first such occurrence in any March since 1946, and was followed by the warmest April ever on record. Winter weather conditions in Toronto can vary greatly from one year to another and is significantly affected by global weather patterns such as El Niño–Southern Oscillation (ENSO). El Niño winters are generally mild and dry, with decreased snowfall (such as the winters of 2009–2010 and 2015–2016), while La Niña winters tend have more precipitation, with more snowfall (the aforementioned winters of 1999 and 2008 were La Niña), but are sometimes above average in terms of temperatures. ENSO Neutral years are more unpredictable, such as 2013-14 which was the coldest, longest winter in recent memory. Summer Maximum temperatures typically range from with moderate to high humidity, proximity to Lake Ontario and the other lakes contribute to summer humidity but far away sources like the Gulf of Mexico also factor in. The subtropical jet can move well north of the area in the midsummer influenced largely by the strength and position of the Bermuda-Azores High and ridging over the North American continent, in some summers, it is a dominating influence. Lake effect cooling varies greatly depending on location within the city and strength and direction of localized winds, variation in water temperature and currents due to recent weather, the most pronounced effect on the Toronto Islands and also on the immediate lakeshore such as in The Beaches, and Scarborough Bluffs, hence the term "cooler by the lake". This is less a factor in the west and northwest parts of the city. Temperatures over occur on average seven days each year (three consecutive days of such temperatures is defined as a heat wave, which occur in most summers but maximums rarely exceed , which last occurred on July 21, 2011). However, coupled with high humidity, the humidex value can rise well above during these heat events, creating great discomfort. Nighttime temperatures generally hover close to in the city but during hotter spells remain closer to . Summer heat episodes are usually broken by cooler, drier periods that can be brief. Thunderstorms are also a regular occurrence and can pop up quickly, especially west and north of the city in areas more prone to the "lake breeze front" or "lake breeze thunderstorms" phenomenon, in which intense, sharply defined squall lines develop quickly on summer afternoons inland, amplified by localized variation in wind patterns between the Great