Marine Weather API for the Caribbean

Marine Weather Challenges in the Caribbean

The Caribbean spans from the Bahamas and Puerto Rico through the Lesser and Greater Antilles to Trinidad, covering over a million square miles of tropical ocean. Its consistent trade winds make it one of the most popular sailing grounds in the world, but understanding the nuances of Caribbean marine weather is what separates a comfortable passage from a difficult one.

Trade wind consistency makes the Caribbean ideal for sailing, but variations matter — 15kt is a pleasant beam reach, while 25kt demands reefed sails and careful passage planning. The difference between a relaxed day on the water and a white-knuckle crossing often comes down to timing and knowing exactly what conditions to expect.

Inter-island passages create their own microclimates with accelerated winds and steep seas. The Anegada Passage between the Virgin Islands and St. Martin, and the Mona Passage between Puerto Rico and the Dominican Republic, are notorious for challenging conditions that differ significantly from the sheltered waters nearby.

Saharan dust events reduce visibility and affect marine conditions across the eastern Caribbean, typically peaking between June and August. These events can cover hundreds of miles and persist for days, impacting everything from navigation safety to dive visibility.

Get a Marine Forecast for the Caribbean

Here's how to get an AI-powered marine weather forecast for San Juan, Puerto Rico — a major departure point for passages to the Virgin Islands and beyond:

curl -X POST https://api.sealegs.ai/v3/spotcast \
  -H "X-API-Key: your_api_key" \
  -d '{
    "latitude": 18.4655,
    "longitude": -66.1057,
    "start_date": "2026-03-15",
    "num_days": 3,
    "vessel_info": {
      "type": "sailing",
      "length_ft": 42
    }
  }'

{
  "summary": "Good three-day window for a 42ft sailing vessel
    departing San Juan for the Virgin Islands. Trade winds
    holding steady at 15-18kt from the east-southeast with
    comfortable 4-5ft seas on a 7-second period. These are
    classic Caribbean trade wind conditions — reliable and
    well-suited for a beam reach westward. No tropical
    disturbances in the forecast. Expect slightly rougher
    conditions in the Vieques Passage before settling into
    the lee of the islands.",
  "daily_forecasts": [
    {
      "date": "2026-03-15",
      "classification": "Good",
      "confidence": 0.91,
      "wind_speed_kt": 16,
      "wind_gust_kt": 21,
      "wave_height_ft": 4.5,
      "wave_period_s": 7
    }
  ]
}

The same request works for any Caribbean coordinates. Use 18.3358, -64.9307 for St. Thomas, 18.0179, -63.0473 for St. Martin, or 13.1939, -59.5432 for Barbados.

Key Weather Parameters for the Caribbean

The API returns a full set of marine weather parameters. For Caribbean waters, these are especially important:

• Wind speed & direction — Trade wind strength dictates daily sailing plans; direction shifts indicate approaching weather systems or tropical waves
• Wave height & period — Long-period Atlantic swell (8-12 seconds) vs short-period trade wind chop creates very different comfort levels, even at the same wave height
• Swell data — North Atlantic swell wraps around islands creating different conditions on windward vs leeward sides; critical for anchorage selection
• Visibility — Saharan dust events (June through August) can reduce visibility significantly across the eastern Caribbean

Use Cases in the Caribbean

Popular Caribbean Coordinates

Here are commonly used coordinates for Caribbean marine forecasts:

• San Juan, Puerto Rico — 18.4655, -66.1057
• St. Thomas, USVI — 18.3358, -64.9307
• St. Martin — 18.0179, -63.0473
• Antigua — 17.1274, -61.8468
• Martinique — 14.6415, -61.0242
• Barbados — 13.1939, -59.5432
• Aruba — 12.5211, -69.9683