Where Does the Water Come From?

When it rains here in the Northern Plains, where does the moisture that makes the rain come from?  Actually, there is enough moisture in the air for rain most of the time.  Storm systems tend to make the most of available moisture because of convergence (air flowing together from different directions).  Also, slow-moving storms are able to generate more rain than ones that quickly pass.  But some weather systems are just better at making rain than others and a lot of this has to do with how much water vapor is available for rain.  If you follow air streamflows into our region, you will see that source regions of that moisture include both the Pacific Ocean and the Gulf of Mexico.  Storm systems really laden with moisture usually have some sort of connection to these regions. A lot of the time, it is the Gulf that brings us the moisture, but sometimes we get very moist air from the tropical regions of the Eastern Pacific.       Meteorologist John Wheeler