Lake Erie Displacement

December 23-24, 2007

Introduction
The Great Lakes are a important natural resource and a study in contrasts. The largest, Lake Superior, contains approximately 3,000 cubic miles of water, has an average depth of 500 feet (1,332 feet at its deepest) and nearly 2,800 miles of shoreline. Its capacity is sufficient to hold all of the water in the other Great Lakes plus three additional Lake Eries. At the other end of the spectrum is Lake Erie, which holds a mere 199 cubic miles of water, has an average depth of 62 feet (210 feet at its deepest) and 871 miles of shoreline.

The variations in the depth of Lake Erie, as shown on the bathymetry chart (above), are tremendous. In contrast with the deep northeastern basin, Erie's extreme western basin is quite shallow. In addition to being a wonderful recreational resource, Lake Erie's role as a commercial shipping byway is a vital component of the region's economic health. The areas near the mouth of the Detroit River, gateway to the upper lakes, and the port of Toledo, Ohio are located in the lake's shallow western end and are therefore highly susceptible to short-term fluctuations in water levels. Precipitous declines in water level cause expensive delays for commercial shippers and ruin outings by leaving the family boat high and dry.

Short-term fluctuation in water level are the result of the combination of the southwest to northeast orientation of the lake, the western basin's shallow nature, and the prevailing weather conditions. The profile of Lake Erie published by The Great Lakes Information Network (GLIN) indicates the record difference in the water level from the west end of the lake to the east end was an astounding 16 feet! Even at levels far below this record, fluctuations in water levels of just a few feet present hazards to both commercial and recreational mariners.

Displacements and Seiches
The most significant and long-lasting water level fluctuations, referred to as displacements on Great Lakes forecast graphics, occur in response to strong, prolonged winds blowing across the surface of the lake. Particularly noteworthy displacements occur when the wind is aligned along the long axis of the lake, southwest to northeast in the case of Lake Erie.

Wave heights grow in response to areas of high and low pressure in the trough of the wave. Graphic from The Comet Program.

The process of displacement begins on a very small scale as capillary waves (wavelengths less than 1.7 centimeters) form in response to the transfer of wind energy to the water's surface. Although capillary waves form, scientists are uncertain as to the precise dynamics of how these small ripples form on an otherwise smooth surface.

If the wind persists, particularly if its speed increases, form and frictional drag start to promote the development of larger waves. In contrast to the small capillary waves which are restrained by the surface tension of the water, the restoring force of waves having larger wavelengths is gravity.

As shown in the graphic to the left, the height and steepness of the waves both promote, and increase in response to, the development of localized areas of low and high pressure in the wave's trough. The area of high pressure on the windward side helps to lift the leading edge of the wave's crest, while the area of low pressure on the leeward side works to deepen the trough. These pressure patterns in the wave's trough help sustain and increase wave heights.

Of course, the overall height of waves is restricted by the combined interaction of wind speed, duration of the wind, depth of the water in which the waves form and distance over which the wind and water interact, known as fetch. A nomogram for determining the approximate wave height from these variables may be found here. If these wave development variables favorably interact for a sufficient period, a considerable volume of water can be moved from one end of the lake to the other.

In the case of Lake Erie, the shallowness of the western basin is exacerbated while flooding occurs at Buffalo, New York located at the lake's northeastern tip. A demonstration of this process can be viewed on this graphic from the University of Wisconsin Sea Grant Institute.

A seiche (pronounced "saysh") is defined by the National Weather Service (NWS) as a "standing wave oscillation of water in large lakes usually created by strong winds and/or a large barometric pressure gradient." In the Great Lakes, the term seiche is used generically to describe an increase in water level along a shore, referred to here as a displacement. While this paper will adhere to the strict definitions, seiches and displacements are closely related. As the force of displacement-creating wind relaxes, its ability to maintain the unnatural distribution of water in the lake falters and gravity steps in to restore order. However, the level of the lake does not immediately reach equilibrium, instead it sloshes back and forth - a seiche -- while gravity progressively dampens the oscillations. Depending upon the magnitude of the displacement, the seiche -- although steadily weakening -- may persist for a day or two.

A Perfect Setup
The weather forecast issued early on December 23, 2007 suggested that conditions would be perfect for the development of a significant displacement of water later in the day and into Christmas Eve.The HPC Daily Weather Map for December 23, 2007 (below) showed a low pressure system centered over Lake Superior with its attendant cold front reaching into the Gulf of Mexico.

