The role of a weather router in cruising

Practical: Meteorology

With state-of-the art modern communications, expert advice on passage planning is only an email away. Weather routers Jeremy Davis and Amanda Delaney explain what they do and how it can help the cruising sailor.

As all mariners know, weather information is critical to having an enjoyable trip. Correctly planning for weather conditions can make the difference between an uneventful cruise and a life-threatening situation. In recent years, available meteorological information has multiplied tremendously on weather-dedicated websites, government websites, television stations and radio broadcasts. Making sense of all this data has become more difficult for the average sailor on who or what you should trust. The question becomes, who is accurate and who is not?

Fortunately, there are options available in utilising the best weather data for an upcoming trip. Using a weather routing company can take the pressure off from combing through the numerous amounts of data presented. Weather routing services do all of the grunt work; condensing everything you need into an easy readable report, that takes into account your itinerary, your weather constraints, along with current and forecast conditions, resulting in the best possible departure time and route.

Rules of thumb

Weather routing information has been used for centuries, starting with “rules of thumb” — lessons learned by experiencing weather features at certain times of the year. However, in the past decade not only has weather technology expanded tremendously, but so too has the ease of sharing information from a weather routing company to a client. Satellites are now able to determine wind speeds along the ocean surface twice a day, and can now measure ocean currents and sea surface temperatures to the nearest tenth of a metre per second, and tenth of a degree respectively. Ships, buoys, and coastal stations provide an instant, real-time array of data that would have been unimaginable just a few decades ago.

Instead of weather forecasts and routing advice being relayed in the shortest amount of text possible, in order to save on communication costs, now, full reports detailing a weather synopsis, routing recommendations, winds, seas, swells, currents, precipitation, cloudiness, and forecaster comments can be easily transmitted. These highly customised forecasts, along with forecast charts, can be sent instantly over email and fax, reaching the client much faster than in recent years.

Weather Routing Inc

One such company that provides weather routing services is Weather Routing Inc, based in New York, for whom we work. Weather Routing (WRI) has been in business for nearly 50 years, providing customised forecasts and routing advice to mariners all over the world, including the Australia and New Zealand regions. WRI specialises in developing relationships with mariners, allowing for a personalised forecasting service that is difficult to find using online weather websites. Meteorologists constantly review forecasts, with courtesy updates sent if needed, to ensure users of the highest quality products.

Besides customised forecasts, WRI has developed a supplemental website called SeaWeather, available at www.seaweather.net On SeaWeather, forecast charts, sea surface temperatures, wave height analysis, worldwide tropical information, satellite and radar imagery, and much more can be viewed. The website is constantly being updated with new services. Sailors in the Australia and New Zealand region can utilise the Southwest Pacific section, which has close-up regions of Australia, New Zealand and New Caledonia, for trip planning purposes.

Tropical surveillance emails

Active clients of WRI, along with premium Seaweather clients, can elect to receive free tropical surveillance emails for the Indian Ocean and Coral Sea. Associated with

WRI tropical surveillance are automated alerts that are sent if a specific location becomes threatened.

Companies such as Weather Routing Inc are also using new platforms to interact with customers. WRI has their own page on Facebook (www.facebook.com/weather.routing) where members can post questions, share weather stories and photos, and more. Short weather headlines are “tweeted” on WRI’s Twitter page (www.twitter.com/weatherrouting).

Climatology of the Aus/NZ region and routing challenges

The Australia and New Zealand region features some of the world’s most variable weather — from tropical cyclones and strong winter storms, to tranquil areas of high pressure. The wide variety of weather systems throughout the year creates a myriad of routing challenges. A common knowledge of the climatology across Australia, New Zealand and the surrounding waters offers mariners an idea to choose the best time to make short coastal trips, or longer sea basin journeys. Climatology also educates mariners to avoid certain areas during different seasons. Let’s explore some of the weather features and routing issues throughout the year, and why it is important to use a weather routing service.

Main weather features

There are two main weather features that are responsible for the weather across northern Australia and the adjacent tropical waters. These are the equatorial trough (low pressure) and a semi-permanent high-pressure ridge. These weather features are present all year and define the seasons depending on their movement.

The equatorial trough extends from west to east from the Indian Ocean across Indonesia to the international dateline.

On satellite imagery, this trough is defined by the presence of areas or clusters of showers and thunderstorms along or near its axis (generally where NE trade winds from the north converge with SE trades further south), and typically track from east to west within the trough. During the winter, this trough reaches its northern zenith between 10 degrees N and 05 degrees S. With the trough found so far north, generally dry weather usually prevails over the tropical waters and northern Australia. During the spring the trough will migrate southward and by the summer will range between the equator and 10 degrees S. Numerous showers and thunderstorms will track across the northern Coral Sea and northern Australia. These showers and thunderstorms will diminish during autumn when the equatorial trough shifts to the north again.

Over the southern tropical waters, a semi-permanent ridge of high pressure can be found extending from 155 degrees E to the international dateline. Light and variable winds can be found along the ridge axis and near the centre of the ridge. Areas of high pressure will also move across portions of Australia behind cold fronts before merging with this ridge east of Australia. The ridge will shift north or south around the same time as the equatorial trough. During the winter, the ridge can be found between 15S and 30S, and interaction between the ridge and the equatorial trough will allow for enhancement of E-SE trade winds across the Coral Sea and across the northern Australian waters (with speeds often approaching and reaching minimal gale force during much of the season). “Brief” (usually lasting 1-2 days) lulls in these enhanced trades occur in more N-NE-E winds as cold front approach and pass off to the south.

Ridging

During spring, ridging begins its slow “progression” south, reaching its southernmost point during the summer over the northern Tasman Sea between 25 degrees S and 35 degrees S. As this occurs, and as the equatorial trough moves further south (over the northern tropical waters), trade winds subside across this region. This ridge will return northward during autumn.

