WES WEEK THREE: 23-27 September 2002

THE AUTUMNAL EQUINOX -- The Autumnal Equinox will occur on the afternoon of Saturday, 22 September 2001 (at 2305Z, or 5:05 PM EDT, 4:04 PM CDT, etc.). At that time the noontime sun will appear directly above the equator, representing one of the two times during the year for such an occurrence, with the other being at the vernal equinox in March. Within the next several days the length of daylight will become noticeably shorter than that of nighttime.

In recent years Great Lakes water levels have declined significantly to near historic lows. Lakes Superior, Michigan, Huron, and Erie are experiencing their lowest water levels in 35 years with serious implications for lake-based activities.

From 1997 to 2001, Lakes Michigan and Huron dropped by 104 cm (40.8 in.) and Lake Erie dropped by 96 cm (37.6 in.), encompassing the greatest three-year drop in lake levels since continuous records began in 1860. This dramatic decline in lake level is due to a combination of weather conditions in the Great Lakes drainage basin. Rainfall, snowfall, and air temperature during winter and spring govern water levels of the Great Lakes in spring and summer. Spring melting of the winter snow pack in the drainage basin is an important contributing factor as is air temperature that ultimately governs evaporation rates. During 1997, 1998, 1999, and 2000, lower than normal precipitation and higher than normal temperatures reduced the winter snow pack, decreased discharge on rivers flowing into the lakes, and accelerated lake evaporation. Although input of water into the Great Lakes was above the long-term average in October 2001, lake levels remain well below average because of less than the usual snowmelt in the Spring of 2001 and less than normal winter ice-cover in 2001-02. As of January 2002, ice cover on the lakes has been limited to shorelines and embayments and is below the long-term average. (More open water translates into greater winter evaporation.) As of 8 February 2002, Lake Superior's water level is 10.2 cm (4 in.) below its long-term average and 25 cm (10 in.) above its level at this time last year. The water level of Lakes Michigan and Huron is currently 38 cm (15 in.) below average and 20 cm (8 in.) above last year. (The Straits of Mackinac that link Lakes Michigan and Huron are so wide and deep that the levels of these two lakes are essentially the same.) Lake Erie is 5 cm (2 in.) below its average level and 15 cm (6 in.) above this time last year. Lake Ontario's level is also 5 cm (2 in.) below average and 13 cm (5 in.) above its level this time last year.

Low lake levels have adversely impacted commercial navigation, marinas, recreational boaters, and electric power facilities. Shallower than usual water requires expensive and environmentally damaging dredging to keep shipping channels navigable and ports open. Even with more dredging some channels cannot accommodate the draft of heavily laden freighters. Reduced carrying capacity impedes transport of grain, coal, ore, and other raw materials to processing facilities and markets. In 2000, Lake Carriers transported 5% to 8% less cargo, sending prices higher. If storm winds cause near-shore water levels to fall, marinas, docks, and boat ramps may be temporarily inaccessible. The Great Lakes supply most of the potable water for communities and cooling water for coal-fired and nuclear electric power plants located along their shores. A drop in water level may require costly repositioning of intake pipes. On the positive side, lower lake levels translate into broader beaches and wetland habitats and a respite from shoreline erosion.

For more information on past, current, and anticipated Great Lakes water levels, go to http://www.glerl.noaa.gov/data/now/wlevels/, a web page maintained by the Great Lakes Environmental Research Laboratory (GLERL). Records of Great Lakes water levels constitute one of the longest high quality hydrometeorological data sets in North America, dating back to about 1860. Lake level measurements are collected and archived by NOAA's National Ocean Service.

Concept of the Week: Questions

1. With rising temperatures, the rate of evaporation of water [(increases)(decreases)].
2. A trend toward snowier and colder winters and springs is likely to cause levels of the Great Lakes to [(rise)(fall)].

• 25 September 1939...A west coast hurricane moved onshore south of Los Angeles bringing unprecedented rains along the southern coast of California. Nearly five and a half inches of rain drenched Los Angeles during a 24 hour period. The hurricane caused two million dollars damage, mostly to structures along the coast and to crops, and claimed 45 lives at sea. "El Cordonazo" produced 5.66 inches of rain at Los Angeles and 11.6 inches of rain at Mount Wilson, both records for the month of September. (David Ludlum) (The Weather Channel)
• 25 September 1997...While much of San Bernardino and Inyo Counties in California are desert, between 1 to 2 inches of rain from former Pacific Hurricane Nora fell across a large portion of these counties. State highway 190 was washed out between Panamint Springs and Stove Pipe in Death Valley National Park. Flooding also shut down many other park roads. (Accord Weather Calendar)
• 25-26 September 1962...Torrential rains from severe thunderstorms over the hilly regions to the west and north of Barcelona Spain resulted in flooding, resulting in much damage. Many buildings collapsed. Tarassa was particularly hard hit with approximately 500 deaths. (Accord Weather Calendar)
• 26-27 September 1997...Thirty hours of rain that totaled between 8 to 14 inches fell across Pinellas and Hillsborough Counties in Florida, resulting in 5.3 million dollars in damage. In St. Petersburg, a 23 year old woman survived being swept 12 blocks in an underground drain before being dumped into a retention pond. (Accord Weather Calendar)
• 28 September 1986...Flash flooding swept a car into a swollen creek from off the Kamehameha Highway on the Hawaiian island of Oahu. The driver held onto some branches until help arrived, but the strong current stripped him of his clothes and watch. (Accord Weather Calendar)
• 28 September 1986...Torrential rains and floods were responsible for South Africa's worst natural disaster in Natal. As much as 35 inches of rain fell between the 25th and 29th to the northeast of Empangen, resulting in 317 deaths and 163 missing persons. Topsoil from some farms was completely washed away leaving only bedrock. (Accord Weather Calendar)

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Prepared by AMS WES Central Staff and Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2002, The American Meteorological Society.