EXPLANATION OF THE 7-DAY SIGNIFICANT FIRE POTENTIAL PRODUCT

For those of you who monitor this product, it is important to ensure that it is being interpreted correctly.

To begin with, it is very important that the user understand that although weather is a major contributor to large fire potential, this product is NOT a weather forecast! It is a LARGE FIRE POTENTIAL FORECAST!

The purpose of the product is to estimate the daily large fire risk across southern and central California for the next 7 days by assessing the following:

1. Daily probability for occurrence of a new large fire and/or,

2. Daily potential for significant new growth on an existing large fire

The product is based on a statistical model which uses fuel moisture inputs from the NFDRS (WIMS) and various gridded weather inputs from weather models. This data is processed through a series of equations that yield forecasts of Fuel Dryness Level (DL) as well as probabilities (some objective and some subjective) of certain critical weather conditions for each of the next 7 days. When appropriate combinations of DL and weather triggers are expected, a “High Risk Day” is designated on the Chart to warn of a significantly higher than normal chance for a “Large Fire”.

A “Large” fire has been defined for each Predictive Service Area based on a statistical analysis. The definitions are as follows:

Eastern Sierra – 250 Acres

Central Sierra – 50 Acres

Southern Sierra – 300 Acres

Sierra Foothills – 175 Acres

Central Coast Mtns & Vlys – 300 Acres

Central Coast – 50 Acres

South Coast – 200 Acres

South Central Mountains – 100 Acres

Southern Mountains – 125 Acres

Deserts – 3,000 Acres

Information is portrayed on the chart by a color code and, when appropriate, a symbol as follows:

Fuel Dryness (DL) is represented on the chart for each day and for each Predictive Service Area as one of three colors. DL is determined by combinations of ERC and F100.

1. Green – Indicates a DL which historically has not resulted in any realistic chance of large fires.

2. Yellow – Indicates a rather “normal” summertime dryness that typically will not result in large fires unless accompanied by a critical trigger event (often weather related).

3. Brown – Indicates very dry fuel levels which result in a much higher than normal chance of large fires when accompanied by a critical trigger event. A low to moderate threat for large fires exists in the absence of a trigger.

“High Risk Days” are special days when conditions (i.e. dry fuels in conjunction with a critical trigger event) exist that historically have yielded a significant chance for a large fire. They will be designated on the chart in RED with a symbol designating the trigger as follows:

1. Red and a lightning bolt – An expected combination of dry fuels and a lightning trigger. It must be remembered that this designation will only show on the chart when an appropriate amount of lightning and an appropriate DL is expected. This is NOT simply a lightning forecast.

2. Red and a HD – An expected critical combination of dry fuels and an unseasonably hot and dry air mass. While this condition does not start fires, it often produces a favorable environment for new starts to become large. Thus this trigger can result in significant growth on existing fires but for most areas, it correlates poorly with new large fires.

3. Red and a W – Represents dry and windy. Again as stated in number 2 above, wind does not necessarily start fires (although in some extreme cases it could), it rather produces a favorable environment for new start to become large.