As the boats sail southeast into the Atlantic, the winds are expected to stabilize with the building North Atlantic high; however, the teams will have to keep their eyes on not only the long term planning, but also the smallest local weather features.
The North Atlantic high, also called the Azores High or the Bermuda High is a semi permanent feature that moves around the North Atlantic.
The position of the high determines the strength and reliability of the NE’ly trade winds. If the teams sail too close to the center of the high, the winds will drop off to a glassy calm.
Likewise if they stay too far from the high, the trade winds will be weaker and less reliable.
Over the last week the North Atlantic high has been weak.A series of stronger lows moving east into Europe had forced the high west; however, as the boats head south the high is expected to re-develop around 35N and 25W.
Over the next 4 days, the high will build, and a central pressure of over 1030mb is expected by Thursday.
As the high develops, the pressure gradient will build and the corresponding strength of the trade winds will increase. Over the next 24 hours (Tuesday) the high is expected to build to 1026mb and the winds will build out of the NNE with the pressure gradient around the leading edge of the high.
As the high develops it will move northeast, and the boats will move southwest, causing the winds to veer right. By Wednesday the teams will see a more ENE’ly flow around the south side of the high. The winds speeds are expected to be in the mid to high teens over the next two days.
There is reasonably good agreement between the various forecast models, meaning the navigators will have more confidence in the wind forecast. However even the trade winds will keep the navigators on their toes looking for the best breeze.
Within the trade winds there are often bands of stronger wind that could make or break a team’s performance. These bands, sometimes visible as streaks of clouds, are formed by convective heating.
The bands are caused when heated air rises, cools as it spreads out, and then sinks. The rising air (locally lower pressure), or sinking air (locally higher pressure) makes for lighter winds at the surface.
However, between these localized systems the surface winds are notably stronger. Often the rising air causes clouds; however, bands of stronger winds can form even if no clouds are present.
The winning teams will have to not only sail fast, paying attention to the very localized weather patterns, but also plan ahead for the best approach to the Doldrums.
Jennifer Lilly