An atypically early major stratospheric sudden warming in mid-Dec 1998 resulted in an abnormally warm and weak polar vortex through most of the 1998--99 winter. The first major warming in nearly 8 years, it was only the second major warming observed before the end of Dec, and strongly resembled the previous Dec 1987 major warming in several characteristics atypical of major warmings later in winter. 3D mechanistic model simulations reproduced most characteristics of the Dec 1998 major warming, including the magnitudes of zonal mean easterlies and temperature increases and the 3D evolution of the flow, paving the way for more detailed future studies of the mechanisms involved in this unusual event. ¿ 1999 American Geophysical Union