The omega blocking condition and extreme rainfall in Northwestern Iran during 25 - 28 October 2008

Heavy precipitation plays a significant role in arid and semi-arid regions of Iran. In order to understand the effect of blocking high system on rainfalls in northwest Iran during 25 - 28 October 2008, meteorological conditions including pressure, wind fields and temperature at multiple levels of the atmosphere were analyzed. Sea level pressure, the 1000-500 hPa thickness, perceptible water, relative humidity, temperature, u and v components of wind at 850 hPa, geopotential height at 500 hPa and relative vorticity of u and v were obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis dataset. Location and displacement of the atmospheric systems such as cyclones, anticyclones, fronts and wind fields were identified using synoptic charts. Daily rainfall data obtained over 50 weather stations. Results indicate that the existence of a blocking high over the northern portion of the Caspian Sea caused the activity of two accompanying low pressure systems in which the western low resulted in excessive and intensive rainfall over the study area for 4 days. The low pressure was built by suitable wind patterns in the underlying levels (850 hPa). Moreover, modeling the outputs of 24-hour rainfall by the Geographic Information Systems (GIS) indicates that during the examined period northeastern parts of the region received the maximum rainfall.