In this paper, synoptic patterns associated with maximum precipitation over the Khorasan region, between 1985-2000 with the computation of some forcing functions are studied and classified into three types.
Type A: Consists of the anticyclones that move from the Scandinavian Peninsula and central-Europe in an east and south-east direction during cold seasons, with relatively high speed and cold cores. They first affect the Caspian Sea area and then the Khorasan region, and cause little of precipitation and very cold weather which lasts for 24 to 36 hours in the region. For this type the maximum amount of precipitation for 24 hours recorded was 16mm in Goochan city.
Type B: includes steering frontal cyclones from east of the Mediterranean Sea that after passing the central part of Iran, affect the Khorasan region, and if these systems combine with a low pressure system from the Red Sea, that is a form of inverted thermal trough, which developed due to the effects of forcing functions they produce more precipitation than type A, but with a mild temperature. For this type maximum amount of precipitation recorded for 24 hours was 16mm in a lower latitude compared to type A, in Ferdos city.
Type C: includes cold core anticyclones that move from a high latitude (similar to type A) simultaneously with the steering cyclones (or trough) from the Mediterranean or the Red Sea (similar to type B) and theses two systems impact over Iran and produce a strong temperature and pressure gradient along the northern part of Iran, that makes a frontal zone over the Khorasan region and therefore heavy rain and snow is expected to fall over the region. For every type, the group which had considerable precipitation compared to other cases, their relative vorticity, absolute vorticity and advection are computed on 500hpa level. The maximum amount of precipitation in this period of study recorded for 24 hours was 40 mm in Mashhad city (of type C) at time of 1998.2.10. At this time relative vorticity is order of magnitude about , and its advection at 1998.2.9, is order of magnitude . So when a low pressure from a lower latitude, that has sufficient humidity and moves to the center of Iran , impact with a high pressure from upper latitude that has very cold air over Iran we will have high precipitation in the Khorasan region.