US Marine Weather & Hurricanes

Prerequisites

• The basics of wind, pressure and fronts.
• Knowing what a knot and a nautical mile are.
• Being able to use a VHF radio to listen to a broadcast.

Learning Objectives

This lesson covers the weather part of the ASA 103 Coastal Cruising standard, for US waters. By the end of it you will be able to:

• Name the main National Weather Service (NWS) marine forecast products and where to get them.
• Explain what Small Craft Advisories, Gale, Storm and Hurricane Force Wind Warnings mean, in knots.
• Recognise a Special Marine Warning and act on it.
• Plan a cruise around the Atlantic hurricane season.
• Explain why wind against current makes the Gulf Stream dangerous.
• Prepare for summer fog on the West Coast and afternoon thunderstorms in the Southeast.

Who Forecasts US Marine Weather

The National Weather Service, part of NOAA, issues all official US marine forecasts and warnings.

• Local Weather Forecast Offices cover the coast and the Great Lakes.
• The Ocean Prediction Center (OPC) covers offshore and high-seas waters in the Atlantic and Pacific.
• The National Hurricane Center (NHC) in Miami issues tropical cyclone forecasts, and its Tropical Analysis and Forecast Branch covers tropical offshore waters.

Forecasts are organised into zones, each with a code such as ANZ050. Learn the zone codes for where you sail, because broadcasts and websites list forecasts by zone.

The Main Forecast Products

Always check the time and date of issue. NWS warns that linked data may not be the latest forecast.

How to Receive Forecasts

• NOAA Weather Radio: continuous broadcasts on the VHF WX channels (WX1 to WX7). Any marine VHF can receive them near the coast.
• VHF channel 22A: the Coast Guard announces urgent marine information and warnings on channel 16, then reads them on 22A.
• NAVTEX: run in the US by the Coast Guard. Stations broadcast routine forecasts four times a day, and warnings at any time, out to about 200 nautical miles from shore. There is no fee. There is no US NAVTEX on the Great Lakes, where the Canadian Coast Guard covers much of the area.
• Internet: weather.gov/marine, the NHC and OPC websites, and phone apps that show NWS zone forecasts.

Advisories and Warnings

NWS uses a ladder of advisories and warnings, based on sustained wind or frequent gusts.

Gale, Storm and Hurricane Force Wind Warnings are used when the wind is not directly linked to a tropical cyclone. For tropical systems, NHC issues Tropical Storm and Hurricane Warnings instead.

Small Craft Advisory: It Depends Where You Are

There is no single national threshold for a Small Craft Advisory. Each NWS region sets its own:

• Eastern (Maine to South Carolina, Lakes Erie and Ontario): 25 to 33 kt, with lower thresholds of 20 to 25 kt in some areas; or seas of 5 to 7 ft or more.
• Southern (Georgia to Texas, and the Caribbean): 20 to 33 kt, or seas of 7 ft or more for more than 2 hours.
• Central (Great Lakes from Minnesota to Ohio): 22 to 33 kt, or waves over 4 ft.
• Western (Washington to California): 21 to 33 kt, or waves over 10 ft or too steep.
• Alaska and Pacific islands: higher thresholds again.

"Small craft" has no exact size. For a 25- to 40-foot cruising sailboat with a new crew, a Small Craft Advisory is a strong reason to stay in port.

Special Marine Warning

A Special Marine Warning (SMW) covers short, violent events, usually lasting up to 2 hours:

• Thunderstorm winds or gusts of 34 kt or more.
• Hail of three-quarters of an inch or larger.
• Waterspouts.
• Squall lines and strong cold fronts with winds of 34 kt or more.

If you hear one for your area, reef or drop sail, get everyone into life jackets, and head for shelter or open water away from lee shores.

The Atlantic Hurricane Season

The Atlantic hurricane season runs from June 1 to November 30, with most storms from mid-August to mid-October. The Eastern Pacific season starts earlier, on May 15.

• Tropical storm winds are 34 to 63 kt. Hurricane winds are 64 kt or more.
• NHC's advice to mariners is clear: avoid the 34-knot wind field of a hurricane. Do not plan to sail through the edge of it.
• Older books teach the "1-2-3 rule": add 100, 200 and 300 nm to the 34-kt wind radius at 24, 48 and 72 hours to find the area to avoid. NHC no longer endorses it now that forecasts are more accurate. The current tool is NHC's Tropical Cyclone Danger Graphic, based on wind-speed probabilities.
• Charter companies in Florida and the Caribbean restrict or cancel bookings in hurricane season, and insurance policies often require the boat to be out of the hurricane zone or hauled out.

If a storm threatens, the best plan is to be somewhere else. Decide early, because marinas, haul-outs and hurricane holes fill up days in advance.

