The probability of occurrence is extremely high. Warnings are issued based on eyewitness reports or clear signatures from remote sensing devices such as doppler radar and satellite. Lead-times for thunderstorm type events are generally 30 minutes or less. Lead-times for winter storms and river floods, may be 6 to 18 hours.
Watches generally cover larger areas than warnings.
However, with larger storms, such as winter storms, the entire watch area may be affected. Severe thunderstorm and tornado watches are usually issued 1 to 3 hours before the event begins. With flash floods, lead-times may be 3 to 12 hours. For winter storm watches, lead-times are usually 12 to 36 hours. An advisory is issued for weather that is expected to be a disruption to the normal routine and an inconvenience, but it is not expected to be life-threatening.