A 10-Yr Climatology Relating the Locations of Reported Tornadoes to the Quadrants of Upper-Level Jet Streaks

Observations and numerical model simulations associate rising motions below the right-entrance and left-exit quadrants of an upper-level straight jet streak with the development of convection and severe weather. The occurrence of tornadoes in relation to the jet quadrants is investigated for the continental United States for the spring months of 1990–99. Tornadoes occurred primarily within the two exit quadrants, with the left-exit quadrant favored over the right-exit quadrant. While fewer tornadoes were located below the two entrance quadrants, the right-entrance quadrant was favored over the left-entrance quadrant. For those days on which many tornadoes occurred (“outbreak” days), a greater percentage of tornadoes occurred below the left-exit and right-entrance quadrants than for those days on which only a few tornadoes were reported. Composite diagrams are presented to clarify the relationship between the quadrants of a jet streak, severe weather, and synoptic features such as low pressure centers and frontal boundaries.