What is the Fire Weather Index?

The FWI is a composite 0–100 score combining drought code, heat, wind, and fuel flammability. It's used across boreal and temperate forests as the primary operational proxy for ignition potential and expected fire behavior.

What do the FWI bands mean?

0–34 low: routine operations. 35–59 moderate: pre-position resources, monitor drought code. 60–79 high: restrict ignition sources, stage crews, issue advisories. 80+ extreme: red-flag posture, community warning, likely evacuation staging.

How is FWI different from a wildfire risk map?

A risk map is structural — it scores an area based on fuel, topography, and long-run climate. FWI is a live weather-driven score that changes daily. Both matter; the map tells you where, FWI tells you when.

How far ahead can FWI be projected?

Operationally, 5–14 days is standard from ensemble weather models. For planning — species shift, thinning, prescribed-burn windows — a 20-year FWI trajectory is what our forestry world model outputs.

What raises FWI the fastest?

A stalled ridge that pushes heat-day counts up, a precipitation shortfall over the prior 60–90 days that lifts drought code, and a wind event on cured fuel. Any two of those together is when the score jumps into the extreme band.

Fire Weather Index explained: bands, thresholds, and decisions.

Fire Weather Index explained.

One number, four inputs, four bands. Here's how to read the FWI in the field — and the thresholds that trigger real operational decisions.

The four inputs

Drought code. A rolling seasonal moisture deficit — the deep-fuel dryness that governs whether an ignition holds.
Heat. Recent heat-day count and the current day's maximum temperature.
Wind. Sustained wind and gust — the multiplier on spread rate and spotting distance.
Fuel flammability. A species-mix–weighted score of how ignitable the standing fuel is right now.

Reading the bands

Bands are not linear. The jump from moderate to high is roughly 3× the change in expected fire behavior; high to extreme is another 3×. Treat 60 as the operational-posture line and 80 as the red-flag line.

What ArchipelAI gives you

The Forest Health workspace shows an FWI per unit, projected 20 years forward, with today's value scrubbable against the trajectory. See it live in the forestry demo.

Common questions

What is the Fire Weather Index?: The FWI is a composite 0–100 score combining drought code, heat, wind, and fuel flammability. It's used across boreal and temperate forests as the primary operational proxy for ignition potential and expected fire behavior.
What do the FWI bands mean?: 0–34 low: routine operations. 35–59 moderate: pre-position resources, monitor drought code. 60–79 high: restrict ignition sources, stage crews, issue advisories. 80+ extreme: red-flag posture, community warning, likely evacuation staging.
How is FWI different from a wildfire risk map?: A risk map is structural — it scores an area based on fuel, topography, and long-run climate. FWI is a live weather-driven score that changes daily. Both matter; the map tells you where, FWI tells you when.
How far ahead can FWI be projected?: Operationally, 5–14 days is standard from ensemble weather models. For planning — species shift, thinning, prescribed-burn windows — a 20-year FWI trajectory is what our forestry world model outputs.
What raises FWI the fastest?: A stalled ridge that pushes heat-day counts up, a precipitation shortfall over the prior 60–90 days that lifts drought code, and a wind event on cured fuel. Any two of those together is when the score jumps into the extreme band.

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