Seasoned boaters know first-hand that weather conditions on the Great Lakes can change from pleasant to terrifying very quickly. From dangerous lightning to high winds, thunderstorms can ruin an outing in many ways. However, reading the marine forecast before leaving the dock and regularly checking weather radar along the way will dramatically increase your chances of avoiding uncomfortable or dangerous conditions.

A Wide Range of Marine Weather Forecasting Products
Each day, the forecasters at your local National Weather Service Weather Forecast Office (WFO) produce a wide range of text products designed to keep you safe on the water. Your local WFO is also responsible for issuing marine weather watches and warnings when severe weather threatens.

There are many NWS text products that are important to recreational boaters:

• Nearshore Marine Forecast: the wind, wave, and general forecast for waters within five nautical miles of shore.
• Offshore Marine Forecast: the wind, wave, and general forecast for waters beyond five nautical miles of shore.
• Area Forecast Discussion (AFD): the marine section of an AFD contains the forecaster’s analysis of the short-term weather pattern and their reasoning behind the published forecast. It is written in plain language for recreational boaters.
• Hazardous Weather Outlook (HWO): a description of the potential for severe weather in the forecast area for the current and next day. It is written in plain language for the general public. In contrast to the cryptic phrase Showers and thunderstorms likely that regularly appears in the marine forecast during the summer, the HWO offers a detailed description of where, when and what type of severe weather is expected.
• Small Craft Advisory (SCA): while criteria vary across WFO’s, this warning product is issued when wind and wave conditions are expected to be hazardous for small boat operators. When it comes to SCAs, we all operate a small craft.
• Marine Weather Watch: a Watch is issued when conditions are favorable for the development of severe weather. You should remain alert for rapidly changing weather conditions during a Watch.
• Marine Weather Warning: a Warning is issued when dangerous conditions are occurring or imminent. A Warning means immediate action is required to protect you and your passengers.
• Gale Warning: sustained winds, or frequent gusts, ranging from 34 to 47 knots.
• Storm Warning: sustained winds, or frequent gusts, ranging from 48 to 63 knots.

All of these products are readily available on each WFO’s home page. Many are also available on the weather band of your VHF radio, mobile apps, and SiriusXM Marine.

In addition to their suite of text products, the National Weather Service Great Lakes Portal website provides access to marine weather forecasts in a graphical format. Images showing predicted wave heights, wind speed, wind gusts, water temperature, and precipitation type are available five days into the future.

Doppler Weather Radar Is Your Best Resource
The most effective resource for monitoring the location, intensity, evolution, and movement of thunderstorms is Doppler weather radar. Doppler weather radar is a land-based technology with an effective range of 120 nautical miles. The National Weather Service (NWS) and Environment Canada (EC) operate several Doppler weather radar stations that service the Great Lakes.

By emitting and capturing reflected energy pulses, radar stations collect a wealth of useful information about a thunderstorm, such as its overall size, height, strength, and evolution over time. In addition, by detecting the Doppler shift, the station can determine the thunderstorm’s speed and path.

Reflectivity—The Most Common Type of Imagery
The most common and readily available type of radar imagery is reflectivity (Figure 1). Reflectivity, measured in dBZ, is the amount of energy that is reflected by raindrops, hailstones, and other particles within the thunderstorm.

Figure 1: Base reflectivity from NWS Milwaukee at 9:37 pm on May 2, 2018.

Values of dBZ vary dramatically based on the size, number, and type of particles in the storm. Less than 20 dBZ typically indicates mist, dust, insects, and other small particles. Extremely small particles, such as cloud droplets, are too small to be detected by the station. As a result, clouds and early-stage thunderstorms (before they contain raindrops or hail) do not appear on NWS radar imagery. Reflectivity values associated with rainfall usually range from 20 dBZ to 50 dBZ, depending on the number and size of the raindrops. Hail is usually present when dBZ values are greater than 50.

Reflectivity imagery (particularly an animation), is used to assess the location, size, intensity and overall movement of individual thunderstorms, thunderstorm clusters, squall lines, tornados, and waterspouts.

Getting Data Underway
There are many websites and smartphone/tablet applications that provide access to weather observations, forecasts, and Doppler weather radar. For boaters, the challenge lies in access to this vital data when their journey takes them beyond the range of wi-fi and cellular internet. (Cellular internet is typically unavailable beyond 5 to 7 nautical miles from shore.)

SiriusXM Marine (SXM) overcomes the limitations of cellular internet by delivering a wide variety of NWS and EC products such as weather observations, forecasts, and Doppler weather radar directly to your boat’s chartplotter via satellite.

Figure 2: Thunderstorms across northern Lake Huron on a Garmin chartplotter.

Overlaying reflectivity radar imagery directly on your chartplotter improves situational awareness by allowing you to view a chart, navigational aids, approaching thunderstorms, and your vessel’s location on a single zoomable image (Figure 2). The SXM transmission also includes cloud-to-ground lightning strikes and detailed thunderstorm attributes, such as speed and direction of movement, storm height, and maximum value of dBZ. When a severe thunderstorm or marine weather warning is issued, the area covered by the warning and the text of the warning are also available directly on your screen. SiriusXM Marine is a good investment if monitoring the weather and avoiding an unexpected encounter with thunderstorms is your goal.

Thanks to improvements in weather forecasting products, navigational equipment, and communication technology, it has never been easier to access marine forecasts and monitor thunderstorms before and during a trip on the water.