An exceptional southerly Foehn in the Alborz Mountains in northern Iran is investigated by using a combination of observations, reanalysis, and simulation data. A synoptic analysis is used as well as a high-resolution numerical modeling to clarify the Foehn event at different scales. The event resulted in an extensive and high-intensity fire in the Gilan and Mazandaran forests in northern Iran. The results indicate that a mechanically driven Foehn occurred in the Alborz Mountains during December 16–18, 2005. On the synoptic scale, the Foehn event occurred due to the presence of high pressure over the interior regions of Iran and lee cyclone over the southern Caspian Sea, with a strong south–north pressure gradient across the Alborz Mountains. In the mesoscale, the results suggest that mountain waves generated over the northern slopes of the Alborz Mountains are the primary source of the localized southerly wind maximum around the lee side of Alborz. A numerical simulation reveals that strong meridional surface pressure differences along with southerly flow, which is blocked upstream of the Alborz Mountains, result in higher nonlinearity and create large-amplitude vertically propagating mountain waves over the Alborz Mountains. The study also indicates that the wave-breaking region on the lee side with a critical level ranging from 600 to 400 hPa is responsible for reflecting the mountain wave energy back to the ground and for creating severe downslope wind (Garmij) in leeward side of Alborz Mountains. The Foehn event first appeared early on December 16, due to a wave-breaking at 550 hPa in western part of the Alborz Mountains.

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