Oahu Climate

by Rick Shema

I. Climate on Oahu:

The prevailing northeasterly trade winds are the dominant factor that governs Oahu’s pleasant climate and all the Hawaiian Islands. The persistent trades mean steady winds in summer months. In combination with warm seawater temperatures gives Hawaii fabulous sailing conditions and an ideal location for Kenwood Cup. Average wind speed recorded at Honolulu International Airport for August is 12 kts from the east-northeast direction. Recent seawater temperatures have measured between 76-78 degrees F (24-26 degrees C). Average air temperature range from 88 degrees F (31 degrees C) to 74 deg F (23 degrees C) with moderate humidity of 53% during the day. Even the warmest months are comfortable with blowing trade winds. The island has two mountain ranges separating the moist windward side from the drier leeward coast. The Koolau Mountains extends along the northeastern side and to the west lays the Waianae Mountains. Both mountain ranges serve to block trade wind moisture. As a result, showers occur almost daily on the windward side while on the leeward side showers are light. They are more frequent during evening hours. On occasion, trade winds shut down and winds become light and variable. Under these conditions, it can become hot with high humidity. A land-sea breeze regime may be established as the winds tend to be thermally driven. On leeward coasts, wind direction then becomes southerly. This is referred to as “Kona Winds”. Interior clouds often extend higher in vertical height than normal trade wind clouds. Heavy rain and thunderstorms are possible, especially after the second day of Kona Winds. It is rare for weather to be severe enough to interfere with scheduled events.

 

II. Race Course Winds and Seas

Wind speed and direction are typically governed by a High pressure center to the north of the Hawaiian Islands during the summer. Two additional effects are time of day and funneling of winds between the islands of Oahu and Molokai (Kaiwi Channel). Most racing will be conducted offshore Waikiki in the afternoon. This area and time of day is characterized by moderate east-northeast winds 18-25 kts. At night and early morning, the winds reduce by approximately 5-12 kts. Usually by late morning, winds increase again due to the additional forcing effects from the sun. In a particularly strong pressure gradient, the winds can exceed 35 kts. These conditions are often accompanied by higher gusts above 40 kts. Because of the mountain ranges, the wind direction in the channel is fairly consistent ranging from 060 to 070 degrees True even when isobaric pattern would other wise indicate a different direction. However, when trades are particularly easterly, the wind range becomes 080 to 090 degrees True. Advantageous wind shifts are seen closer to shore, as the winds build pressure to windward of the Koolau mountains and funnel down toward the leeward side through mountain ridges and valleys. This is particularly true sailing towards Koko Head during the short and long distance offshore races. In the vicinity of north shore Molokai, winds have a noticeable bend to the east as they come under the influence of steep ocean cliffs 4,000 ft (1200 m) high. This veering wind can be used to gain tactical advantage. During summertime Kona conditions, winds can be light and variable. In these times, it has been advantageous for boats to seek favorable winds near shore due to thermal effects. Sometimes favorable winds are associated with cloud lines. Clouds on the leeward side often have localized cells that contain increased wind speed as a result of downdrafts.

Sea heights are mostly driven by local winds. Trade wind swells in the race area can attain heights of 8-12 ft (2.5-3.5 m). When winds are in the 35-40 kt range, a fully arisen sea can attain 20 ft (6.2 M).

III. Currents, Surf, and Tides

Currents are not well defined around the Hawaiian Islands. They are near impossible to predict as they vary with complex effects of tides, bottom features, eddies, winds, and other physical processes. Studies completed in past years have been inconclusive. Surface currents are modified by the shapes of the islands and, close to shore, by the tides. Eddies in the lee of islands, especially west of the Big Island of Hawaii and along the south coast of Oahu, are common and probably result from high winds funneled between the islands. The best method to differentiate current direction and speed is visual observations of buoys and onboard instrumentation. However, trends from one day to the next may be inferred.

Tides are mixed with two highs and two lows of different heights each day. Spring tides do not exceed 3.0 ft (.90 m). Tidal currents have been observed flooding westward from Makapuu Point to Honolulu. An inshore, eastward countercurrent accompanies the flood current. Current speeds can exceed one kt. During ebb tides, current generally runs eastward. 

Northern Hemisphere summer means high surf along south facing shores from winter storms in the Southern Hemisphere. Swells generated by distant storms affect south facing channels and harbor entrances. Although these conditions provide excellent surfing opportunities from Diamond Head to Barbers Pt, waves produced from these storms can break in channel entrances creating a dangerous situation when entering and leaving port. Surging currents are also produced in harbors. Caution is advised.

