Failures in a large electric power system comprising several transmission hardware and substation equipment are inevitable. It is clear that the number of annual failures depends on the system topology itself as well as on the environmental conditions. One of the primary reasons of major transmission line outages are severe weather conditions. The stress created by severe weather is much higher than in normal weather and increases with the severity level of the weather, leading to increases in line outage rates. Thrace part of Turkish National Power System is more important from the point of service reliability as being the most industrialized region of the country. This paper is devoted to the quantitative effects of severe weather on the outage statistics of 154 kV Turkish National Power Transmission system of the region. Moreover, a weather dependent failure rate model was proposed for that part of the system.