The Newsletter of the Buffalo Forecast Office
Page 6
Forecasting Wind and Waves on the Great Lakes --
The Importance of Local Effects
maximum speeds during the late afternoon and may be 5 to 10
Whether you are planning a leisure boat ride or looking to wind
knots faster and opposite in direction to the winds well off-
surf the waves, it pays to be ready for what you are likely to
shore. This can result in higher waves right along the lakeshore,
encounter when you go out on the water. The National
while conditions in the middle of the lake may be calm. The lake
Weather Service in Buffalo prepares marine forecasts for the
breeze circulation will remain intact until the land begins to cool
nearshore waters (within 5 nautical miles of shore) of Lake Erie
around sunset.
and Lake Ontario and the open waters of Lake Ontario. These
forecasts are an excellent way to get an outlook for the condi-
A less familiar, but important influence on the wind speed over the
tions that are likely to develop over the lakes. However, the
lakes is a concept knows as atmospheric stability. Atmospheric
wind and waves that you actually experience at your site are
stability is related to how the temperature changes at different
often the result of local interactions between the water and the
heights in the atmosphere. For example, during the Spring and
surrounding land. These interactions can be quite complex and
early Summer, a layer of warmer air typically lies over the colder
may result in wind and wave conditions that vary considerably
lake surface. This layer of warmer air above the colder water cre-
from those that were forecast. Therefore, one of the biggest
ates a stable temperature profile known as a marine inver-
challenges for the marine forecaster is to determine whether
sion. The marine inversion acts like a lid which prevents stronger
these local influences will be strong enough to overcome the
winds aloft from mixing down to the lake. This can result in rela-
prevailing winds. Since the waves on the lake are primarily
tively calm winds and waves on the lake, even though it may be
driven by the winds, an accurate wind forecast is essential for
quite breezy a few miles inland. These calm conditions are ideal
an accurate forecast of the waves.
for boaters, but can be a source of frustration for sailors or wind
surfers looking for a bigger thrill.
During the late Summer and Fall, the opposite situation occurs
when colder air from Canada begins to flow over the warmer
lakes. The warmer air at the lake surface has a tendency to rise
and be replaced by the colder air aloft. This unstable temperature
profile results in greater mixing which allows stronger winds aloft
to be transported down to the lake surface. This will produce gust-
ier winds and higher waves on the lake, especially during the early
morning hours when the temperature difference between the lake
and the colder air is often greatest. The point to emphasize with
atmospheric stability is that it can weaken or strengthen the pre-
vailing winds in a way that may take boaters by surprise.
Finally, there is the impact of thunderstorms on the wind and
waves. The Great Lakes, on average, experience 30 to 40 days
with thunderstorms each year. These storms can produce strong
wind gusts, very heavy rain, frequent lightning and possible hail.
These threats can come upon you quickly and unexpectedly, es-
pecially on days where there is a lot of haze, making it difficult to
see developing storms. Thunderstorm downdrafts can generate
Local topographical features such as the shape and steepness
high waves even when the main storm is several miles away from
of the shoreline, the presence of islands or cliffs, or your loca-
you. Therefore, boaters should always be alert to sudden
tion along a peninsula can all have a significant impact on alter-
changes in wind direction and speed on days when thunderstorms
ing the prevailing winds. For example, Lake Erie gets narrower
are forecast. Boaters on relatively shallow lakes, such as Lake
at its eastern end. This causes westerly winds to channel and
Erie, are especially susceptible to the dangers posed by thunder-
speed up, resulting in higher waves. This is especially true dur-
storms since the waves respond to changes in the wind more rap-
ing the late Fall when strong gales can generate high waves
idly.
along the eastern shores of the lake. Along the eastern
stretches of Lake Ontario, the presence of cliffs and bluffs can
Hopefully, this brief article has given you a little better understand-
cause the wind to change direction, which can result in choppy
ing of how challenging it can be to forecast the wind and waves on
water near the shoreline. Islands and peninsulas act as obsta-
the Great Lakes. However, if you are aware of your local condi-
cles to the wind which can change its direction and speed.
tions and understand how they can influence the prevailing winds,
Another influence familiar to boaters and residents near the
you should be able to fine tune the marine forecast so that you
lakes is the development of the lake breeze. On warm sunny
can anticipate some of the changes that may develop on the
days, the land heats up more quickly than the surrounding wa-
lakes.
ter. The resulting difference in temperature between the lake
and the land generates a wind circulation where cooler air
Tom Paone, Forecaster, Marine Focal Point
above the lake moves toward the shore and replaces the rising
air over the warmer land. The lake breeze tends to reach its