars indicate the elevations exceeded by 75% and 25% of simulations. The magenta "maximum" tide height indicates the elevation exceeded by 1% of simulations - a possible, but unlikely level. Based on these projections, La Jolla should see high tides of approximately 6.6 to 7.6 ft and as high as 7.8 ft if King Tides coincide with a storm event. The highest tides are expected Christmas Eve and Christmas Day.

Figure 3. Projected tide elevations at La Jolla tide station, December 2015.

Table 2. Projected timing and likelihood of King Tide flooding at La Jolla, December 2015.


Water Level Projection (ft NAVD88)

Day
Hour
Predicted
Lower
Projection
Upper
Max
Probability
(Tide > 6.1 ft)
20
5
5.2
5.4
5.8
6.3
6.6
15%
21
5
5.6
5.8
6.3
6.8
7.0
67%
22
6
6.0
6.2
6.7
7.2
7.4
99%
23
7
6.3
6.5
6.9
7.4
7.7
100%
24
8
6.4
6.6
7.0
7.6
7.8
100%
25
8
6.4
6.5
7.0
7.5
7.8
100%
26
9
6.1
6.3
6.8
7.3
7.5
99%
27
10
5.8
6.0
6.4
6.9
7.2
84%
28
10
5.3
5.5
5.9
6.4
6.7
23%


San Francisco King Tide Projections

The projected number of King Tide days for the month of December at San Francisco is 6, with a range of 3 to 9. The week of Christmas is projected to see 5 consecutive days with high tides exceeding 7.0 ft NAVD88 (approximate King Tide threshold). The red dates indicate days where greater than 50% of the simulations exceeded the King Tide threshold.

Figure 4. Timing and likelihood of King Tide Flooding at San Francisco tide station, December 2015.

The figure below shows the projected range of tide elevations. The red error bars indicate the elevations exceeded by 75% and 25% of simulations. The magenta "maximum" tide height indicates the elevation exceeded by 1% of simulations - a possible, but unlikely level. Based on these projections, San Francisco should see high tides of approximately 6.9 to 7.4 ft and as high as 8.4 ft if King Tides coincide with a storm event. The highest tides are expected December 23-24.

Figure 5. Projected tide elevations at San Francisco tide station, December 2015.


Table 3. Projected timing and likelihood of King Tide flooding at San Francisco, December 2015.
Water Level Projection (ft NAVD88)
Day
Hour
Predicted
Lower
Projection
Upper
Max
Probability
(Tide > 7 ft)
20
7
6.3
6.3
6.8
7.4
7.8
20%
21
8
6.6
6.6
7.0
7.6
8.0
49%
22
8
6.8
6.8
7.3
7.9
8.3
80%
23
9
6.9
6.9
7.4
8.0
8.4
91%
24
10
6.9
6.9
7.4
8.0
8.4
90%
25
11
6.8
6.8
7.3
7.9
8.3
88%
26
11
6.6
6.6
7.1
7.7
8.1
56%
27
12
6.3
6.3
6.8
7.4
7.7
22%

I plan on monitoring the tide stations this week and will report on the results! And remember, if you do visit the coast this week, please be careful and don't drive through standing water.






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