DEEP: Forest Fire Weather

Forest Fire Weather Information

Red Flag Warnings

Red Flag Warnings are issued by the National Weather Service (NWS), which predicts weather and forecasts warnings nationwide. Connecticut is divided between three different National Weather Service stations. Predictions for Hartford, Tolland and Windham counties are made in Taunton, MA; predictions for Litchfield County are made in Albany, NY and predictions for Fairfield, New Haven, Middlesex and New London counties are made in Brookhaven, NY.

A Red Flag warning is a warning to the fire fighting community that if there is a fire, the weather conditions can be expected to cause erratic fire behavior. Red Flag warnings are not a fire danger rating and they are not synonymous with High, Very High or Extreme fire danger. Red Flag warnings are issued when winds will be sustained or there will be frequent gusts above a certain threshold (normally 25 mph). In addition, relative humidity needs to be below 30% and precipitation for the previous 5 days has to have been less than 1/4-inch.

Forest Fire Danger Rating

The DEEP Division of Forestry issues Forest Fire Danger Ratings for Connecticut. A National Fire Danger Rating system that utilizes two indexes is used in Connecticut. The "spread" of a fire is predicted with the Spread Index, which is a numeric rating that corresponds with how fast a fire travels in 'Chains per Hour' (a chain is 66'). For example, if a prediction is made that the Spread Index will be 19, it means the fire is predicted to spread 1254 feet (19 x 66') in an hour.

Connecticut also uses a Build Up index that measures drought. It is a relative scale that is based upon past precipitation.

The 5 Forest Fire Danger Ratings or Class Days are:

Rating or Class Days

Spread Index

Build Up Index

LOW

0-10

0-22

MODERATE

11-15

23-44

HIGH

16-29

45-59

VERY HIGH

30-39

60-74

EXTREME

> 40

> 75

In addition, there are three vegetative conditions that may exist.

  • The first stage is a cured stage, meaning the fuels are in a cured state, exposed to full sunlight (early springtime).
  • The second stage is a transition stage (late spring/fall), meaning the upper tree canopy is partially leafed out
  • The third stage indicates full leaf out (summer).

There are also three very important weather factors affecting fire start, spread and the fire weather danger.

  1. Wind - the most important factor; it dries out fuels and drives the fire.
  2. Relative humidity - affects fuel moisture.
  3. Precipitation.

Forest Fire Seasons in Connecticut

Spring Fire Season: Normally mid-March to mid-May

After the snow melts in the northeastern part of the U.S. we enter into a traditional spring fire season. This is the time of the year when deciduous trees are bare and the warm spring sun heats up the forest fuels. Forest fuels are made up of anything that burns; typically grasses, leaves, twigs, branches and decaying material in the soil. As the days grow longer and sun gets hotter, the fuels that are most exposed dry out very fast. Grasses, twigs, and very small branches are called '1-hour fuels'. That is, they can take on atmospheric conditions within an hour. Consequently we can receive precipitation and if the sun comes out and a breeze picks up, the fine fuels can be available for burning within an hour. Larger fuels take longer to dry out. Typically fires that start this time of the year burn just the surface leaves and can spread very fast. Generally they cause little, long term damage to the forest.

During the spring, the Spread Index usually drives the fire danger. Wind is most critical.

Summer Fire Season: Normally mid-May through September

After the trees are fully leaved out we enter a different fire season. The Build Up Index is the driving factor with past precipitation (drought) being critical. Forest fuels dry slowly because of lower temperatures in the shaded woods with correspondingly higher humidity. Remember, temperature and relative humidity have an inverse relationship. The vegetation is growing and sucking moisture from the soil. When the woods get dry enough and a fire gets started the fire tends to grow more slowly than a spring fire but tends to burn deeper into the ground. Fires that burn deeper into the ground burn organic matter in the soil (including tree roots), are more difficult to suppress, and cause extensive mortality to vegetation.

Fall Fire Season: Normally October through snow fall

Fall fire season takes on some of the characteristics of both the spring and the summer. Falling leaves are dry but not quite cured. We go back to the 'transition stage' in fire danger predictions. The sun is getting lower and is diminishing in drying capacity. Fires can spread rapidly.

Open Burning:

Open burning of brush is allowed in Connecticut if a resident has a permit from the local open burning official. There are conditions attached to that permit that restricts its use when there is an increased potential for degradation of air quality or when the Forest Fire Danger is high or above.

Fire Weather Forecasts:

Starting in early spring, the CT Division of Forestry begins monitoring the weather as it relates to Fire Danger. During the Spring Fire Season and at other times of the year when the fire danger is high or above, we broadcast daily predictions for fire danger for 1:00 pm. The predictions are normally out before 7:00 am.

For further information, please contact the Division of Forestry at 860-424-3630 or deep.forestry@ct.gov.

Daily Forest Fire Danger Report

Content Last Updated on April 3, 2014