A mathematical model is proposed for estimating the wind loads caused by the passage of a downburst through horizontal structures, such as bridge decks. The model simulates the three-dimensional flow fields of stationary and environmentally-driven downbursts. It is based on radial and vertical shaping functions of the velocity components, following a path developed for aircraft structures. The radial shaping function of the horizontal velocity is defined with attention to reproducing the local increase in velocity observed in the average field measurements. In addition, the coefficients of an existing empirical vertical shaping function of the horizontal velocity are re-calibrated to better fit the available field data. The shaping functions of the vertical components are derived by imposing mass conservation.The model allows for predicting the large angles of attack on the decks during the passage of downbursts with small to medium diameters. The occurrence of such large angles of attack also implies a change in the aerodynamic design procedure of bridge decks, for which wind tunnel tests are commonly conducted only for angles of attack in the range of about ±10°. The aerodynamic coefficients of a NACA profile and a pseudo-rectangular section are used due to the absence in the literature of aerodynamic coefficients of bridge decks for very large angles of attack. Several runs are conducted by varying the downburst properties and the deck height above the ground to provide results useful for the aerodynamic design of bridges. The results confirm the key role played by the vertical component of the velocity in developing the wind loads on the decks during the passage of a downburst. Non-negligible wind loads are obtained, which are not predictable with the existing approaches. The need for conducting dynamic analyses of flexible bridges is also emerging.