One part of North-western Iran is Zanjan Prov. It receives the moisture of western weather systems. These systems are affected from latitude, height and the direction of mountains. The east-ward replacement of western and short air waves that are separated from Mediterranean one as a principal wave and so, the recurrence of trough and ridge can obtain enough moisture for the Heavy Rainfalls of Zanjan Province (hereafter HRZP) from Mediterranean as nearest sea one. Our aim was the determining of precipitable weather system that caused HRZPs. As a hypothesis we bring up Mediterranean Sea is the principal resource for heavy rainfalls in Zanjan Province. In first stage, we extracted the daily rainfall amounts for 9 weather stations inside and outside the province during spring season including April, May and June. When the daily tables of rainfalls set based on equal to 30 mm/day and more, we could determine 13 heavy rainfalls. Results showed April had most heavy rainfalls. In second stage, we determined 8 HRZPs based on their duration, the cumulative amounts of weather stations during peak days and the frequency of recorded ones. During final stage, we designed daily weather charts based on atmospheric data including slp, hgt, air, rhum, shum, uwnd and vwnd from NCEP/NCAR. Programming was accomplished in C++ software and modeling in GrADS and Surfer ones. The analysis of designed models showed water vapor has flowed from Mediterranean Sea toward study area in lower troposphere (750 HPA) during rainy days. So, we found the cores of maximum relative humidity (more than 80 percent) over study area during rainy days. It was detected that the most of HRZPs happened during April and the Mediterranean Sea is the principal resource of water vapor for them.