Rain’s threatening to fall this weekend. Here’s why that’s not a bad thing.

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JACKSONVILLE, Fla. – Rainfall this year has been about normal, but it is from mid-April to mid-June that we must closely watch the dryness of both the surface vegetation and soil moisture. Each has an important influence on fire conditions.

Fire Index and KBDI

The Fire Index is the ability for spontaneous (from sparks from rail cars or form careless outdoor burning) fires. Even though it might seem obvious, the drier the conditions, the more easily fires can rapidly burn.

The Keetch Bryam Drought Index (KBDI) is more about soil conditions. It’s very possible to see surface vegetation dry out quickly under the spring and summer sunshine, which is typically associated with low afternoon humidity levels. But soil moisture can still stay moist. This moist layer underneath can slow the spread of fires but more importantly reduce the sustainability of ongoing fires due to the higher soil moisture content.

When the KBDI is very dry, it’s possible for fires to burn downward into the soil through roots and biomass. So, instead of fires being very manageable during the overnight hours when winds are light and humidity levels increase, they smolder. Then, as the sun dries the atmosphere the next day, fires quickly spring back to life. This was the condition that developed in late April 1998, just as the worst fire season in Florida’s known history exploded across the state.

Currently, we are in a pattern of moderately dry surface vegetation (moderate fire index) and moist soil conditions (moist KBDI). Put these conditions together and we are below the threshold of significant fire danger. That being said, we’ll need more rain (possibly this weekend) to keep the fire danger in check.

Blue for moist soil conditions

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