Class 1

The fire danger is Low and fires are unlikely to start. Underground fires and fires in heavy fuels may smolder.

Class 2

The fire danger is Moderate and the probability of ignition exists in light flashy fuels. However, the fire will have a slow rate of spread and the fire intensity is usually low.

Fire Danger Sub-Classes

The next three classes are divided into three sub-classes. Most agencies do not do this and is unique to the New Hampshire system.

Class 3

The fire danger is High and the probability of ignition is high. Fine fuels in open areas and on sunny slopes will ignite readily and will spread rapidly. We are likely to have 1+ acre fires statewide. This class is divided into three segments:

Low Class 3: In this segment the fire danger is high, but the fire behavior is more like a Class 2 fire.

Class 3: As defined above.

High Class 3: In this segment the fire danger is higher, the fire intensity is building, and the fire is spreading rapidly.

Class 4

The fire danger is Very High and the probability of ignition is very high. All sources of ignition will start fires easily. Fires will burn very intensely and will have high rates of spread causing control problems. There may be spotting, torching of trees, and deep burning except in early spring, which will cause mop-up problems. Firefighter safety is of utmost concern. We are likely to have several large fires statewide.
May be associated with "Fire Weather Watch" and a "Red Flag Warning".

Class 4

This class is divided into three segments:

Low Class 4: In this segment the fire danger is very high, but the fire behavior is more like a high class 3 fire.

Class 4: As defined above.

High Class 4: In this segment the fire danger is moving towards the extreme, the burning is very intense, and the fire is spreading very rapidly. Spotting and torching is occurring and the fire behavior is becoming a problem.

Class 5

The fire danger is Extreme and the probability of ignition is of a concern because any ignition source will start a fire. Fires will spread rapidly and be very difficult to contain. Spotting, intense burning rapid rates of spread, and high probability of running crown fires may be observed.The possibility of extended attack fires is of great concern as these fire likely will not be extinguished until after this burning period. We are likely to have many large , problematic fires statewide.
May be associated with "Fire Weather Watch" and a "Red Flag Warning".

Class 5

This class is divided into three segments:

Low 5: In this segment the fire danger is extreme, the fire behavior is starting to become a problem. Small fires build quickly, individual trees may be torching, spotting is occurring because of lofted burning embers. Containment may be a problem because of firefighter safety concerns.

Class 5: As defined above.

High Class 5: Fire behavior is extreme. Erratic winds, fire whirls, and spotting are a few of the many problems associated with fires. Containment may not be possible because of safety concerns. The fire may be causing updrafts and a large column or plume.

Fire Weather Watch
(possible "Red Flag Warning")

The National Weather Forecaster is near the "50% confidence level" that "Red Flag" conditions will occur within the next 12 hours.

Those conditions for vegetative Stage 1 and Stage 2 (cured and transition) are:

Wind......sustained or frequent gusts above 25 mph
RH..........less than 30%
Rainfall...less than 1/4" during the previous 5 or more days

Those conditions for Stage 3 (green) are:

Wind......sustained or frequent gusts above 25 mph
RH..........less than 30%
Rainfall...less than 1/4" during the previous 8 or more days
Fuels......Keetch Byram Drought Index is above 300

Red Flag Warning

The National Weather Forecaster is confident that "Red Flag" conditions will occur.

Not all of the conditions have to be met, just a majority of them.