The Lightning Activity Level (LAL) is a measurement of cloud-to-ground lightning activity observed (or forecasted to occur) within a 30 mile radius of an observation site.
The LAL is part of the National Fire Danger Rating System (NFDRS) and consists of two reports. The first report covers the period from the previous day’s observation until midnight (referred to as Yesterday’s Lightning) and the second report covers the period from midnight until the present day’s observation time (referred to as Morning Lightning). Each report is assigned a number on a scale of 1 to 6 which reflects the frequency and character of the lightning. The scale for 1 to 5 is exponential, based on powers of 2 (i.e., LAL 3 indicates twice the amount of lightning of LAL 2). LAL 6 is a special category for dry lightning (see description below) and is closely equivalent to LAL 3 in strike frequency.
The Lightning Activity Level on a scale of 1 to 6 as described below:
LAL 1: No thunderstorm or building cumulus clouds observed.
LAL 2: A single or few building cumulus clouds with only an occasional one reaching thunderstorm intensity observed. Thunderstorms or lightning need not be observed for this activity level to be assigned; however at least one large cumulus cloud must be present.
LAL 3: Occasional lightning (an average of 1 to 2 cloud-to-ground strikes per minute) is observed. Building cumulus clouds are common; thunderstorms are widely scattered.
LAL 4: Frequent lighting (an average of 2 to 3 cloud-to-ground strikes per minute) is observed. Thunderstorms are common and cover 10 to 30 percent of the sky. Lightning is primarily of the cloud-to-cloud type but cloud-to-ground lightning may be observed.
LAL 5: Frequent and intense lightning (cloud-to-ground strikes greater than 3 per minute) is observed. Thunderstorms are common, occasionally obscuring the sky. Moderate to heavy rain usually precedes and follow the lightning activity. Lightning of all kinds (cloud-to-cloud, in-cloud and cloud-to-ground) is characteristically persistent during the storm period.
LAL 6: A dry lightning situation. Low lightning flash rate observed (less than one to three cloud-to-ground strikes per 5-minute period per storm cell passage). Scattered towering clouds with a few thunderstorms; bases of the clouds are high. Virga is the predominate form of precipitation.
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National Wildfire Coordinating Group
http://www.nwcg.gov/term/glossary/lightning-activity-level-(lal)
