Frequently Asked Questions About London Ontario Weather

London Ontario receives an average of 194 cm of snowfall annually based on the 1981-2010 climate normal data. The snowfall is distributed primarily from November through March, with December, January, and February being the heaviest months, each typically receiving 32-46 cm. However, annual totals vary considerably from year to year. Some winters produce over 250 cm of total snowfall, while milder winters may see only 150 cm. Lake-effect snow from Lake Huron contributes significantly to these totals, particularly in areas northwest of the city. The snow season typically begins in November with the first measurable accumulation and extends through late March or early April. Individual snowstorms can deposit 15-30 cm within 24 hours during major winter weather events.

September offers the most consistently pleasant weather for visiting London Ontario, with average high temperatures around 21°C, lower humidity than summer months, and beautiful fall foliage beginning to develop. July and August provide the warmest conditions with temperatures averaging 25-27°C, ideal for outdoor activities, though these months also bring higher humidity and frequent afternoon thunderstorms. June represents another excellent option with comfortable temperatures around 24°C and longer daylight hours, though precipitation levels are relatively high at 88.5 mm average. May can be pleasant with spring blooms and temperatures near 19°C, but weather remains more variable with occasional cool periods. For those who enjoy winter activities, January and February offer reliable snow cover and winter sports opportunities, though temperatures average -2°C to -10°C and require appropriate cold-weather preparation.

Yes, London Ontario and the surrounding region experience tornadoes, though they occur less frequently than in the central United States tornado alley. Southern Ontario averages 12-15 tornadoes annually, with several affecting the broader London area each decade. Most tornadoes in this region are rated EF-0 or EF-1 on the Enhanced Fujita Scale, with wind speeds below 180 km/h, but stronger tornadoes do occur occasionally. The most significant recent tornado near London was an EF-2 that struck Mount Carmel on June 17, 2009, with estimated winds of 190 km/h. The tornado season in southwestern Ontario peaks during June and July when warm, humid air masses interact with cooler air aloft, creating atmospheric instability. Environment Canada issues tornado watches and tornado warnings when a tornado has been spotted or indicated by radar. Residents should have a severe weather safety plan and know how to access weather warnings through radio, television, or smartphone weather apps.

Seven-day weather forecasts for London Ontario have improved significantly over the past two decades but still show declining accuracy as the forecast period extends. Temperature forecasts for days 1-3 are typically accurate within 2-3°C about 80-85% of the time, while days 4-7 show accuracy declining to 60-70% with potential errors of 4-5°C or more. Precipitation forecasts are less reliable, with day 1-2 forecasts correctly predicting whether precipitation will occur about 85% of the time, dropping to 60-65% accuracy by day 7. The forecast skill depends heavily on the weather pattern, with stable high-pressure systems allowing better extended predictions, while rapidly changing conditions associated with storm systems reduce forecast accuracy. Environment Canada updates forecasts multiple times daily as new observational data and model runs become available. For critical planning decisions, checking forecasts daily as the event approaches provides the most reliable information, and users should pay attention to forecast confidence indicators when available.

Lake-effect snow in London Ontario occurs when cold Arctic air masses move across the relatively warmer waters of Lake Huron, located approximately 60 km northwest of the city. This process typically happens from November through February when lake water temperatures remain 10-15°C warmer than the overlying air. As cold air moves across the open water, the lower atmosphere warms and picks up moisture through evaporation. This moisture-laden air becomes unstable and rises, forming clouds and precipitation. When the wind direction is from the northwest or west-northwest, these snow bands move directly over London, depositing heavy snowfall in narrow corridors. Snow rates can exceed 5 cm per hour in intense lake-effect bands. The phenomenon continues until either the lakes freeze over (reducing moisture availability) or the air mass modifies and becomes too warm. Lake-effect snow is highly localized, with some areas receiving 20-30 cm while locations just 10-15 km away see minimal accumulation, making these events challenging to forecast precisely. For more information on lake-effect phenomena, see the Great Lakes Environmental Research Laboratory.

Thunderstorms in London Ontario occur most frequently from May through September, with peak activity in July and August. The region experiences an average of 15-20 severe thunderstorm days annually, defined as storms producing winds over 90 km/h, hail 2 cm or larger, or heavy rainfall exceeding 50 mm. Most thunderstorms develop during afternoon and evening hours between 2 PM and 10 PM when daytime heating creates maximum atmospheric instability. However, nocturnal thunderstorm complexes can occur when upper-level disturbances move through the region regardless of time of day. Spring thunderstorms in May and June often form along cold fronts and can be particularly severe, sometimes producing tornadoes. Summer storms frequently develop from local heating and lake breeze convergence zones. A typical thunderstorm season sees several severe weather watches and warnings issued by Environment Canada. Lightning poses a significant hazard during these storms, with southwestern Ontario recording multiple lightning-related injuries and fatalities each decade. The thunderstorm season ends in September as atmospheric temperatures cool and instability decreases.