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Compare-Contrast-Connect: Swell Forecasting From Weather Patterns

NGSS Science and Engineering Practices

Scientists use mathematical models, computer models, and experience to predict the formation, duration, and projection of atmospheric storms as well as the subsequent effect of those storms on wave and swell formation. Much of the data needed for wave forecasting is freely available to the public.

These sites are among the valuable sites for weather and wave forecasting:


SF Fig. 4.9. Compass image of O‘ahu, Hawai‘i

Image courtesy of David Kimball

SF Table 4.1 is a detailed surf forecast for O‘ahu, Hawai‘i. Read this forecast and notice how the scientist, Patrick Caldwell, uses the global weather patterns to predict surf, wind, and weather for the north, south, east, and west shores of O‘ahu. Refer to SF Fig. 4.9 for a map of the relevant compass headings.

SF Table 4.1. Swell forecast posted on Friday, October 15, 2010

Summary: trending up for northern shores mid next week.


Detailed: mid Friday on northern shores has small breakers from 345-020 degrees. Heights should drop on Saturday.


A jet stream ridge in the NW Pacific and dominant troughs in the gulf of Alaska has favored small, NNW to NNE surf in Hawai‘i over the recent two weeks.


Fresh to strong breezes behind a front on Tuesday in the 345-360 degree band nosed to within about 500 nautical miles (nm) north of Hawai‘i. Short-period energy from this source is producing small breakers on Friday. Surf from this direction is expected to drop on Saturday.


The associated front transitioned to a shear line at about 30°N latitude, with fresh to strong breezes stretched along an east to west ribbon of area well to the NE of Hawai‘i. Models showing it weakening on Saturday. The Mokapu buoy is showing an increase from 20-45 degrees on Friday. Surf from this direction should keep small breakers for select spots of the north shore over the weekend, while most spots should stay flat to tiny. The NNE windswell should fade out by Monday.


A weak low pressure hugging the Aleutians over the last 24 hours NNW of Hawai‘i aimed mostly NE of the state. Small surf from 340-360 degrees is possible next Tuesday.


Models show an amplifying jet trough along the longitude of Hawai‘i this weekend, with an associated surface low pressure gaining storm-force winds near the Aleutians. The low center is predicted to track east as a front pushes towards Hawai‘i. This is similar to patterns of late September that brought moderate to marginally high surf to Hawai‘i. Gales are expected to reach to about 40°N, or about 1200 nm NNW to N of Hawai‘i by Monday.


Surf is expected to steadily rise Wednesday morning from 330-350 degrees, to marginally high levels by the afternoon. High surf is expected to peak on Thursday from NNW to N and drop off steadily with small breakers by late Friday. Strong to near gale breezes are modeled to nose to within 500 nm of Hawai‘i by next Tuesday, and add short-period small breakers from 000-030 degrees next Thursday into Friday. Models show the jet stream ridge in the NW Pacific being replaced by a trough around Tuesday, which should spell a return to NW swell activity locally during the following weekend. It is too early for specifics.


Mid Friday on eastern shores has small to moderate breakers. The 8-second windswell from the east has faded, as a similar episode fills in the NNE to NE, affecting select more northerly-facing locations. Surf should stay about the same on Saturday.


See the latest NWS state weather forecast discussion regarding the gentle trades expected during the upcoming period. Such speeds are too low to create breakers from the local winds. Gentle to moderate trades late next week with low easterly windswell.


Mid Friday on southern shores has small breakers from 160-180 degrees. This episode is expected to drop on Saturday as a new episode builds from 200-220 degrees.


Extratropical cyclone activity returned to the Tasman sea a week ago Friday into Saturday. A small episode is expected to build on Saturday and stay about the same from 200-220 degrees on Sunday, then drop early next week.


This low pressure system tracked SE of New Zealand last Sunday. The ascat satellite showed a broad area of severe-gale to storm-force winds and wave models showed a wide area of seas over 8 m. The limiting factor was the proximity to New Zealand, which creates shadowing for Hawai‘i and reduces the fetch width. Models keep this episode in the small surf bracket. However, this forecast is leaning higher for safety. Surf could build into the small to moderate bracket on Sunday from 185-200 degrees, peak on Monday at moderate levels, and drop back to small levels by Tuesday from the same direction. Flat to tiny conditions are expected on southern shores late next week into the weekend.


Long-range forecasts are subject to high uncertainty.

This forecast was produced through the collaborative efforts of the National Weather Service (NWS) and the National Coastal Data Development Center (NCDDC). Go to National Weather Service Weather Forecast Honolulu Office or additional resources.

NWS Forecaster DONALDSON and Pat Caldwell, NCDDC


Question Set

Use the compass map of O‘ahu and the detailed forecast to answer questions 1–6.

  1. What instruments or sources of information were used to create this surf forecast?
  2. From which direction (in degrees) is the largest swell expected during this forecast period? Which side of O‘ahu would you go to if you wanted to surf this swell? On which day?
  3. The forecast says that a small surf episode is expected from 200–220 degrees on Sunday. Which side of O‘ahu would you go to if you wanted to surf this swell?
  4. Why do you think that that the forecast states that a storm being close to New Zealand would reduce the size of the swell reaching Hawai‘i?
  5. Why are “gentle trade winds too low to create breakers”?
  6. Why do you think the forecaster would write, “Long range forecasts are subject to high uncertainty”?
  7. SF Fig. 4.10 shows the global wave height and direction forecast for October 18th, 2010. Describe how this image agrees and/or disagrees with the predictions made by the forecaster (the predictions were made on October 15th).

Image caption

SF Fig. 4.10. This map of global ocean conditions on October 18th, 2010, shows significant wave height (average height of the tallest one third of all waves) in feet, corresponding to the color legend at the top of the figure, and the peak wave direction as white arrows.

Image copyright and source

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.