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Distribution of tornadoes vs. derechos vs. mid-level storm relative flow and Bulk Richardson Number
Research done at SUNY Albany for eastern New York indicates that mid-level storm relative flows of greater than 20 kts are favorable for tornadoes, while storm relative flows of less than 20 kts are more favorable for straight-line winds (derechos). It has been hypothesized that strong mid-level storm relative flow tends to blow cores of heavy precipitation down-shear away from the area where the updraft is trying to develop low-level rotation. When storm relative winds are weaker, stronger downdrafts are likely to lead to strong outflows near the storm updraft, disrupting the tornadogenesis process.