Bermuda sailing forecasts

I was lucky enough to go racing in Bermuda a week or so ago. I asked my Dad who used to be (and still is to a degree) a meteorologist who in his career spent some time in the area. Here are his recommendations for understanding the general situation with some more recommendations from myself.

The synoptic picture

Bermuda sits at about 32°N, 65°W — some 10° north of the tropics, in a subtropical maritime climate: warm, humid and wet fairly evenly through the year. The quasi-stationary Azores high to the north usually governs the wind, so the prevailing direction is south-west or west. Two things set it apart from higher latitudes. The troposphere is deeper, so convection — showers and thunderstorms — runs taller. And the geostrophic balance is looser: the wind does not always follow the isobars, so a surface chart tells you less here than it would in the English Channel. The island is small, so the sea breeze is weak. When I was there the Azores high was not in charge — the tail ends of fronts off the United States were crossing the area, bringing showers and shifting winds. Hurricane season opens in June but rarely bites until later in the summer, and extra-tropical storms reach Bermuda too.

A human forecaster’s discussion is the most undervalued resource in sailing weather: it tells you what the models are arguing about and which one the forecaster trusts today.

• Bermuda Weather Service (weather.bm). This is the local authority: marine forecast, L.F. Wade airport observations and a written discussion. Read the discussion every time.
• UK Met Office, St George’s. A second national service’s forecast for the east end of the island — a useful cross-check against weather.bm.
• NOAA Ocean Prediction Center, Atlantic. The unified surface analysis and 24/48/96-hour surface forecasts for the whole basin. This is the synoptic frame Bermuda sits inside.
• NHC Atlantic Tropical Weather Outlook. Non-negotiable from 1 June to 30 November. Bermuda is in the hurricane belt; in season, check the 7-day graphical outlook first.

OPC Atlantic unified surface analysis (live, always current). Public domain, NOAA/NWS.

NHC Atlantic 7-day tropical weather outlook (live, always current). Public domain, NOAA/NWS. Blank outside hurricane season.

The model forecasts

Apps turn the synoptic picture into a point forecast — a wind speed and direction for your spot, hour by hour. That precision is seductive and partly false: a single arrow hides the disagreement between the models behind it. So read the spread, not the arrow. Where ECMWF, GFS and ICON converge you can plan around the wind; where they diverge the honest forecast is “uncertain”, and the move is to stay conservative and re-check nearer the time. My post on reading model disagreement covers how to do that.

• PredictWind. Shows the model spread clearly and runs its own downscaled models (PWE, PWG) on top of ECMWF and GFS. The spread display is the point; that same post covers the proprietary models.
• Windy. Fast comparison of ECMWF, GFS, ICON and AIFS over the same point. Switch models on one forecast and watch what changes.
• Windy ECMWF ensemble. The spread of the ECMWF members at a point is a better honesty check than any single deterministic run.
• Earth Nullschool. An animated global model on a rotating globe. Turn on surface streamlines to read the flow rather than the isobars, and switch parameters to taste.

The observations

The models are a hypothesis; the observations are the test. That test matters more here than at home: with the geostrophic balance loose and showery fronts drifting through, a tidy pressure chart can mislead, so the live numbers are what you trust. Bermuda’s are sparse but good.

• L.F. Wade International (TXKF). The island’s anemometer and METAR, via aviationweather.gov and weather.bm’s observation page. For the next few hours of wind, read the TAF: the terminal aerodrome forecast is the closest thing to an hour-by-hour wind outlook.
• NDBC Station 41049 (South Bermuda). The offshore buoy about 50 nm south, ndbc.noaa.gov. Wind, pressure and wave height clear of island effects.
• NASA Worldview. Near-real-time satellite imagery over the island, worldview.earthdata.nasa.gov. Shows the cloud and convection the models are trying to predict — useful for spotting a line of showers before it reaches you.
• Royal Bermuda Yacht Club. Great Sound racing notices and any club anemometer data via rbyc.bm.

Background reading

• Wind Strategy (Houghton & Campbell). The science of why the wind does what it does.