The National Weather Service (NWS) provides mariners with continuous broadcasts of weather warnings, forecasts, radar reports and buoy reports over VHF-FM and SSB radio for the Great Lakes. Reception range for VHF is usually up to 40 miles from the antenna site. There are almost no areas on the inland waterways where a good quality, fixed-mount VHF cannot pick up one or more NOAA broadcasts.

Most U.S. weather broadcasts are made on WX-1 (162.55 MHz), WX-2 (162.40 MHz) and WX-3 (162.475 MHz), although four other frequencies are also used for weather broadcasts. Broadcasts are continuous and are updated hourly. Urgent weather warnings may be broadcast at any time as needed.

The U.S. Coast Guard also broadcasts special forecasts and warnings on VHF-FM. The broadcasts first will be announced on VHF Channel 16, with the actual forecast usually being given on Channel 22A.

Weather forecasts heard on the Canadian VHF channels present land conditions in metric, while nautical conditions are provided in knots. For ease of conversion, a 10-kilometer windspeed is roughly equivalent to 6 mph, or 5.4 knots. Also, each forecast is given in alternating French and English.

Marine Forecast (MAFOR) Codes 

In addition to "plain language" weather information on Great Lakes Forecasts, the National Oceanic and Atmospheric Administration (NOAA) also uses a coded marine forecast called MAFOR. The code begins with a date/time group, followed by the name of the lake to which the forecast applies, followed by one or more five-figure codes. The MAFOR code is not an exact duplicate of the plain-language forecast issued in the Open Lake Forecasts for each lake. Forecasters may issue the MAFOR either for an entire lake or for a segment of a lake. It will reflect the predominant conditions over that area for the 24-hour period. 

Following are groups 2 through 4, representing time, wind direction, wind force in knots and weather forecast, respectively. (There are also groups for Visibility and State of Sea.)

Note that each group number is always preceded by the number 1, which is Group 1 (e.g., 12646, 14755). It is wise to write the numbers as you hear them on VHF, then decipher the code later. Higher numbers often imply unfavorable conditions.

Group 2—Time or Period of the Forecast

0: conditions at beginning of forecast

1: valid for 3 hours

2: valid for 6 hours

3: valid for 9 hours

4: valid for 12 hours

5: valid for 18 hours

6: valid for 24 hours

7: valid for 48 hours

8: valid for 72 hours

9: occasionally 

Group 3—Wind Direction

0: calm

1: northeast

2: east

3: southeast

4: south

5: southwest

6: west

7: northwest

8: north

9: variable

Group 4—Wind Force (knots) based on Beaufort Scale

0: 0-10

1: 11-16

2: 17-21

3: 22-27

4: 28-33

5: 38-40

6: 41-47

7: 48-55

8: 56-63

9: 64 and above

Group 5—Weather Forecast

0: good to moderate visibility

1: risk of ice (23 to 32 degrees F)

2: strong risk of ice (below 23 degrees F)

3: mist

4: fog

5: drizzle

6: rain

7: snow, or rain and snow

8: squalls with or without rain

9: thunderstorm

Example:

MAFOR for Lake Superior: 12646, 14755 

Forecast valid for the first six hours, wind west at 28 to 33 knots, with rain. Next 12 hours, winds northwest 34 to 40 knots, with drizzle.

The NWS also produces a Great Lakes Marine Alert Message (MAW) whenever storm force or greater winds are included in any portion of any Great Lakes forecast. The MAW will be produced no later than 30 minutes after such an indicator is detected.

You can find the MAFOR weather forecasts at www.nws.noaa.gov/os/marine/gtlakes.htm. Note that MAFOR weather forecasts are no longer broadcast for Georgian Bay and the North Channel. If you still wish to receive them, you must now contact the Canadian Coast Guard in these areas.

Weather Patterns

The general Great Lakes weather pattern is summertime high-pressure systems of fairly long duration, punctuated by lows coming through from the southwest. Air temperatures are cooler on the upper lakes than on the lower lakes, yet on bright, sunny days, the temperature occasionally climbs into the 90s. Daytime temperatures average in the 70s, with nights in the low 50s. Bear in mind that these are land temperatures; it is colder on the water. It is possible to have chilly days and downright cold nights anywhere on the lakes. Rainfall varies from season to season, but July and August tend to be the driest months. Thunderstorms during the summer leap up quickly, so keep a careful weather watch. Given the right conditions, any one of the lakes can build up a steep, uncomfortable chop with lightning and even waterspouts in what seems like no time; waves as high as 25 feet were recorded on Lake Michigan during Hurricane Sandy in 2012. In October 2014, lake buoys recorded a 21.7-foot wave height during a fall storm that brought the first snow of the year.

Seiche

The weather phenomenon called a "seiche" (from a French word meaning "to sway back and forth") can occur on the lakes without warning. Variations in air pressure, where lows move rapidly across the lake, sometimes aided by strong winds create changes in water level, which bounce from the eastern shore back to the western shore, somewhat like water splashing in a bathtub. The result is a strong "tidal" wave that may raise or lower water level by several feet within minutes. Large seiches greater than 3 feet can occur in the Great Lakes, sometimes causing boat groundings. Lake Erie is most prone to seiches due its shallow depth and its basin orientation toward prevailing storms, but records show that a seiche has occurred on all five of the Great Lakes. The point is that seiches can and will occur, and you must prepare yourself to handle them.

Fog

Between late spring and late fall the shore can be shrouded in fog when the land surrounding the lakes heats up much warmer than the water. This warm season fog occurs when moisture in the warm air condenses as it flows over the cold lake. Like ocean fog, lake fog can descend suddenly and last for days. Duluth, Minnesota, has an average of 52 days of heavy fog each year. Although most likely to roll in late at night or in the early morning, and burn off by mid-day, fog can occur at any time and any place.