Extreme North Shore Surf - December 15, 2004

Summary:

A large low pressure complex developed in the northwest Pacific, off the coast of Russia, beginning late on December 11. Pressure in this storm system fell to 964 millibars (28.47 inches) and produced winds approaching hurricane strength (65 knots or 74 mph) in a small area on the south side of the main low (satellite and pressure analysis). Meanwhile, a large area of 40 to 50 knots (45 to 60 mph) winds blew from the northwest over an area almost 1500 miles in length. Such strong winds over such a large area, called a fetch, produced wave heights over 40 feet. The energy in these waves then moved southeast, away from the strong winds and toward Hawaii, becoming what is referred to as a swell. This image (streamlines and great circle) highlights the "fetch" area (red box) pointed toward Hawaii. Since the earth is round, swells appear to move in a curved route when looking at a "flat" map. These routes are referred to as "Great Circles". To determine whether or not a swell will impact Hawaii, we look for wind directions (yellow streamlines in the image) blowing parallel to a great circle path. This was exactly the situation that set up on December 13.

As a swell propagates, the energy within it dissipates resulting in progressively smaller swell wave heights. Over the course of the 500 to 800 miles the swell traveled before reaching Hawaii, approximately 50% of the energy was dissipated, meaning the swell had diminished to a little over 20 feet. This is still very significant with respect to actual surf heights along the shoreline and reefs. As the swell energy approaches shore and moves into shallower water, the wave begins to slow down, however wave period stays constant. This results in the wave becoming narrower and taller, a process is called "wave shoaling". Think of it like a chain-reaction wreck. The first car stops, but the cars behind it keep coming. The net effect is to increase the height of the breaking wave, or surf, over what was the original open ocean swell height. There are a lot of other factors (such as underwater slope, swell direction, height and period) at play that determine the exact amount of wave shoaling at a particular location. The general rule of thumb in Hawaii is that height of breaking surf is about double the swell height.

The swell began reaching Buoy 1 about 200 miles northwest of Kauai during the day on December 14. The swell height peaked at 26 feet just before midnight HST (10 UTC) December 15 and then began to fall (buoy chart). Travel time to the islands from the buoy for such a swell are roughly: 5 hours to Kauai, 8 to Oahu, 11 to Maui and 14 to the Big Island. Thus the highest surf occurred on Kauai before daybreak, around sunrise on Oahu and late morning and afternoon on Maui and the Big Island. The darkness made it impossible to get observations from Kauai. Of course as the swell moved down the island chain, it slowly dissipated. The buoy offshore of Waimea Bay, Oahu reported a peak swell height of about 19 feet.

 

Surf Reports:

The following are a few reported surf heights from across the state:

Picture of a 30-35 foot wave at Waimea Bay taken by NWS Meteorologist Bob Ballard

 

Damage Reports:

Some overwash on roads on the north shore of Kauai was reported, but no roads were closed. Similar overwash occurred on Oahu around Haleiwa and on the west side around Makaha. This resulted in road closures for several hours, which in turn resulted in a major traffic jam due to all the people trying to view the large waves. No significant damage has been reported from other islands.

On a side note, the last major surf event nearing this magnitude occurred back in January 1998. In that event, the swell height at Buoy 1 reached 27 feet (just 1 foot higher than this event) and surf heights of up to 50 feet occurred on Kauai and Oahu. The major difference was the longevity of that event, nearly 24 hours. This allowed major beach erosion to take place resulting in significant damage to roads and some homes along the shore.

 

National Weather Service Statements/Warnings:

Numerical models were in good agreement before the storm even developed and ultimately modeled the storms development and movement well. Therefore forecaster confidence was high, allowing our office to spread the word early on about the event. A number of statements were issued concerning the surf event. Following are a sample of products initially mentioning the surf potential: