A method of determining whether one or more navigable elements of a navigable network within a geographic area are still affected by a precipitation weather event. A quantity parameter is associated with each segment of an electronic map representing the navigable network and indicates the amount of precipitation remaining on the navigable element or portion thereof represented by the segment. The value of the quantity parameter increases in the presence of any precipitation weather event, and decreases ac cording to a predefined function with respect to time in the absence of any such precipitation weather event. A speed of travel of one or more devices along the navigable element or portion thereof represented by a segment is determined and compared to an expected speed of travel for the segment; the value of the quantity parameter being decreased based on the comparison. The value of quantity parameter can be used to generate weather events and/or attribute the cause of a congestion event as being the precipitation weather event.