The The High-End cLimate Impacts and eXtremes (HELIX) project calculated fire indexes values at 1.5 and 2 degree resolution the McArthur Forest Fire Danger Index (FFDI) (Noble et al, 1980) equation is as follows:

FFDI = 1.25 * D * exp [ (T - H)/30.0 + 0.0234 * V]

Where:
- D = drought factor,
- T = Temperature (ºC),
- H = humidity (%), and
- V = wind speed (km hr-1).

The drought factor (D) is calculated as follows:

D = (0.191 * ( I + 104) * (N + 1)^ 1.5) / (3.52 * (N+1)^1.5 + P -1)

Where :
- P = precipitation (mm day-1),
- N = number of days since last rain, and
- I is based on Keetch-Byram drought index.

This represents the moisture in the upper soils layers that denotes flammability of organic matter (Keetch and Byram, 1968). The HELIX Project used a varying soil moisture to calculate the deficit compared to the field capacity at a depth of 1m.

References:

Keetch, John J.; Byram, George M. (1968). A Drought Index for Forest Fire Control. Res. Pap. SE-38. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station. 35 p. https://www.srs.fs.usda.gov/pubs/rp/rp_se038.pdf

Noble, I. (1980). McArthur's fire-danger meters expressed as equations. Australian Journal of Ecology, 5, 201-203. Re-published in July 2006 DOI: 10.1111/j.1442-9993.1980.tb01243.x