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Part II. A Case Study of the Surge of August 3, 1960



Surface weather observations and barograph traces from the region surrounding the Great Lakes were analyzed for the hours 0300 to 1300 cst, August 3, 1960. Synoptic and time-section analyses of the surface weather show the squall line which moved across the southern part of Lake Michigan between 0900 and 1000 cst. An isochrone analysis of the pressure jump also is presented.

Records from several water-level recording gages in the Southern Basin of Lake Michigan are transcribed, and show clearly the surge which was produced by passage of the squall line over the Lake.

From transit times at eight stations in the region surrounding the Southern Basin, the average speed and direction of motion of the squall line have been determined. Lake-level data for the surge caused by the squall line of August 3, 1960, are compared with the lake levels computed by Platzman for a squall line moving with similar speed and direction.

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The dates of incidence of extreme wind tide on Lake Erie have been determined for the 20-year period 1940 through 1959, for all cases in which the difference in lake level between Buffalo and Toledo exceeded 6 feet. A frequency-intensity analysis shows that a set-up in excess of 10 feet may be expected once every 2 years. Extreme wind tides occur mainly in the 6-month period October through March; more than 70 percent of the cases fall in the three months November, December, and January. November is the month of most frequent incidence, having more than one-third of the total number of cases in the period studied.

The observed seasonal variation of extreme set-up frequency is interpreted as a reflection of the seasonal variation of storm frequency and of storm-track location. Secondary, but important factors are: seasonal variation of storm intensity and seasonal variation of thermal stability of the atmospheric boundary layer. The tendency for marked temperature stratification to be present in the Lake during the summer probably inhibits set-up to a significant degree during that season.

Storms which produce extreme set-up follow very similar paths during the 24 hours preceding the time of maximum set-up. However, no clear-cut relation was found between source region and set-up intensity, apart from a tendency for Alberta Lows to yield slightly smaller set-up. Brief descriptions are given of the storms of March 22, 1955, and November 17, 1955.

Finally, the writers report that no relation was found between frontal speed and set-up intensity, and infer that resonant coupling with the Lake does not contribute significantly to set-up magnitude.

It should be noted that storms accompanied by strong easterly winds, which produce high water on the western end of the Lake, have not been studied in this investigation.

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