“Has there ever been a tornado in Josephine or Jackson County? What conditions would it take for that event to happen?”
As far back as reporting goes there has never been a tornado in Josephine or Jackson county, but there have been tornadoes in Oregon. In fact there have been 403 tornadoes in the Northwest and 102 have occurred in Oregon since 1951. While most were EF0 to EF1 strength, 6 tornadoes were EF2 or EF3’s. When tornadoes do form in Oregon it is due to a deep upper-level dip in the jet stream or closed-low pressure systems; this can provide cold air aloft and vertical wind shear.
There are a several reasons Oregon doesn’t see tornadoes often, including the steep terrain which makes it difficult for different air masses to meet and create instability. Another reason is the presence of the cold Pacific Ocean which also creates a more stable atmosphere, unlike in the Midwest and Southeast that have the warm Gulf of Mexico fueling in warm moist air at the surface. This air mass is able to collide with a cold dry air mass aloft when a strong cold front moves through creating the strong instability and vertical wind shear needed to form large tornadoes. The ingredients that make the Midwest ideal for tornado formation are a low warm moist layer meeting with a dry mid-layer and a cold dry air mass above, all moving in different directions creating the vertical wind shear to form tornadoes.