Abstract

In the period 1680–1988, 107 significant tornadoes in the Fujita scale categories F2–F5 have occurred in France. These include 49 such events in the historical period 1680–1959, and 58 events in the modem period 1960–1988. Estimates of the temporal and spatial climatological distributions of significant tornadoes in France have been developed that suggest

  1. June and August are the months with the greatest number of such tornadoes;

  2. 1600–1700 UTC is the interval in which occurrence is most likely, with a secondary maximum in likelihood between 1800 and 1900 UTC;

  3. the northwestern quarter of the country is the region where a significant tornado is most likely to occur. A second, much smaller area with several observations is evident in the far south-center portion of France, near the Mediterranean coast;

  4. two significant tornadoes can be expected in France each year;

  5. the mean area stricken by such a tornado is about 4 km2;

  6. France has a mean risk probability of a significant tornado occurring at a point of about 1.5 × 10−5 per year, a value some 15 times lower than the Great Plains of the United States.

During the cold season November–March, tornadoes are most frequently observed in northwest France. During the warm season April–October, they axe most frequently observed in the interior of the country.

An examination of the meteorological situations associated with 21 cases suggests that there is a distinctive synoptic pattern for each season. Analyses of tornado-producing situations show that French tornadoes usually occur where an unstable surface layer of modified air that originated in the Mediterranean is overlaid at midlevels by maritime air originating over the Atlantic. The instability in the surface layer develops while the air is in southern France. Such situations are characterized by high degrees of conditional instability between surface and midlevels in the troposphere as reflected in a steep lapse of wet-bulb potential temperature. However, case studies of individual events suggest that for a tornado to occur, the instability of the surface layer must be further enhanced by local warming and moistening The appearance of a secondary low-pressure center on or near a cold front advancing from the west is an additional favorable condition for the onset of severe weather.

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