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USE BUOYS TO PREDICT EXACT SWELL ARRIVAL TIME
OF NORTH PACIFIC SWELLS

(For Central & North California ONLY)

These tables help you to establish the swell arrival time on Northern California beaches (relative to San Francisco) providing you know roughly what the swell direction is.   Once the swell begins to hit one of the outer buoys listed below (like 46003), write down the time and period of the first reading.  Then select the appropriate table based on the estimate swell direction, browse over to the period column, select the period of the swell that hit buoy 46003, and browse over to the travel time column that corresponds to that period reading and write that value down.  For example, if a 20 second period swell hit 46003 at 10 AM on Monday from 270 degrees, then that swell should hit San Francisco 51.3 hours later, or at 1 PM Wednesday.

Swell Angle: 270 Degrees

Hit Order

Buoy Number

Distance from Pillar Point (nmiles)*

Period
(sec)

Travel Time
(Hrs)**

Buoy Distance from Pillar Point Travel Line ***

1

46003

1600

@13

78.9

850 nmiles N of focus

     

@14

73.3

 
     

@17

60.4

 
     

@20

51.3

 
     

@25

41

 
           

2

46006

750

@13

37

300 nmiles N of focus

     

@14

34.4

 
     

@17

28.3

 
     

@20

24

 
     

@25

19.2

 
           

3

46036

650

@13

32

600 nmiles N of focus

     

@14

29.8

 
     

@17

24.5

 
     

@20

20.8

 
     

@25

16.7

 
           

4

46002

450

@13

22.2

450 nmiles N of focus

     

@14

20.6

 
     

@17

17

 
     

@20

14.4

 
     

@25

11.5

 
           

5

46059

380

@13

18.7

150 nmiles N of focus

     

@14

17.4

 
     

@17

14.3

 
     

@20

12.2

 
     

@25

9.7

 
           

6

46014

71

@13

3.5

Pt Arena

     

@14

3.3

 
     

@17

2.7

 
     

@20

2.3

 
     

@25

1.8

 

 

Swell Angle: 290 Degrees

Hit Order

Buoy Number

Distance from Pillar Point (nmiles)*

Period
(sec)

Travel Time
(Hrs)**

Buoy Distance from Pillar Point Travel Line ***

1

46003

1700

@13

83.7

200 nmiles N/E of focus

     

@14

78

 
     

@17

64.1

 
     

@20

54.5

 
     

@25

43.6

 
           

2

46036

850

@13

41.9

350 nmiles N/E of focus

     

@14

39

 
     

@17

32.1

 
     

@20

27.2

 
     

@25

21.8

 
           

3

46006

730

@13

36

240 nmiles S/W of focus

     

@14

33.5

 
     

@17

27.5

 
     

@20

23.4

 
     

@25

18.7

 
           

4

46005

700

@13

34.5

290 nmiles N/E of focus

     

@14

32.1

 
     

@17

26.4

 
     

@20

22.4

 
     

@25

18

 
           

5

46002

525

@13

25.9

0 nmiles off focus

     

@14

24.1

 
     

@17

19.8

 
     

@20

16.8

 
     

@25

13.5

 
           

6

46059

360

@13

17.7

134 nmiles S/W of focus

     

@14

16.5

 
     

@17

13.6

 
     

@20

11.5

 
     

@25

9.2

 
           

7

46014

102

@13

5.0

75 nmiles N/E of focus

     

@14

4.7

 
     

@17

3.8

 
     

@20

3.3

 
     

@25

2.6

 

 

Swell Angle: 315 Degrees

Hit Order

Buoy Number

Distance from Pillar Point (nmiles)*

Period
(sec)

Travel Time
(Hrs)**

Buoy Distance from Pillar Point Travel Line ***

1

46036

900

@13

44.3

0 nmiles off focus

     

@14

41.3

 
     

@17

34

 
     

@20

28.9

 
     

@25

23.1

 
           

2

46005

680

@13

33.5

75 nmiles N/E of focus

     

