During the cold season within the Northern Hemisphere, derechos generally develop within a pattern of southwesterly winds at mid levels of the troposphere in an environment of low to moderate atmospheric instability ( caused by heat and moisture near ground level or cooler air moving in aloft and measured by convective available potential energy ) and high values of vertical wind shear ( within the lowest of the atmosphere ).
22.
Although the formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood, there are six main requirements for tropical cyclogenesis : sea surface temperatures that are warm enough, atmospheric instability, high humidity in lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a pre-existing low level focus or disturbance, and low vertical wind shear.
23.
Forced along the extent of a cold front, the primary squall line raced eastward across Ohio at approximately 60 mph ( 95 km / h ), reaching Pennsylvania by 0200 UTC . As a result of decreased atmospheric instability, the threat for tornadoes and hail had greatly diminished by this time, while the probability for damaging winds remained elevated to high; as a result, watches issued east of Ohio were severe thunderstorm watches rather than tornado watches.