The forecast predicted that this system would become more powerful while it lingered over the upper Great Lakes for a couple of days. The system's cold front was expected to pass over Lake Erie early on December 23, veering the winds from southeasterly to southwesterly direction. The NWS forecast for the open waters of Lake Erie issued at 4:19am EST included a warning regarding storm force winds and the following prediction for the wind and wave conditions:

.TODAY...SOUTH GALES TO 35 KNOTS BECOMING SOUTHWEST AND INCREASING TO
STORM FORCE WINDS TO 50 KNOTS BY AFTERNOON....WAVES 5 TO 7 FEET
BUILDING TO 10 TO 13 FEET.
.TONIGHT...SOUTHWEST STORM FORCE WINDS TO 50 KNOTS DIMINISHING TO
GALES TO 45 KNOTS OVERNIGHT... WAVES 11 TO 15 FEET.
.MONDAY...WEST GALES TO 40 KNOTS DIMINISHING TO 30 KNOTS IN THE
AFTERNOON. ... WAVES 9 TO 12 FEET.

In addition to the marine forecast, the NWS issued a Low Water Advisory at 5:00am EST on the 23rd cautioning mariners of the likelihood that water levels in the western basin would continue to drop, resulting in a potentially significant hazard to safe navigation.

The series of forecast guidance images above were produced by the Lake Erie Operational Forecast System (LEOFS) and predict the displacement of water from one area of the lake to another from 17 EST on December 23, 2007 through 11 EST on December 24, 2007 (each of the images in the time series can be displayed by rolling your mouse over the valid time of the forecast). At the beginning of the forecast period -- December 23 at 17 EST -- the water level at Toledo was anticipated to be 2.5 feet below and 2.5 feet above low water datum at Buffalo. Based upon the LEOFS guidance, the displacement was expected to reach its peak at 4:00 EST on December 24th, with a 6.0 foot decline at Toledo and an increase of 5.0 feet at Buffalo.

Observations
The HPC Surface Analysis chart (below left) at 18Z on December 23, 2007 showed the center of a deep occluded low pressure system over Lake Superior. The system's cold front was near Buffalo, exposing Lake Erie to the system's strong westerly and southwesterly flow.

HPC Surface Analysis from 18Z on December 23, 2008. Click here to open image in new window,	Trace of wind speed, wind gusts and barometric pressure from the C-MAN station on South Bass Island, Ohio. Data from NOAA's National Buoy Data Center. Click here to open image in new window.

As shown on the trace (above right) of wind speed, gusts and barometric pressure observations from the C-MAN station on South Bass Island, the winds were near or above 35 knots from early morning on December 23rd until the morning of December 24th. During late afternoon on the 23rd, a sustained wind of 45 knots and gusts to 52 knots were recorded. An examination of the wind during the afternoon of December 23rd shows that the direction was southwesterly and in near perfect alignment with the orientation of Lake Erie.

Water level readings at Toledo, Ohio and Buffalo, New York from NOAA's Great Lakes Online. Larger versions for Toledo and Buffalo.

The displacement of water during the storm was far less than the historic high of 16 feet, but still quite dramatic. The graphs of the observed water level heights (above) document that the storm resulted in a drop of nearly 5 feet at Toledo and 5 foot rise at Buffalo. A photo gallery of pictures related to the storm is located here.

The Aftermath
No storm lasts forever, and by 18Z on December 24, 2007, the low pressure system had traveled several hundred miles to the northeast. The slackening barometric pressure gradient and the resultant decrease in wind speed spelled the end of the displacement. In response, a seiche formed as gravity worked to restore equilibrium on the lake.

Lake Erie Water Level Displacement forecast at 12Z on December 24, 2007 from NOAA Great Lakes Coastal Forecasting System.

The Water Level Displacement forecast (above) issued at 12Z on December 24, 2007 predicted the water level at three stations on Lake Erie, Toledo (blue), Cleveland (green) and Buffalo (red). Plotting the water level displacement forecast in this fashion makes it very easy to correlate the simultaneous negative displacement at Toledo and the positive displacement at Buffalo. The signature oscillation (sloshing) of water associated with the seiche is clearly evident. The Water Level Displacement forecast issued at 0Z on December 25, 2007 predicted that it would take more than two days for the level of Lake Erie to reach equilibrium.

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© 2008 Mark A. Thornton
www.LakeErieWX.com