Five questions for: Meterorologist Bryan Tilley

This year, we noticed a change to the NOAA marine forecasts. Instead of the familiar "waves 4 to 7 feet," they routinely added "occasionally to 10 feet."

We wondered when this change was made and why. For answers we turned to meteorologist Bryan Tilley, who works on daily weather forecast production; severe weather watches, warnings, and advisories; and marine forecasts and Doppler radar programs for the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS) in Detriot/Pontiac, Michigan.

Bryan Tilley, NOAA/NWS meteorologist. Photo credit: NOAA

HKWT: When was this change made, and why?

BT:  The change you noticed in the Great Lakes wave forecasts was made at the Detroit/Pontiac office in October 2010 after collaboration with the U.S. Coast Guard and commercial shipping interests in the region. Not all NWS offices in the Great Lakes have made this change, as it remains under development. Chicago is the only other office using the terminology. The idea is to better represent the spectrum of wave conditions in a given weather pattern rather than just the significant wave height.

HKWT: What does "occasionally" mean in this context?

BT: Forecasts for Lake Huron, Lake St. Clair, and the Michigan waters of Lake Erie include a "wave" forecast and a "max wave forecast" when the max wave will be five feet or greater. The Chicago office uses "occasionally" to mean the same as "max wave." Our Science Officer developed the methodology and offers the following background: 

Assuming a Rayleigh distribution to the wave spectra (which works well in the Great Lakes), the maximum wave height (1/20th wave) is approximately 147% of the significant wave height (the average of the highest 1/3 waves in the spectrum). As an example, if the significant wave height is 5.5 feet with a dominant period of 10 seconds, the 1/20th (max wave) will be 8 feet and will be observed at any given location roughly every 3 1/2 minutes.

The Rayleigh distribution. Illustration credit: NOAA

HKWT: Has the method of prediction also changed in any way?

BT: The method of prediction has not changed. Waves on the waters of the Great Lakes are simulated with a model based on wind and temperature input by meteorologists. The wave spectrum data is part of the simulation and we are now including it in the forecast wording. 

National Weather Service office in Detroit/Pontiac.

HKWT: What do you hope this change will achieve?

BT: We hope the presentation of the max wave data will give forecast users a chance to evaluate the potential worst case scenario in a given weather pattern for the day. It adds a layer of probabilistic data to the forecast that, hopefully, aids in decision making.

HKWT: When we paddled in the San Juan Islands, we looked at synoptic charts to get a better idea of what weather patterns to expect. Do we have access to those, or to something similar, here? 
BT: NOAA/NWS provides many options on weather maps for individual access. You may find the following links useful:

• Synoptic Maps Produced by NWS Meteorologists
• Computer Model Output