Storm Catastrope Atlantic and Western Europe - Case Study 25 - 28 December 1999

• I. Introduction
• II. Overview
• Summary of typical cloud features
• Lothar and Lothar successor
• Martin
• III. Physical Background
• Baroclinic zone and surface wave
• Upper level jet streak and left exit region
• Relation between upper level jet streak and surface wave
• The role of Potential Vorticity (PV)
• IV. Weather
• V. Numerical Model forecast problems
• VI. References

Introduction

From 25 to 28 December 1999 a series of four intensive storms developed rapidly over the Atlantic and moved into West Europe and then further east. Two of the four storms caused extensive damage over west and central Europe, primarily due to extremely strong winds.

From a satellite meteorology point of view, the events were extraordinary as all of these storms are examples of the Conceptual Model "Rapid Cyclogenesis" but with different intensities. Two storms, named "Lothar" (25 - 26 December) and "Martin" (26 - 27 December) can be regarded (with some differences) as ideal examples of "Rapid Cyclogenesis". A third storm (hereafter called "Lothar successor") initially had the potential for Rapid Cyclogenesis but eventually, developed somewhat less.
The naming "Lothar" and "Martin" is not according to international conventions but was introduced by the German Weather Service (DWD). It is used in this manual for convenience of storm identification.

From a forecaster's point of view, the storms will be well remembered, since the two most intense storms were not well predicted by any of the numerical models. In this report the problems of the ECMWF and Hirlam NWP forecasts will be discussed.

It should also be noted that these storms are good examples of the potential of conceptual models to correct the output of Numerical Models.