presents an unstable atmospheric condition. Warm air usually rises with respect to cooler air. According to Collins (1995) these circumstances create a "firecracker effect in the springtime atmosphere (Collins, 1995, pp. 81-84)."
Atmospheric instability during spring may be evidenced by wind direction. Stable conditions are typically associated with general upper-level flow out of the northwest. With such winds, cold air aloft usually moves over cold air at the surface. In addition, stable conditions may also be indicated by a straight east-west flow aloft. These winds do not "suggest a high likelihood of severe weather (Collins, 1995, pp. 81-84)." In contrast, a strong southwesterly flow aloft does imply that conditions favor storm formation. The key level for such flows is approximately 500 millibars or about 18,000 feet. The wind coming out of the southwest typically originates in Canada. However, as it moves southward, it can turn at the tip of a trough aloft. Once the flow changes direction, it then heads back toward the northeast. This mechanism can ultimately resu
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