Of major Canadian cities, St. John's is the foggiest (124 days), windiest (24.3 km/h (15.1 mph) average speed), and cloudiest (1,497 hours of sunshine). St. John's experiences milder temperatures during the winter season in comparison to other Canadian cities, and has the mildest winter for any Canadian city outside of British Columbia. Precipitation is frequent and often heavy, falling year round. On average, summer is the driest season, with only occasional thunderstorm activity, and the wettest months are from October to January, with December the wettest single month, with nearly 165 millimetres of precipitation on average. This winter precipitation maximum is quite unusual for humid continental climates, which most commonly have a late spring or early summer precipitation maximum (for example, most of the Midwestern U.S.). Most heavy precipitation events in St. John's are the product of intense mid-latitude storms migrating from the Northeastern U.S. and New England states, and these are most common and intense from October to March, bringing heavy precipitation (commonly 4 to 8 centimetres of rainfall equivalent in a single storm), and strong winds. In winter, two or more types of precipitation (rain, freezing rain, sleet and snow) can fall from passage of a single storm. Snowfall is heavy, averaging nearly 335 centimetres per winter season. However, winter storms can bring changing precipitation types. Heavy snow can transition to heavy rain, melting the snow cover, and possibly back to snow or ice (perhaps briefly) all in the same storm, resulting in little or no net snow accumulation. Snow cover in St. John's is variable, and especially early in the winter season, may be slow to develop, but can extend deeply into the spring months (March, April). The St. John's area is subject to freezing rain (called "silver thaws"), the worst of which paralyzed the city over a three-day period in April 1984.