@14

31.2

 
     

@17

25.7

 
     

@20

21.8

 
     

@25

17.4

 
           

3

46006

610

@13

30

468 nmiles S/W of focus

     

@14

28

 
     

@17

23

 
     

@20

19.6

 
     

@25

15.6

 
           

4

46002

500

@13

24.6

150 nmiles S/W of focus

     

@14

23

 
     

@17

18.9

 
     

@20

16

 
     

@25

12.8

 
           

5

46059

310

@13

15.3

242 nmiles S/W of focus

     

@14

14.2

 
     

@17

11.7

 
     

@20

9.9

 
     

@25

7.9

 
           

6

46014

125

@13

6.2

 
     

@14

5.7

 
     

@17

4.7

 
     

@20

4

 
     

@25

3.2

 

* - Distance corrected for swell angle
** - Travel time is based on distance (corrected for swell angle). For example: If a 20 second period swell hits 46006 from 270 degrees, it will take 29 hours for that swell to reach Pillar Point.
*** - This figure provides some indication as to how close the buoy is located to that part of the swell which will eventually strike Pillar Point.

 

APPENDIX A
Swell Profile and Scaling Factors

It is possible to use 'swell measurements' obtained from offshore buoys and convert these measurements to estimate what the swell size will be when it arrives at the San Francisco buoy (46026).

The procedure is:

  • Gather necessary meteorological data to ascertain the swell is in-fact traveling towards your locale
  • Make multiple readings over time at the selected buoy and develop a swell profile (for that buoy). A swell profile is compiled as in the example below (identify the buoy number, date and time the swell was observed, and the actual swell height and period for that time).
  • For each reading in the profile, estimate the swell arrival time using tables above.
  • Estimate the ‘arriving size’ for each data point in the swell profile. This is accomplished by applying the appropriate ‘scaling factor’ (see table below) for the applicable buoy. Simply multiple the actual swell reading by the appropriate scaling factor below:

Example Swell Profile Table

Buoy Number

Date/Time

Comments

Swell Reading

Estimated Arrival Date/Time

Estimated Arrival Size

46006 11/11/97
10:00
Swell Hits 10 ft @ 20 secs    
  11/11/97
12:00
  12 ft @ 20 secs    
  11/11/97
14:00
Swell Maxs 14 ft @ 17 secs    
  11/11/97
16:00
  13 ft @ 14 secs    

 

Scaling Factor Table

OFFSHOREBUOY NUMBER

SWELL ANGLE
(degrees)

SCALING FACTOR
(for 46014, 46042, 46028, 46063)

SCALING FACTOR
(for 46012)

46003

270

.7

.6

 

290

.7

.6

 

315

NA

NA

46006

270

.8

.6

 

290+

.8

.56

 

315

.8

.56

46005

All*

.8

.6

46002

All*

.9

.7

46059

270

1.0

.8

 

290+

1.0

.75

 

315

1.0

.7

* - All swells between 270 and 315 degrees. North of 315 degrees is outside the window for all North CA buoys.

 

Swell Travel Time
This table provides quick-look timings for swells traveling in the NCal Swell window

Distance from North California

11 secs

13 secs

14 secs

17 secs

20 secs

25 secs

600

35

30

28

     
700

41

35

32

     
800

47

39

37

     
900

52

44

41

     
1000

58

49

46

38

   
1100

64

54

51

42

   
1200

70

59

55

45

   
1300  

64

60

49

   
1400  

70

64

53

45

 
1500  

74

69

57

48

 
1600  

79

73

60

51

 
1700  

84

78

64

55

 
1800  

89

83

68

58

46

1900    

87

72

61

49

2000    

92

76

64

51

2100    

96

79

67

54

2200    

101

83

71

56

2300    

106

87

74

59

2400    

110

91

77

62

2500      

94

80

64

2600      

98

83

67

2700      

102

87

69

2800      

106

90

72

2900      

109

93

74

3000      

113

96

77

 

 

2766

.
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