June 19, 2019

Winds of Block

What the summer winds do off Block Island depends on the big picture, but if the sun is shining and there’s dew on the deck, expect spectacular sailing conditions
By Chris Bedford, Photography by photoboat.com

Photo: Douglas McKeige’s crew on Jazz mastered the conditions of Race Week in 2017, winning seven of eight races in the J/88 fleet.

Storm Trysail Club’s Block Island Race Week marks one of the great regattas of the year. Block Island’s location—approximately 8 nautical miles off the Rhode Island coast and 12 nautical miles from the eastern tip of Long Island—­situates it for a unique combination of winds controlled by the large-scale synoptic weather pattern, modified by diurnal winds that develop along the southern New England coast.
Generally, the weather map is dominated by the Bermuda High pressure and lower pressure over North America. This pattern is typical of the summer in the mid-latitudes, where land areas are warmer, thus “holding” lower pressure relative to the ocean, which is relatively cold and favors high pressure.

While this is the average, the pattern does fluctuate, ­mainly in response to frontal systems moving out of central North America—although the frequency and intensity of these systems are typically lower in summer. A Bermuda High can last four to seven days between frontal passages, and it’s not unusual to have an extended period lasting up to 10 days. It is normally associated with fair weather, but can have extended visits from dense fog.

Outside of this pattern, there is a secondary pattern, which can occur in one- to three-day blocks. This pattern is initiated by a cold front moving off the New England coast, followed by high pressure moving into northern and central New England and the Canadian Maritimes. Sometimes, this pattern can result in unsettled and rainy weather, especially if there is low pressure south of New England.

Prevailing Winds
The wind blows from the southwest quadrant more than 60 ­percent (six out of 10 days) of the time in Block Island. Typically, these winds are associated with the gradient flow pattern around the Bermuda High described above, modulated by heating and sea-breeze effects off the Rhode Island shore. Afternoon wind speeds are often in the mid-teens or higher—­especially with a little help from a sea-breeze component.

The second most likely wind direction is from the east/­northeast. This pattern occurs when high pressure is over New England and/or low pressure is south of New England. This occurs about 20 percent of the time (two out of 10 days), and is normally associated with lighter wind, below the mid-teens—except when a stronger low is located to the south.
One other pattern occurs after a cold front moves through. A windshift to the northwest, and sometimes north, will ­follow. It can be characterized by stronger winds in the morning, and lighter winds in the afternoon. Northerly component flow ­normally doesn’t last more than a day or two this time of year.

Sea Breezes
Sea breezes are common on the New England coast. Typically, sea breezes start on the Rhode Island mainland shore in the late morning and work their way offshore to Block Island. It can take a couple of hours for this to happen. Sea breezes are normally a mixture with the preexisting gradient flow. A southwesterly wind is lighter and more right-shifted in the morning. As the sea breeze builds out to Block Island, the wind backs left and strengthens.

In easterly gradient cases, the sea-breeze effects are often minimized. First, weather conditions often limit the amount of heating over southern New England. Second, there is a convergence of wind just off the New England shore, which is detrimental to sea-breeze development. So, the net effect of a sea breeze on an easterly gradient is to veer the wind right (from northeast/east to east/southeast) and ease the windspeed. If the wind is northeast or east in the morning, expect it to veer and ease in the afternoon, possibly transitioning from a promising mid-upper teens breeze to light and patchy.

In post-frontal northwesterly conditions, there is a higher probability of a “die-and-build” sea breeze onset. Northwest/northerly winds in the morning ease and become variable, until a south/southwesterly sea breeze builds out from the mainland coast to Block Island. This often delays the sea breeze and can make it lighter than it would be in cases where it has a favorable southwest-gradient head start.

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