The Development of a Dynamically Based Fire Spread Weather Forecast Index

A predictive index for high fire spread weather conditions has been developed and is being tested by North Carolina State University. The index is different from other well known and widely tested indices, e.g., the Haines index, because it is formulated assuming that scale dependent dynamical forcing processes transport very dry and high potential temperature air into the planetary boundary layer under certain very specific synoptic circulation regimes. At the meso-alpha scale, the preferred transport regime is associated with the right exit region of a balanced transverse ageostrophic and thermally indirect jet streak circulation. This anticyclonically-curved circulation, resulting in strong parcel descent, is associated with the mid-upper tropospheric convergence of very dry air and its downward transport. At the meso-beta and meso-gamma scales of motion, this upper-level balanced ageostrophic circulation phases with an unbalanced mid-lower tropospheric jet streak circulation, which is thermally direct and is located just behind or ahead of a surface cold front. The unbalanced circulation produces a shallow but very strong descending circulation under the upper-level ageostrophic circulation. These 2 circulations couple and act as a conduit for very dry air to reach the convective planetary boundary layer near the surface cold front. The 2 circulations, one hydrostatic and one nonhydrostatic phase to maximize the fire spread index, which is a function of the vertically integrated 2-dimensional hydrostatic and 3-dimensional nonhydrostatic convergence of dry air from the upper troposphere into the convective planetary boundary layer. Two geographically different examples of the index's utility will be shown, one from an isolated East Coast wildfire and one from a widespread California Santa Anna wildfire event.