Pyrocumulonimbus Firepower Threshold (PFT) is a meteorological benchmark that indicates when a wildfire has reached sufficient intensity to generate its own weather system, specifically a pyrocumulonimbus (pyroCb) cloud. PFT is expressed in gigawatts (GW) of total fire power.
A pyroCb forms when intense heat from a fire creates a powerful convective column that reaches the upper troposphere or lower stratosphere. Once a fire generates a pyroCb, it begins to drive its own weather rather than simply responding to ambient conditions. This transition marks a fundamental shift in fire behavior and significantly reduces the effectiveness of suppression efforts.
When a fire approaches or exceeds the PFT, the following conditions become possible:
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Downbursts and erratic outflow winds that can rapidly change fire direction without warning.
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Ember cast over long distances, creating spot fires well ahead of the main fire front.
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Collapse of the convective column during a pyroCb can generate powerful downdrafts, producing sudden and extreme surface wind events.
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Conditions that make aerial operations dangerous or impossible.
The PFT is calculated based on atmospheric instability, specifically the convective available potential energy (CAPE) and the moisture and temperature profile of the atmosphere above the fire. A lower PFT indicates an atmosphere that is more susceptible to pyroCb formation; an atmosphere that is stable or dry aloft requires more fire energy to trigger one.
When forecast fire behavior approaches the PFT for the current atmospheric conditions, it signals conditions under which extreme, self-driven fire behavior becomes possible. This is particularly relevant for utility customers monitoring conditions that could render infrastructure at risk to fires that behave outside of normal spread models.