Further west, we find a thermal trough (low pressure). This trough can be found virtually year round and along or very near the west coast of Australia. Eastward/westward shifts of the trough are the norm, depending upon the presence and timing of additional weather features in the vicinity. For example, if high pressure over the southeastern Indian Ocean were to build eastward near the Australia coast, then the trough will shift inland and intensify. When this scenario occurs, south to southeast winds become enhanced to the west of this trough. As ridging “retreats” offshore over the southeastern Indian Ocean, the trough will move offshore and allow for light weather along the western Australian coast. The trough is usually found from Perth northward to Northwest Cape during the summer. During winter,

the trough will retreat further north and can be found from near Shark Bay to Cape Leveque. This northward location is due to the active cold fronts that impact the southwestern Australian coast during the winter months.

Southern Australia

The gale/storm track and cold fronts are the primary weather concerns across southern Australia and New Zealand. Gales and storms are strongest and found as far north as 40 degrees S during winter. Gale to, at times, storm-force westerly winds will impact coastal areas along the Great Australian Bight and through Bass Strait. These gales/storms will continue eastward and eventually impact New Zealand. Cold fronts during this time of year reach as far north as 28S and move over southern Australia about every two to three days, and then track eastward over the Tasman Sea and southern Coral Sea to New Zealand and south of New Caledonia. Winds will become mainly northwesterly with numerous showers and thunderstorms ahead of cold fronts.

Once fronts pass, winds shift and become west to southwest. Secondary gales can develop over the northern Tasman Sea along cold fronts and track southeastward towards New Zealand and/or merge with the primary gale/storm. These secondary gales can generate large southwest or south swells across the Tasman Sea that can impact the southeastern coast

of Australia and New Zealand. Given the rather expansive coverage of wintertime gales and storms, these swells often reach New Caledonia and other islands over the Coral Sea during the winter.

Transition periods

Spring and autumn are transition periods, with the gale/storm track progressively further south in spring, and beginning its northward progression during the autumn. Gales and storms during these times are found further south between 45 degrees S and 55 degrees S, and are generally weaker than during the winter. Secondary gales over the Tasman Sea still develop but are further south and less frequent. Cold fronts track across Australia and reach New Zealand about every three to four days and generally only reach as far north as 30 degrees S to 33 degrees S.

By summer, the gale/storm track is typically between 50 degrees S and 60 degrees S and cold fronts only extend as far north as 35 degrees S, mainly tracking across the Great Australian Bight and across Victoria and Tasmania every four to five days. These fronts will eventually reach the southern island of New Zealand and are relatively weak during this time of year.

Cyclones

Although late spring through early autumn usually provides ideal weather across most of Australia and New Zealand, there is still one dangerous weather feature that can wreak havoc for travellers . . . the tropics. Tropical cyclones generally from November through April when sea surface temperatures are warmer, and the equatorial trough is further south over the northern Coral Sea, Timor Sea, Arafura Sea and the Gulf of Carpentaria.

Over the Coral Sea, tropical cyclones generally develop between 06 degrees S-15 degrees S and typically take one of two tracks: either a west to west-southwest track and move inland over eastern Queensland or turn southwestward and eventually south upon reaching 20 degrees S to 25 degrees S, then turn more southeastward ahead of a cold front. Upon reaching cooler waters and a more “hostile” environment aloft in more southern waters, tropical cyclones will then tend to transition into a non-tropical (or extratropical) gale or storm. These extratropical systems generally accelerate to the southeast before either dissipating or merging with a cold front.

Some tropical cyclones will develop and track toward and across the Cape York Peninsula and move over the Gulf of Carpentaria and inland over northern Australia. Additional tropical cyclones may develop over the Arafura Sea or Gulf of Carpentaria. Gulf of Carpentaria cyclones generally move inland whereas those developing over the Arafura Sea often track westward towards the

Timor Sea or move inland over the Northern Territory.

Tropical cyclones that form or move over the Timor Sea generally track either southwestward and make landfall along the northern coast of Western Australia or continue west to west-southwestward over the southeastern Indian Ocean. Once over the Indian Ocean the system will generally turn south and southeastward between 20S to 25S and become either extratropical or merge with a cold front.

Across these areas of the world, tropical cyclones will increase in frequency from November through January and the season reaches its peak during February. Thereafter, a decline in development occurs through April as the equatorial trough begins to migrate northward and sea surface temperatures begin to cool across the tropical waters.

What does this all mean?

Overall, the summer (outside of tropical cyclones) provides the best travel weather, with cold fronts remaining further south during that time. The spring and autumn months also allow for periods of good weather, especially closer to summer. However, these transitional seasons are times when fronts need to be monitored for their potency and the potential for secondary gales to develop over the Tasman Sea that will bring inclement weather to New Zealand. Cold fronts though will relax the trade winds across tropical waters and these are the best times to make hops between islands across the Coral Sea.

Winter creates the most challenging weather, and any good weather windows are typically short-lived, especially across the mid-latitudes. Trade winds are strongest at this time and it is generally best to make trips from east to west (for more following winds) or wait for cold fronts to approach and pass south, allowing for those 1-2 day breaks in enhanced trades resultant large swells to slacken.

Knowledge is power

For more information visit http://www.wriwx.com or email wri@wriwx.com.

Amanda Delaney:
Amanda is a senior meteorologist and has been employed with Weather Routing Inc for eight years. She specialises in yacht forecasting and enjoys the challenges of forecasting for different regions of the world.

Jeremy Davis
Jeremy is also a senior meteorologist and has worked at Weather Routing for more than nine years, where he particularly enjoys tropical forecasting and worldwide climatology.