Which of the following is NOT one of the seven phenomena requiring an urgent PIREP classification?

Fog is considered a significant weather phenomenon but does not typically require an urgent PIREP (Pilot Report) classification. PIREPs are primarily focused on reporting immediate hazards that could significantly impact aviation safety, such as severe icing, tornadoes, and low-level wind shear, which present critical and urgent risks to flight operations. These hazards can pose immediate threats to aircraft stability and safety, thereby necessitating prompt reporting and dissemination of information to other pilots and air traffic control. In contrast, while fog can impact visibility and flight operations, it does not usually present the same level of acute risk that would prompt an urgent PIREP.