Abnormal weather is weather that is not typical. Other words for abnormal are atypical, unusual, anomalous, and uncharacteristic. Abnormal weather refers to any situation in which the weather is not what is typically expected. It also can refer to stormy weather.

Each location has an average high and low temperature. High and low temperature near these values indicate that the high or low is near the average (also called normal or mean value). Being significant off from the average high or low for some locations may be typical since an average takes into consideration the extreme values also. Rapid weather changes from fronts can cause the high and low values to waffle significantly. The main point to make is that when weather is referred to as abnormal, it usually is because the weather is not typical for what is expected for this time of year. For example, if on May 15 the average high is 80 F and the average low is 60 F, a day with snow and a high temperature of 30 F on this date is abnormal for what this location would typically have for weather.

Abnormal weather can also refer to stormy weather since for many locations storms are less common than nicer weather. For example, if Monday through Wednesday are warm and sunny and then Thursday is stormy, the weather on Thursday may be referred to as abnormal.