Monday, October 20, 2014

Why Bermuda and Hawaii Observe High Waves but Relatively Low Storm Surges

Hurricane Gonzalo slammed Bermuda with damaging winds on Friday and Saturday.  The strong category-2 hurricane generated winds of 110 mph is it hit the island nation. At the L.F. Wade International Airport, sustained winds reached 93 mph, with gusts to 113 mph, as the southern eyewall hit (Masters 2014).

Gonzalo generated massive waves, which were forecast to reach as high as 30-40 ft (9.1 - 12.2 m). However, storm surge levels were relatively low for such a large, intense storm. As of Saturday, the highest storm surge observation available online was a 6-ft (1.83-m) storm tide that inundated portions of L.F. Wade International Airport (See Saturday’s blog post).

Hurricane Ana generated massive waves along south-facing shores of the Hawaiian Islands

Meanwhile, Hurricane Ana churned in the Pacific this weekend, passing just south of the Hawaiian Islands. The category-1 hurricane produced massive surf, forecast at 12-20 ft (3.88-6.10 m), which was highest along the south-facing shores of O’ahu and Kauai. However, maximum surge levels along south-facing Honolulu harbor remained less than one-half foot high.

Storm surge in south-facing Honolulu harbor remained below one-half foot. This NOAA Tides and Currents graph depicts predicted tide levels in blue and actual water levels in red. Storm surge is the difference between red and blue. 

Why did these two hurricanes generate massive waves, but relatively low storm surge/ storm tide? These coastal profile illustrations provide the answers.

Graphic created by Hal Needham

The image above is a diagram of coastal flooding on mid-oceanic volcanic islands. Although there are many differences between the geology of Bermuda and Hawaii, both island regions trace volcanic origins, as Bermuda sits on the rim of an ancient caldera, and Hawaii owes its origins to a plate moving over a hotspot, which created this island chain. Although the islands have many geological differences, both are isolated island regions, surrounded by relatively deep water.
As strong winds blow over the ocean and push water towards such islands, the energy transported across the ocean remains largely intact until coming close to shore, because the bathymetry, or offshore water depth is relatively deep. This means that waves do not dissipate much energy until they are close to shore, when they finally “feel the bottom” and break. Such coastal areas observe very high waves. However, the deep bathymetry also serves to suppress storm surge levels because underwater currents efficiently redistribute excess water.

This system can be contrasted to shallow river deltas, where offshore bathymetry is shallow (see graphic below). In these areas, hurricanes generate much higher storm surges, but lower waves. Also, the waves tend to start breaking farther from shore, serving to dissipate more wave energy in these areas. Surges are higher in these areas because the shallow bathymetry forms a "small container," so displaced water has nowhere to go but "up." The Mississippi and Louisiana coastline provides a good example of shallow water near a river delta that is vulnerable to hurricanes.

 Graphic created by Hal Needham

The bigger picture of these recent coastal flood events is that coastal profile, such as shape of coastline and bathymetry, greatly affects the height of waves and storm surges. This explains why locations like Bermuda and Hawaii typically observe high waves, but relatively low storm surges.

Masters, J., 2014: Gonzalo Brushes Newfoundland; Ana Drenching Hawaii. Dr. Jeff Masters Wunderblog. Available on the Web at:

Saturday, October 18, 2014

Massive Waves Slam O'ahu Tonight and Build on Kauai through Sunday

Hurricane Ana is generating massive waves that are slamming the south shores of the Hawaiian Islands tonight. The highest waves are impacting the south and east coasts of O'ahu, Kauai and Ni'ihau. The National Data Buoy Center's buoy at Barber's Point, just west of Ewa Beach, O'ahu, reported a significant wave height of 9.8 ft (3 m) and building at 4:56 PM HST.

The significant wave height is the height from trough to crest of the highest one-third of the waves. This means the highest one-third of the waves at Barber's Point are at least 9.8 ft (3 m) high.

The National Weather Service offshore forecast for waters south of O'ahu predicts wind waves of 10 ft (3.05 m) and a SE swell of 12 ft (3.88 m) tonight.

Hurricane Ana is generating enormous waves that are pounding the south shore of O'ahu and Kauai this evening.
Base Map: National Data Buoy Center, Graphics: Hal Needham

However, the NOAA Tides and Currents tide gauge at nearby Honolulu is reporting storm surge levels less than 0.5 ft (15 cm). This is not surprising, however, as hurricanes passing near Hawaii typically generated enormous waves but relatively low storm surges.

Storm surge levels at Honolulu remain less than 0.5 ft (15 cm) high this evening, despite enormous surf pounding O'ahu's south shore. Storm surge in this image is the difference between actual water levels (red line) and predicted tide levels (blue line).
Source: NOAA Tides and Currents

As Ana moves NW, the center of circulation will pass dangerously close to Kauai and Ni'ihau. Expect surf to build along the south and east shores of those islands tonight and Sunday. The National Weather Service offshore forecast south of Kauai predicts a 12 ft (3.88 m) south swell on Sunday.

According to a wave model used by Swell Info, significant wave heights along south and east shores of Kauai and Ni'ihau should exceed 12 ft (3.88 m) on Sunday morning.

Significant wave heights are forecast to build tonight and Sunday morning on south and east shores of Kauai and Ni'ihau. This forecast is for 0800 HST Sunday morning.
Source: Swell Info. See animated model at:

Stay safe everyone and send me pics or descriptions if possible!
~Hurricane Hal

Gonzalo's Storm Surge Inundation Reached at least 6 ft (~ 2m)

Hurricane Gonzalo's storm surge did not cause as much damage as feared, although the surge did damage boats and low-lying infrastructure. The highest storm tide inundation I could find occurred at L.F. Wade International Airport, where AWOS weather sensors were, "damaged due to saltwater inundation" (Stewart 2014). The elevation of the airport is 6 ft (~ 2 m), which means a combined storm tide + waves reached at least this elevation to damage these instruments. (Airport information available at:

A combination of storm tide and waves damaged AWOS weather sensors at L.F. Wade International Airport, where the elevation is 6 ft (~ 2m).
Map: Wikipedia Commons, Graphics: Hal Needham

Bernews posted numerous storm damage photos, some of which show boats washed up on shore. The positioning of the boats suggests the surge in these locations was at least several feet (~ 1 m) high.

Gonzalo's storm surge washed small vessels onshore

Gonzalo's storm surge washed small vessels onshore

This morning, Jim Edds posted photos of damaged causeway sections. Apparently, Gonzalo knocked down six concrete wall sections of the causeway, however, it is not apparent if water or strong winds toppled these sections. Most online photos today suggest that wind damage was more severe than surge/ wave damage.

It should be noted that waves outside the reefs were much higher than storm surge/ storm surge heights inside the reef. As Gonzalo approached Bermuda, seas (open ocean waves) were forecast to exceed 30 ft (9.1 m).

 Photo of causeway damage from Jim Edds (@ExtremeStorms) 

Please feel free to send me any surge pics or observations! My email is  hal"at"

Hurricane Hal


Stewart, 2014: Hurricane Gonzalo Discussion Number 23. 1100PM AST FRI OCT 17 2014. Available on the Web at: