Foehn Climatology and Index for Sofia 1993–2014

Foehn is an extreme weather event caused by high wind gusts and its forecasting is still a challenge. The foehn in Bulgaria is observed on the northern slopes of the mountains. The foehn in Sofia occurs after the air mass passes over Vitosha and Lulin mountains. Foehn days in Sofia are selected using as criteria the surface temperature, relative humidity and wind direction. For the period 1993–2014 there are 136 foehn days. For 55.9% the wind gust is between 14 and 19 m/s corresponding to yellow Meteoalarm code, 14.7% are with gusts 20–29 m/s (orange code), and 1.5% are with gusts over 30 m/s (red code). For the foehn days the upper-air flow at mountain peak Cherni Vrah is in 47.4% of the days from 202⁰ direction and in 36.8% from 225⁰ Orange code days are 55.0% with wind direction 225⁰ at Cherni Vrah, while for red code it is 180⁰. Cherni vrah-Sofia Stability Index is computed and found to have high sensitivity to foehn/no foehn conditions. The case studies in 2010 show that the Index decreased by 20% with foehn start, and increased by up to 29% after the foehn. For the foehn days a manual circulation classification is created with 4 circulation classes. The circulation class I with Mediterranean cyclones passing over Hungary, results in 51.3, 65.0 and 100.0% of the days with yellow, orange and red code, respectively.