Fire Weather Basics

Skyview provides fire weather forecasts and current weather information from local weather stations to help local fire fighters and wish to share some terminology we use in our forecasts and additional information with the rest of you.

Lightning Activity Level or (LAL) Definitions

LAL 1 No thunderstorms
LAL 2 Isolated thunderstorms.  Light rain will occasionally reach the ground.  Lightning is very infrequent, 1-5 cloud-to-ground strikes in a 5 minute period.
LAL 3 Widely scattered thunderstorms.  Light to moderate rain will reach the ground.  Lightning is infrequent, 6-10 cloud-to-ground strikes in a 5 minute period.
LAL 4 Scattered thunderstorms.  Moderate rain is commonly produced.  Lightning is frequent, 11-15 cloud-to-ground strikes in a 5 minute period
LAL 5 Numerous thunderstorms.  Rainfall is moderate to heavy.  Lightning is frequent and intense, greater than 15 cloud-to–ground strikes in a 5 minute period.
LAL 6 Same as LAL 3 except thunderstorms are dry (no rain reaches the ground).  This type of lightning has the potential for extreme fire activity and is normally highlighted in fire weather forecasts with a Red Flag Warning.

* = Information from: Intermediate Wildland Fire Behavior: Student Workbook. National Wildfire Coordinating Group, July 1994

Haines Index* – Haines Index combines the effects of instability and dry air to determine conditions in which an existing fire has the potential to spread rapidly.  It should not be used to determine if a fire will start.  Since wind is not a parameter of the Haines Index, it is best used for plume-dominated fires which can develop extreme surface winds through their own internal heat.  The Haines Index is yet to be tested on wind-driven fires…  When the value is 5 or 6, the probability of extreme fire behavior, including torching, crowning and spotting is significantly increased.  Critical fire behavior is usually low, with minimal fire spread when the values are 4 or less… * = Definition from: http://www.crh.noaa.gov/fsd/firewx/firedef.php

Haines Index

Haines Index Potential for Large Fire Growth
2 or 3 Very Low
4 Low
5 Moderate
6 High

Haines Index values of 4 or higher are days with a dry and unstable air mass, while 4 or lower are typically days with cool/moist and a more stable air mass.  A high Haines Index value in tandem with a high LAL value and strong surface winds can combine for disaster.  This is the worst type of scenario and the Front Range of Colorado is primed for these types of conditions. Winds have been breezy through the spring and summer so far, fuels for fire are dry due to below normal precipitation, the atmosphere becomes more unstable with the hot temperatures of summer and thunderstorms that do manage to develop produce more lightning than rainfall.  Unless changes in the current weather pattern change significantly expect more Red Flag Warnings and the threat for wildland fire to increase further.  It’s not all doom and gloom as monsoonal moisture is late to arrive this year, but should return by late July bringing much needed rainfall to the Front Range of Colorado from scattered afternoon thunderstorms.