What is the lightning activity level (LAL)?
Those of you that are regular users of the fire weather planning forecast issued by our office realize that there are several elements predicted in this product that are tailored to the fire weather community. The lightning activity level (LAL) is one such parameter. But what exactly does an LAL of 3 or 6 really mean?
The lightning activity level is a number developed in order to help land management and fire protection agencies prepare for the possibility of lightning-caused wildland fires. It is designed to indicate both the amount of lightning associated with thunderstorms, if there are any, as well as whether or not there will be wetting rains accompanying the storms.
The LAL extends numerically from 1 to 6. A level 1 indicates that thunderstorms are not expected (even though other precipitation certainly could be). On the other hand, an LAL of 6 indicates dry thunderstorms are forecast with 6 to 10 cloud-to-ground lightning strikes expected in any 5 minute period within those storms. An LAL of 6 indicates a potentially dangerous fire weather situation. This level is forecast only a handful of times a year, if it is at all.
An LAL between 2 and 5 indicates a varying degree of lightning and thunderstorm activity. For example, an LAL of 4 technically means that scattered thunderstorms are expected (with the probability of them somewhere between 30 and 50 percent). It also means that lightning is frequent with 11 to 15 strikes in a five-minute time and that moderate rainfall will accompany the storms. You may be wondering how forecasters can be so specific with the number of strikes expected in a given period. The truth is that they aren’t able to. The LAL is only expected to indicate what the average number of strikes would be with typical thunderstorms at a given level. There can be wide variations in activity across small areas. For example, severe thunderstorms produce a large amount of lightning, but they also normally bring with them heavy rainfall. Dry thunderstorms, on the other hand, may have only a handful of strikes with them – but they can occur in the wrong place at the wrong time and lead to disaster. The best bet for using this parameter is to utilize forecast trends instead of literal values, and to determine your own thresholds for action. An LAL of 6, however, is something that will only be issued if forecasters have high confidence in dry lightning extensive enough to cause wildland fires in a given area. It will often be accompanied by a fire weather watch or red flag warning if it is combined with other critical fire weather conditions. Then again, an LAL of 3 indicates the same amount of lightning as an LAL of 6, but wetting rains of some form are expected to accompany storms at the LAL 3 level. Even so, during very dry conditions an LAL of 3 could be just as important as an LAL of 6.
The forecasters at the NWS use a variety of information sources when computing expected LAL levels including surface observations, information from weather balloons launched twice-daily around the globe (including in Bismarck), satellite and radar images, as well as forecast models. Perhaps the best tool these folks have in predicting LAL (and other fire weather parameters) is their own experience and outright instinct. Even so, forecasting lightning activity level is far from an exact science.