The Gulf Stream and Offshore Forecasts

Prerequisites

• The US forecast system, advisories and the basics of hurricanes, from US Marine Weather & Hurricanes.
• Reading isobars, fronts and highs and lows on a surface chart.
• Using VHF and registering a beacon, from US VHF Radio & Distress Calls.

Learning Objectives

This lesson covers the offshore weather part of the ASA 108 Offshore Passagemaking standard, and supports ASA 106 Advanced Coastal Cruising for sailors making their first overnight passages off the US East Coast. By the end of it you will be able to:

• Find and read the Ocean Prediction Center's surface analyses, forecast charts and wind and wave charts.
• Tell the difference between coastal, offshore and high seas forecasts, and know who issues each.
• Use NOAA's Gulf Stream products to plan a crossing to the Bahamas or Bermuda.
• Explain why wind against the Gulf Stream is dangerous and choose a crossing window.
• Use National Hurricane Center products to plan around the hurricane season.
• Get forecasts once you are beyond phone and VHF range.

Who Forecasts Offshore Waters

Close to the coast, local Weather Forecast Offices issue Coastal Waters Forecasts, usually four times a day. Further out, two national centres take over:

• The Ocean Prediction Center (OPC), part of the National Weather Service, covers offshore and high seas waters of the North Atlantic and North Pacific.
• The National Hurricane Center (NHC) in Miami, through its Tropical Analysis and Forecast Branch, covers tropical waters, including the Gulf of America, the Caribbean and the southwest North Atlantic around the Bahamas. It also issues all Atlantic tropical cyclone forecasts.

The offshore and high seas forecasts are text products, issued four times a day. They describe where each low, high and front is, then give winds and seas zone by zone. Seas are given as significant wave height: the average of the highest third of the waves. Individual waves can be much bigger.

OPC Charts

Text tells you what to expect; charts tell you why. OPC publishes a set of Atlantic charts at ocean.weather.gov:

• Surface analysis: the current position of highs, lows, fronts and isobars, for the West Atlantic, the whole Atlantic and the East Atlantic.
• Surface forecasts at 24, 48, 72 and 96 hours: where those systems are expected to go.
• Wind and wave analysis and forecasts: wind barbs and significant wave height contours, again out to 96 hours.
• Wave period and direction forecasts: useful for judging swell, which matters as much as height.
• 500 mb charts: the upper-air pattern that steers surface systems. A deep trough at 500 mb often means a developing low beneath it.
• Sea state analysis: a hand-drawn chart, updated every 3 hours, showing analysed wave height in 3-foot contours with ship and buoy reports.

How to use them together: read the latest surface analysis first, then step through the 24, 48, 72 and 96 hour forecasts to see how the pattern evolves over your passage. Check the wind and wave forecasts for the times you will be in each area. Then read the text forecast for the zones on your route, because it contains the warnings.

Compare the OPC charts with computer model data (GRIB files, covered in Ocean Routing and Weather Routing). The OPC charts are drawn by forecasters who have already weighed up the models. If the GRIB and the OPC chart disagree, trust the forecaster.

The Gulf Stream

The Gulf Stream is a warm, fast current that flows out of the Florida Straits and north along the US East Coast, then turns north-east away from the coast around Cape Hatteras.

• NOAA gives its average speed as about 4 mph (about 3.5 knots), with surface speeds reaching about 5.6 mph (nearly 5 knots) at its strongest.
• It slows to about 1 mph as it widens further north.
• Its position is not fixed. North of Hatteras it meanders, and it throws off warm and cold eddies that can have their own strong currents, tens of miles from the main stream.
• The edges, especially the western or northern wall, can show a sudden change in water temperature and sea state.

NOAA position products. OPC publishes Gulf Stream analyses at ocean.weather.gov/gulf_stream.php, made from US Navy (Naval Oceanographic Office) data. They are composite charts of the analysed Stream position, and they are usually updated three times a week, not at weekends or on holidays. Download the latest one before you leave and plot the Stream's edges and any eddies on your passage plan. Satellite sea surface temperature images from NOAA are also useful for spotting eddies.

Using the current. Two knots of favourable current adds about 48 miles to a day's run; two knots of foul current takes the same away. Plan your crossing point and angle so you spend as little time as possible in foul current, and use the main stream where it helps.

Wind Against Current

When wind blows against a strong current, the waves get shorter, steeper and higher. In the Gulf Stream, which flows north, any wind with a northerly component builds dangerous seas, sometimes breaking, even in moderate wind. A wind that would give an easy sail in the open ocean can make the Stream frightening in a few hours.

The rules for a crossing:

• Never cross with a northerly, and be wary of a forecast that shows a front arriving with a northerly shift before you are across.
• Southerly or light winds, or winds from the east or west, give the best conditions.
• A cold front passing over the Stream in winter can also bring very rough seas along the edge, as cold air meets the warm water.
• Allow for the set. A boat making 6 knots across a 3-knot current is set north a long way. Aim well south (up-current) of your destination.

Crossing Windows: Bahamas and Bermuda

Florida to the Bahamas. The crossing from South Florida to Bimini or the Grand Bahama Bank is short, but it is all in the strongest part of the Stream. Sailors wait for a settled window with no northerly, often after a front has passed and the wind has gone round to the east or south-east and dropped. Leave early so you arrive on the banks in daylight to see the coral heads.