IV. Squalls

Squalls are small-scale features of disturbed weather embedded in the trade winds. They can be used to gain tactical advantage due to shifting winds and increased velocity ahead of the squall. However, behind the squall there is a definite area of reduced wind that can last for 30 minutes to an hour. Tack or jibe to avoid this area. Correct use of squalls can be a significant factor in gaining considerable distance towards the finish line. Before the squall there is a sudden drop in temperature and increase wind velocity. This is the beginning of the downdraft winds ahead of the squall. Wind speeds can exceed double the value of normal trades. So if you are observing 20 kt trade winds, then you can expect an average of 40 kts in or near the squall. Squalls are strongest at night due to more cooling at the cloud tops.

V. Tropical Cyclones

AMERICAN PRACTICAL NAVIGATOR, by Bowditch, PUB. No. 9, published by National Imagery and Mapping Agency, 1995, contains an excellent description of tropical cyclones, detecting their presence, and avoidance procedures. Although it is not likely that a tropical cyclone will adversely affect yachts sailing in Kenwood Cup Races, it is a good idea to review this valuable information wherever sailing in the tropics.

A. The Season: In the Northern Hemisphere, tropical cyclone season begins in June and ends in November. Most of the activity occurs in August and September. In Eastern North Pacific, tropical cyclones usually originate between 10N and 25N. In the early part of the season, they tend to form between 90W and 110W. By August, the western boundary extends as far as the Hawaiian Islands. The number of tropical cyclones in Eastern North Pacific averages 15 for normal sea surface temperatures. Five or six of these will become hurricanes. Hawaii’s Weather Guy (https://www.weatherguy.com ) will post on the website the latest tropical cyclone warnings. 

B. La Nina: La Nina is characterized by unusually cold ocean temperatures in the equatorial Pacific, as compared to El Nino, which is characterized by unusually warm ocean temperatures. Both of these phenomena oscillate from a La Nina (cold phase) to El Nino (warm phase). Currently, we are in a La Nina cycle. It is likely that cold phase conditions will gradually weaken as summer ends and near normal or slightly cooler than normal ocean conditions will be present in the tropical Pacific. The unusually cooler water currently in the Equatorial Eastern Pacific is correlated with the tendency towards decreased tropical activity. However, with the cold phase apparently waning, it is expected that this year’s tropical cyclones will number near normal to slightly more than normal.

C. Easterly Waves and Remnants of Tropical Cyclones: The most likely effect of tropical cyclones may come from the remains an earlier tropical cyclone or easterly wave. These disturbances disrupt the normal trade wind flow causing havoc to racers and cruisers alike. As the disturbance travels westward near the islands the winds back to the north. On the east side, the winds veer towards the east-southeast. The associated weather ahead of the disturbance is generally good visibility with few clouds. The weather then changes, being characterized by high humidity, heavy showers, thunderstorms, and rain with gusty winds. If a tropical disturbance or easterly wave happens to be in the vicinity of the Hawaiian Islands, then get ready for a disruption of normal trade winds and deteriorating weather. Only after the disturbance moves westward, will normal trades resume. They can be detected on weather maps as a bump-like pattern in the isobars. To the west of the trough-line, the winds have a more northerly component. This is especially true when the trades are more easterly with a pronounced wave. To the south of the disturbance, the isobars are typically spaced further apart and that means lighter winds.

VI. Tsunamis

Tsunamis originate from underwater seismic activity. Many submerged earthquakes generate small sometimes, undetectable waves. Occasionally, large waves form of disastrous proportions and can be very hazardous to life and property primarily near coastal areas. The Hawaiian Islands have experienced significant damage from tsunamis and are particularly vulnerable to those originating in the north and the southeast Pacific Ocean. A 55 ft wave was reported off Makapuu Point, Oahu during a 1946 tsunami. A 30 ft wave resulted in two deaths on the Big Island. The National Oceanic and Atmospheric Administration manages a tsunami warning system that issues warnings for the Hawaiian Islands. Civil Defense and other authorities will broadcast those warnings to the population. Police cars and Civil Air Patrol airplanes will patrol along the shorelines sounding sirens. Local radio and TV broadcasting stations will interrupt programs to provide the latest warning information and instructions. When a tsunami warning is received, people should vacate waterfront areas for higher ground. The local telephone book indicates which coastal areas are to be avoided. Boats should head for deep ocean water provided time permits safe sortie from anchorages and harbors. 

VII. References:

1. AMERICAN PRACTICAL NAVIGATOR, by Nathaniel Bowditch, PUB. No. 9, National Imagery and Mapping Agency, 1995.

2. WEATHER FOR THE MARINER, third edition, William J. Kotsch, RADM, U.S. Navy (retired), Naval Institute Press, 1983.

Rick Shema (Weatherguy.com is a U.S. Navy meteorology and oceanography officer and is the meteorologist in charge of the weather committee for Kenwood Cup 2000. He offers weather routing services for racers and cruisers worldwide. Rick can be contacted by email at Rick@weatherguy.com.