This unit on lightning activity level is divided into 4 parts. Click on the subject below to take you directly to that part.
| DEFINING LAL |
LIGHTNING ACTIVITY LEVELS (LAL)
In the western U.S. lightning is the greatest single cause of forest fires. A critical element used to determine fire danger in the National Fire Danger Rating System (NFDRS) is a lightning risk factor...as seen by the flowchart of the NFDRS model. LAL levels are also used to estimate the expected number of new fires for a rating area. A predicted LAL value for one forecast zone may include several fire-danger rating areas within that one zone.
Lightning Activity Level (LAL) is a numerical rating ranging from 1 to 6 that represents observed or forecast frequency and characteristics of cloud-to-ground (CG) lightning for a fire zone. The scale that determines the amount of activity is exponential and based upon a power of 2. For example, a LAL of 3 indicates twice the lightning as LAL 2. LAL of 4 is twice that of 3 and so forth. LAL 6 is special and rare - and only used for dry thunderstorms that create severe fire problems. LAL of 1 indicates no lightning during the specified rating period.
When the NFDRS was developed in the 1970's, there was little or no guidance available for forecasting the area density of CG lightning strikes. To this day, there is no coherent thunderstorm model available that covers both the dynamics of cumulus cloud growth and the electrification processes within thunderstorms. However, based on 3 years of weather and lightning observations, a correlation was found between the amount of CG lightning and maximum height of radar echoes. LAL index levels were assigned to specific ranges of radar heights according to their relative frequency of occurrence. Table 1 shows related meteorological parameters associated with specific LALs.
The basic rating area used to represent the amount of CG lightning strikes during any rating period is 2500 square miles - or a circle around the observation point with a radius of about 28 miles . When NFDRS was developed, the assumption was made that this is the smallest area a generalized forecast could be made. Also determined, was that 2500 square miles is about the largest area that lightning activity can be effectively observed from a surface observation point - such as a forest fire lookout.
The LAL Guide was developed with both the forecaster and observer in mind. The forecaster can verify LAL forecasts by using available radar data (max echo height, precipitation coverage and intensity), pilot reports, satellite data, lightning data, etc. Field observers on the otherhand record the amount of observed lightning and help verify LAL forecasts by descriptions of cloud and storm development, rate and amount of observed CG lightning and area coverage of the storms. In 1985-86 a study done in Boise found that observers missed 25% of actual lightning strikes compared to that detected by the old BLM lightning detection system.
LAL 1 - NO THUNDERSTORMS are expected - and NO lightning will occur in the forecast zone.
LAL 2 - 5 CLOUD TO GROUND LIGHTNING will occur with increasing areal coverage (and/or increasing lightning strikes) in the forecast zone. Wetting rain showers generally occur with LAL 3-5. In general, storms become more intense with higher LALs. Note from table 2 that with an LAL of 5, over 50% of the forecast area will receive NO measurable rain.However some of the larger lightning caused fires were those that went undetected by areal reconnaissance - and smoldered for up to 2 weeks before becoming active and being detected.
LAL 2 and 3 are more representative of summer air mass thunderstorms - while LAL 4 and 5 occur with squall lines or fronts.
LAL 6 - DRY LIGHTNING describes this rare but very significant event. In such situations cloud bases tend to be high - generally above 15,000 ft MSL, with no significant wetting precipitation. In most high-based thunderstorms the frequency of cloud-to-ground lightning strikes is low...yet a very high number of these strikes start fires because the sub-cloud base environment is very dry and fuel moistures in the smaller fuels is very low. Some of the larger lightning-caused fires are those undetected by IR areal reconnaissance lights.
Whenever the potential exists for dry lightning, the forecaster should issue a RED FLAG WARNING or a FIRE WEATHER WATCH. In several areas of the western states, the first day of a thunderstorm outbreak (after an extended dry period), has the highest potential for new lightning caused wildfires. As the respective pattern progresses, more moisture is normally advected into the region - reducing the risk of LAL 6. However, in areas of the western mountain states, extended periods of dry lightning activity does occur. Keep in mind that in the western mountainous states, dry lightning only occurs on approximately 1-2% of all lightning days.
RETURN TO TOP* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Predicted LALs should reflect the maximum level of activity expected in a forecast area. A prediction of average LAL would be no more valuable to the fire manager than the average overnight low to a citrus grower. Fire agencies believe that for verification purposes, a single observation of a lightning strike anywhere in a forecast zone is verification of LAL 2. If a higher or lower LAL activity than forecast is observed, the forecast is scored as a "miss."
The selection of forecasted LALs is by nature subjective. For example, a forecaster may find that cloud and storm development is most useful in forecasting tomorrow's LAL, while radar echoes are best for verifying LAL forecasts. A forest dispatcher may find observations of cloud development most reliable when confirmed by lightning counts or flash rates from a nearby lookout.
Forecasting LAL is similar to forecasting thunderstorm coverage for a public forecast zone. Using similar tools such as model guidance, satellite, and radar, forecasters can compare current conditions to the synoptic scale pattern and determine how the pattern will change for the next 24 hour period. Based upon that decision, the forecaster may alter the observed LAL value accordingly. Fire weather offices that are collocated with fire agencies have advantages to receiving lightning display data in real time. Such data allowed forecasters to forecast LAL using the frequency and distribution of cloud-to-ground lightning strikes.
Two decision trees have been created to help forecasters use appropriate LAL levels. The first decision tree will look at radar echo heights with cloud development while the second decision tree looks at Cloud to Ground Lightning strikes for a rating period for a 2500 square mile area.
In western Washington, the highest number of lightning strikes occur during the month of June and July. Graph 2 shows a chart of lightning activity for western Washington for 1991 to 1996. The most number of strikes occur during the month of July, however the maximum number of days with lightning occur during June. Therefore one can reason there is more lightning in storms in July than June.
TYPES OF LAL FORECASTS
NFDRS LAL forecasts include two different values. The first value is a forecast that represents the level of activity from the observation time (2PM LDT/LST) to midnight of the same date. The second LAL value represents the forecast LAL from midnight to 2pm LDT/LST of the next day.
****** Another LAL forecast forecasters will issue is the value in the land management forecast. That value is valid from 5am of the following day to 5am to the next day.
RESOURCES for obtaining lightning activity vary. Distribution of lightning data from the National Lightning Detection Network NLDN (owned and operated privately) by Western Region Headquarters is currently providing lightning data to most WSFO and WFOs. WRTA No. 97-20 summarizes this NLDN system - and some of the inherent errors in the system. Particularly, an average of only 80% of all lightning strikes is recorded with this system. Accuracy in detection decreases to about 60% near the coast and U.S. borders - due to the configuration of the detection network.
By the end of this summer WRH will have assisted in developing an overlay for RAMSDIS of fire weather zones that will aid in determining CG lightning strikes. Upon completion of that, a more regionally consistent and objective method to determine LAL may become operational.
The Missoula Fire Labs are currently working on better ways to determine LAL. One part of their research involves changing the cloud-to-ground lightning strike values per 2500 square miles to a flux density weighted value.
An example of lightning strike displayis from theALBERTA FOREST SERVICE that illustrates 24 hours of lightning data valid until 0800 MDT 26JUN97. Northern WA, ID and MT can be seen at the bottom of the image.
RETURN TO TOP