East Coast to Bermuda. A passage from the Chesapeake, New England or North Carolina crosses the Stream far out, where it meanders. Plan to:

• Check the latest Gulf Stream analysis and the OPC charts for the timing of fronts.
• Cross the Stream where its axis is narrowest or where an eddy helps, and avoid crossing as a cold front arrives.
• Pick a window of at least the passage length plus a margin. Bermuda is several days away for most cruising boats, so the forecast you leave on will not be the forecast you arrive in.

Hurricane Season and NHC Products

The Atlantic hurricane season runs from June 1 to November 30. The basics, including wind strengths and why the old 1-2-3 rule has been retired, are in US Marine Weather & Hurricanes. For offshore passages, know these NHC products:

• Tropical Weather Outlook: issued four times a day from May 15 to the end of the season. It shows areas that might develop, with the chance of formation in the next 2 days and the next 7 days.
• Advisories and the forecast cone: once a storm forms, NHC issues forecasts of track and strength. The cone shows the probable track of the centre. It is sized so the centre stays inside about two-thirds of the time, based on recent forecast errors. Strong winds and big seas often reach well outside it.
• Tropical Cyclone Danger Graphic: the area mariners are advised to stay out of, based on wind-speed probabilities.
• Offshore and high seas forecasts carry hurricane and tropical storm warnings for each zone.

Planning around the season: most offshore deliveries between the US, the Caribbean and Bermuda are timed for before June or after November. If you sail in season, keep the passage short, watch the Tropical Weather Outlook daily, and have a bolt-hole plan for each leg.

Getting Forecasts Offshore

Beyond VHF and phone range, you need another link:

Choose at least two independent methods. A satellite unit can fail, and an HF radio needs a licence (see the VHF lesson) and practice.

Your 406 MHz Beacon

Every offshore boat should carry a 406 MHz EPIRB, and each crew member a PLB if possible. In the US, registration with NOAA is required by federal law and is free, at beaconregistration.noaa.gov. Registration lasts two years: NOAA sends a reminder, but it is your job to renew it and update it when your boat, phone numbers or emergency contacts change. Before an offshore passage, add the trip details and crew list so rescuers know who they are looking for.

Worked Example: A Window to the Bahamas

You plan to sail from Fort Lauderdale to Bimini in a 36 ft sloop in March.

1. Monday. A cold front is forecast to pass on Tuesday. The OPC surface forecast shows a high building behind it.
2. Tuesday. The front passes with a north-westerly, then a northerly. The Stream is rough; you stay put.
3. Wednesday. The coastal and offshore forecasts give north-east 15 knots, easing. The 48-hour wind and wave forecast shows seas dropping.
4. Thursday. The forecast is east to south-east 10 knots, seas 2 to 3 ft, no front for three days. That is your window.
5. Plan. The current runs north at up to a few knots in the middle of the Stream. You aim well south of Bimini and leave before dawn so you arrive in good light.
6. Before leaving. You download the latest Gulf Stream analysis and the Tropical Weather Outlook (quiet in March), check the EPIRB registration is current, and file a float plan.

Common Mistakes

• Crossing the Gulf Stream with any northerly in the forecast.
• Ignoring the set and arriving miles north of the destination, down-current.
• Using a Gulf Stream chart from last week; the stream and eddies move.
• Trusting a GRIB file over the OPC forecast, or near a tropical storm.
• Treating the hurricane cone as the danger area, when the winds reach well beyond it.
• Leaving with only one way to get forecasts at sea.
• Forgetting to renew the beacon registration every two years.

Summary

• Local offices issue coastal forecasts; OPC and NHC issue offshore and high seas forecasts.
• Read OPC surface analyses and 24 to 96 hour forecasts, wind and wave charts and 500 mb charts together, then the text for warnings.
• The Gulf Stream averages about 3.5 knots and can approach 5. Use NOAA's position analyses, updated about three times a week.
• Never cross the Stream with a northerly. Wait for a settled window and allow for the set.
• The hurricane season runs June 1 to November 30. Use the Tropical Weather Outlook, the cone and the Danger Graphic.
• Have at least two ways to get forecasts offshore. Radiofax may be ending; satellite is now the main route.
• Register your 406 MHz beacon with NOAA and renew it every two years.

Check Your Understanding

1. Which NWS centre issues offshore and high seas forecasts for most of the North Atlantic? The Ocean Prediction Center, with the National Hurricane Center covering tropical waters.
2. What wind direction makes the Gulf Stream most dangerous, and why? Any northerly, because it blows against the north-flowing current and builds short, steep, breaking seas.
3. How often does OPC usually update its Gulf Stream analysis? About three times a week, not on weekends or holidays.
4. What does the NHC forecast cone show? The probable track of the storm's centre, which stays inside it about two-thirds of the time. Hazards can extend well outside it.
5. How long is a NOAA beacon registration valid? Two years. It must then be renewed, and it should be updated whenever details change.

Sources

• https://ocean.weather.gov/gulf_stream.php
• https://ocean.weather.gov/Atl_tab.php
• https://ocean.weather.gov/product_description/abtrssa.php
• https://oceanservice.noaa.gov/facts/gulfstreamspeed.html
• https://www.weather.gov/marine/forecast
• https://www.weather.gov/marine/uscg_broadcasts
• https://www.weather.gov/media/notification/pdf_2026/pns26-48_commentsHF__RadioFax_Broadcast.pdf
• https://navcen.uscg.gov/node/228
• https://www.nhc.noaa.gov/aboutcone.shtml
• https://beaconregistration.noaa.gov/RGDB/faq