The Gulf Stream

The Gulf Stream is a strong, warm current that flows north along the US East Coast from Florida. Sailors crossing it to the Bahamas, or using it on the way north, must respect it.

• Wind against current: when the wind blows against a current, waves build quickly into steep, short, dangerous seas. In the Gulf Stream, a north wind blowing against the north-flowing current is the classic danger. Wait for settled weather with no northerly component before crossing.
• The current sets you north, so plan your heading to allow for it, or you will arrive well north of your destination.
• In winter, cold air flowing off the coast over the warm Stream can build large waves along its edge.

If you are caught in steep seas, BoatUS advises: don't take them head on, and don't put them on the beam. Slow down so you have enough power to steer but not too much speed, and take the waves just off the bow.

West Coast Fog

In summer, the US West Coast, and San Francisco Bay above all, gets thick fog. Cool ocean water chills the air above it into a marine layer of low cloud and fog, often a few hundred to 2,000 feet deep. As the land heats up, strong westerly afternoon winds pull the fog through the Golden Gate.

• Fog can arrive in minutes and is common on summer mornings and evenings.
• Expect strong afternoon winds in the Bay, often 20 kt or more.
• Carry radar or AIS if you can, sound fog signals, and know your position at all times.

Afternoon Thunderstorms in the Southeast

In summer, Florida, the Gulf Coast and the Chesapeake Bay often see afternoon and evening thunderstorms. In Florida, the sea breeze pushes inland during the day and triggers storms as it meets the land heating. NWS forecasts often say "isolated" storms in the morning, becoming "scattered" in the afternoon.

• Plan to be at anchor or in harbour by early afternoon in summer.
• Watch for towering cumulus clouds building in the heat of the day.
• A storm brings sudden, strong gusts, often from a new direction, plus lightning and heavy rain that cuts visibility.

Worked Example: Planning a Day Sail in Chesapeake Bay

It is July. You plan a day sail from Annapolis.

1. At 0700 you check the Coastal Waters Forecast for your zone: southwest 10 to 15 kt, waves 1 to 2 ft, scattered thunderstorms after 2 pm.
2. There is no Small Craft Advisory, so conditions are fine for your crew.
3. You plan to be back on the mooring by 1400, before the storms.
4. At 1300 you hear a Coast Guard notice on channel 16 and switch to 22A: a Special Marine Warning for gusts to 40 kt. You drop sails and motor in.
5. The squall hits at 1340 while you are tied up. A late start or an afternoon sail would have put you in it.

Common Mistakes

• Reading a forecast without checking when it was issued.
• Assuming "Small Craft Advisory" means the same thing in every region.
• Crossing the Gulf Stream with a north wind because "it's only 15 knots".
• Planning a charter in the Caribbean in September without hurricane cover.
• Ignoring a Special Marine Warning because the sky is clear where you are.
• Still teaching the 1-2-3 rule as the current NHC method.

Summary

• The NWS issues US marine forecasts by zone: Coastal Waters, Offshore Waters and High Seas.
• Get them by NOAA Weather Radio (WX channels), VHF 16 and 22A, NAVTEX, or online.
• Small Craft Advisory is about 20 to 33 kt, depending on region. Gale is 34 to 47 kt, Storm is 48 to 63 kt, Hurricane Force is 64 kt or more.
• A Special Marine Warning means a short, violent event with 34 kt or more. Act at once.
• The Atlantic hurricane season runs June 1 to November 30. Avoid the 34-kt wind field.
• Never cross the Gulf Stream with a north wind. Expect summer fog in San Francisco and afternoon thunderstorms in the Southeast.

Check Your Understanding

1. What wind range does a Gale Warning cover? 34 to 47 knots.
2. Is a Small Craft Advisory the same everywhere in the US? No. Each NWS region sets its own wind and sea thresholds.
3. You hear a Special Marine Warning for gusts of 45 kt. What should you do? Reef or drop sail, put on life jackets, and head for shelter or safe water.
4. When does the Atlantic hurricane season run? June 1 to November 30.
5. Why is a north wind dangerous in the Gulf Stream? It blows against the north-flowing current and builds steep, dangerous seas.

Sources

• https://www.weather.gov/marine/cwd
• https://www.weather.gov/marine/gmdss
• https://www.weather.gov/marine/offtafbmz
• https://www.nhc.noaa.gov/aboutgraphics.shtml
• https://www.nhc.noaa.gov/prepare/marine.php
• https://www.nhc.noaa.gov/marinersguide.pdf
• https://www.boatus.com/app/views/201812/ocean-wave-basics.asp
• https://www.weather.gov/tbw/SB_Methodology