Maximum wind gust associated with heavy showers or thunderstorms. This method was developed by Ruud A.A.M. Ivens of the Royal Netherlands Meteorological Institute (KNMI) and was originally detailed in the paper Proc. Symp. Mesoscale Analysis & Forecasting, Vancouver, Canada 17-19 August 1987, ESA SP-282 (August 1987).

It has been proven to be accurate and reliable for use in operational forecasting, particularly in the European area when other techniques originating from the USA produced excessively high velocities.

The technique is a statistical method developed using multiple regression. The following parameters are considered:

• The difference between maximum daytime temperature (T_x) and θ_w at 850 hPa: If the showers developed over the sea, sea surface temperature is used instead of T_x.
• \(T_x-\theta_w500\) and \(\theta_w850-\theta_w500\)
• The square roots of the above parameters.
• Wind velocity U at 850 and 250 hPa.

There was a marked difference in the maximum gust in relation to upper air data in the case of \(T_x-\theta_w850<9 ^\circ C\) or \(T_x-\theta_w850\geq 9 ^\circ C\). As a result of this multiple regression equations were derived separately.

$$if \quad T_x-\theta_w850<9 ^\circ C:\quad$$

$$FF_{max}=14.9+0.976*U850+1.27(\theta_w850- \theta_w500)$$

$$if \quad T_x-\theta_w850\geq 9 ^\circ C:\quad$$

$$FF_{max}=15.9+0.174*U850\sqrt{T_x- \theta_w500} +0.057*U250\sqrt{T_x-\theta _w500}+0.92*(\theta _w850-\theta _w500)$$

where FF_max is the maximum gust in knots, T_x the maximum daytime temperature (deg C), U850 and U250 the wind at 850 and 250 hPa (kts) respectively and θ_w850, θ_w500 the wet bulb potential temperature at 850 and 500 hPa (deg C) respectively.

Note that should any of the expressions within the square root operators evaluate to less than zero (highly unlikely in real atmospheric conditions), then the expression will be set to zero internally to avoid arithmetical errors.