I was playing around with some basic FWI/FBP calcs in the 'FWI calculator' in RedApp and noticed that the ISI values appear to be off for high-wind situations.
The issue is almost certainly the footnote on p. 32 of the STX-3, and reiterated in the GLC-X-10.
Basically, the footnote specifies a different calculation for ISI when WS > 40 km, to avoid massively high ISI values under stronger winds. In essence it creates an logistic rather than exponential growth curve under these conditions. Prometheus has this calculation implemented, but RedApp does not appear to.
This needs to be fixed.
For example, I was playing with the following indices:
FFMC 94.2, WS 55 km/h (other indices don't matter in this case).
This produces a standard daily ISI, according to RedApp 2.12 of 125.9. A similar situation calculated by hand (or in Prometheus 6.1) produces an ISI of about 82.
With WS of 70 km/h, the RedApp ISI is 268.1. The adjusted value should be closer to ISI 89 by my calculations.