NOAA's chief surf forecaster in Hawaii is Pat Caldwell. (NOAA – the National Oceanic & Atmospheric Administration). NOAA's in-depth forecasting keeps us all informed of approaching big-wave episodes, major weather/wave developments and related safety issues.

Exclusively for bigwaveshawaii.com, Pat explains the North Shore's exactly why the North Shore is home to the big-wave phenomenon. You can find Pat's daily forecast at: www.prh.noaa.gov/hnl/pages/SRF.php

*NOAA - National Oceanic & Atmospheric Administration

Why is O‘ahus North Shore the world's big wave magnet?

How big do the waves get?

Why does it get so big?

Why do surfers choose the North Shore?

The famous surf in Hawaii occurs due to various favorable environmental conditions, from the geographic position in the middle of the vast Pacific Ocean just on the fair weather side of the main storm track, to the ideal seabed shape for making the perfect peak, to the wave grooming, consistent offshore trade winds, to the small tidal ranges, and finally to the comfortable, “hang loose” tropical weather. It's ripe for picking October through April.

No one doubts that Hawaii is surfer's “Mecca” for the reasons explained below. Surf climatology for the north shore shows the average heights during peak season (November-February) around 16 feet (5 m) top to bottom on the shoreward side of the wave at the highest point of cresting.  But during peak season, there are on average about 20 days with enormous heights of 36-50 feet face (11-15 m) at the highest peaks on the outer reefs, while Waimea Bay holds roughly 25-35 feet (8-11 m) on the face.
 

Growing waves need pushing winds from a common direction over an area of open ocean, called a fetch, for a sufficient amount of time, or duration. To make extreme surf, the four most important criteria are wind speed, fetch size, duration, and proximity to the target, ie., Hawaii. 

For creation of giant surf, winds thrashing the ocean surface must be nearly hurricane force, or 50-70 knots, for several days over a long area the distance of California eastward to Colorado, or over 1000 nm. Winds spiral inward toward the center of a storm's low pressure center.  A given bird's eye (or satellite) snapshot of surface winds shows a circular nature of winds around the center of low pressure.  Only a portion of those winds are aimed at a forecast target, such as Hawaii.  At any given time, the fetch length aimed at Hawaii is typically too short to produce giant surf, even if the speeds are mega strong.  However, there is a special condition that makes giant surf possible.

The growing seas and swell under the storm moving toward Hawaii have a speed of about 20-25 knots.  If the storm tracks at similar speeds in a direction toward Hawaii, which is common as storms leave the active generation zone off Japan in winter and steer eastward, then the same growing, eastward moving swells continually receive wind energy.  This is called a "captured" or "trapped" fetch, and allows the extraordinary wave heights to develop.

As swells leave the storm, the wave heights rapidly decrease, resulting in a drop in half the size over the first 1000 nm of travel.  Thus for giant surf to arrive in Hawaii, the storm track needs to be close to the islands.  Ideally the storm's fetch reaches to within about 300 to 500 nm before veering away from the islands, bringing in the giant surf, but keeping the local weather fair, giving ideal surf conditions.

If the storms get too close, the surf becomes "victory at sea", with overlapping waves of differing wave direction and periods arriving simultaneously, making for unfavorable surf.  At peak winter in January into early February, about 75% of the episodes either have light winds, trades, or southerlies making for ideal surf, with the other 25% arriving with strong westerlies to northerlies, creating havoc.

Avid surfers like large waves.  Hawaii is close to the northern Pacific winter storm track leading to an abundance of high surf from October through April. Hawaii is a surfer's paradise for more reasons than just the size of the waves.  Other factors are critical too, such as wave shape, accessibility, number of surfing spots, local weather, and tides.

The shape of the wave determines the quality of the ride. The surfer desires a wave with a wide open shoulder, or blue water, where he/she can race toward safety from the steep section of the breaker where the white water hurls shoreward, better known as a "curl" or "tube".

What makes the shape of the waves so desirable in Hawaii?  Scientists call it geomorphology-- no I'm not talking with a mouthful of peanut butter, it's just a fancy word for the shape of the coastal sea floor.

The first important ocean bottom characteristic in Hawaii is the short distance from deep ocean to the breaking zones on the reefs and beaches.  The narrow coastal sea floor shelf and steep slope from deep to shallow water result in a minimal loss of wave energy prior to breaking.  This leads to "island juice"-- wave power in surfer lingo.

The second aspect of the seabed in Hawaii is the complex arrangement of underwater hills and valleys adjacent to shore. These were created by ancient lava flows, streams, erosion and extinct and growing coral reefs. Waves entering the coastal zone bend toward the shallowest areas. Scientists call it refraction.  This occurs because the depth determines wave speed in the shallow waters.

Consider a long wall of water moving shoreward toward a reef with deep water on either side.  At the shallowest location on the reef, the wave slows down.  But the wall on either side in the deep water moves ahead.  A bird's eye view would show a seaward bowed pattern of the originally straight wall centered on the “hill” or shallowest area.

At some moment in time, the wave face in the shallowest area becomes steep and unstable, causing a crest to form, then hurling forward as white water, or referred to by surfers as a “pitching lip.” The surfer begins his/her ride just seconds before the crest throws shoreward.  The surfer angles to the left or the right, depending on which side has the most desirable form and a safe exit, away from the turbulent white water of the shallower waters, toward the blue wave face or "shoulder" in the deeper waters on the edge of the reef. 

This second aspect of sea floor shape also determines the accessibility. One can safely and easily approach the take off zone by paddling in the deep, blue waters, called the channel, on one or both sides of a sea floor hill or reef, where the turbulent white water occurs.

In contrast, long sandy beach breaks found around the world, and well known up and down the east coast of the USA, have a seabed shape that is consistent in depth with distance from the beach, that is, the depth contours run parallel to shore. In these locations, there are no channels to access the surf, making it a wearisome task to punch through the breakers and reach the outside takeoff zone.

In Hawaii, with such a complex pattern of "hills" and "valleys" in the nearshore area, a large number of surf spots result.  There are hundreds of excellent places to ride waves on the north shore, with each having a favorable incoming wave direction, height, and period, or time between successive waves, that make that place "turn on and go off", surfer terms for optimal surf.

Several other geographical aspects make the surf so exceptional in Hawaii. Waves typically are generated by storms that track from Japan eastward toward California or Alaska.  This results in the most common wave direction during Fall through Spring from the northwest. The coast from Haleiwa to Kahuku on the north shore faces directly into the most frequent swell episodes, allowing greater focusing of the swell energy on the reefs and thus the highest surf heights possible for the given wave pattern.

Also, since the coast faces northwest, and the prevailing trades are from the east, the local winds blow "offshore" or against the waves, resulting in the most desirable wave form for surfers.

Tidal ranges in Hawaii are small, about 2 feet (0.6 m) during spring tides. With the small tidal variation, the surf is more consistent throughout the day. Many other big wave venues around the world are at locations with extreme tidal ranges, making the window of opportunity for the best waves even narrower.

Since the storm track is usually north of 25N latitude, and Hawaii is at 20N latitude, Hawaii typically remains in fair weather when the waves arrive, as opposed to higher latitude locations like Oregon that mostly get their biggest waves along with stormy gales, making junk surf.  The tropical latitude of Hawaii makes for the balmy, warm weather year round, ideal for “hanging loose” on the beach, and “cutting loose” in the surf.