Big Waves in Hawaii. Why?

 There is a lot of excitement in Hawaii today, as large waves from the northwest reach the north coast of Oahu, making possible the Eddie Aikau Big Wave Invitational, a surfing competition only open to the most elite surfers.    This competition REQUIRES waves reach  at least 40 ft in height at the venue:  Waimea Bay.

NOAA has a buoy just offshore and so far it has only reported a significant wave height of around 6 meters (20 ft).   But the highest waves can substantially exceed the significant wave height and waves can grow as they feel bottom close to the shore. (The significant wave height is the average of the highest third of the waves).

NOAA runs a computer forecast model that predicts significant wave height and direction, called Wave Watch 3.   Here is its forecast for 10 AM.   The arrows show the wave direction (from the northwest), with significant wave heights of 15-20 ft.

What was the source of these waves?

Well, to get big waves you need three things (in addition to water!).  Strong winds, a long fetch for the winds to act on the water, and substantial duration of the wind interacting with the water surface.

To get perfect big waves on the NW Oahu coast generally requires a strong low pressure center to move west to east north of the islands.     

And you need the low to follow the right path.   Too far away and the waves have a chance to decline before reaching Seattle.  To near the islands and conditions are too stormy and unpleasant at the venue.   

To put it another way, you need the storm in on goldilocks track.

In this case, a low pressure center at 10 PM last night was in the perfect position northwest of the islands and was usually strong (indicated by the shading, which shows the difference of the pressure from normal).  Goldilocks indeed.

Now, it case some of my Northwest readers are disappointed regarding the big wave action reaching the Hawaiian Islands, I have some good news.   Some very big waves should reach our coast on tomorrow (Monday, see below).  Surfing anyone?