Map from Google - Niagara_Falls

In the Niagara Falls area the climate is continental, with cold winters and warm summers. The average temperature of January is -4 °C (25 °F), that of July is 22.5 °C (72.5 °F). Although we are on the 43rd parallel, in winter the weather is cold because the North American landmass cools down a lot, and outbreaks of polar air masses are frequent, to the point that on colder winters the waterfalls can even freeze.
Here are the average temperatures of the Niagara District Airport, at Niagara-on-the-Lake, Ontario, Canada.
Average temperatures - Niagara District
Niagara DistrictJanFebMarAprMayJunJulAugSepOctNovDec
Niagara DistrictJFMAMJJASOND
Min (°C)-8-7-3391418171371-4
Max (°C)-1051319242726211582
Min (°F)181927374857646355453425
Max (°F)303241556675817970594636

Precipitation amounts to about 950 millimetres (37.5 inches) per year, and is well distributed over the seasons: in fact, weather fronts can pass in all months, due to the continual clash between warm and cold air masses, and in the only season in which they are less frequent, ie summer, thunderstorms can erupt in the afternoon, adding more rain, so much so that summer is as rainy as autumn, at least in quantity. Here is the average precipitation.
Average precipitation - Niagara District
Niagara DistrictJanFebMarAprMayJunJulAugSepOctNovDecYear
Niagara DistrictJFMAMJJASONDY
Prec. (mm)707070757590658595759095955
Prec. (in)2.82.82.8333.52.63.33.733.53.737.6
Days13121212121191010121314140

In winter, from late November to March, precipitation occurs mainly in the form of snow (there are on average around 50 days with snow per year). Snow normally covers the ground from mid-December to early March. In a year, about 180 centimetres (70 in) of snow fall; sometimes a snowfall can be seen already in October, and in spring, still in April. In winter, real snow storms can occur, accompanied by gusts of blizzard. During the most intense outbreaks of cold air, the temperature can drop to around -25 °C (-13 °F).
However, given the unstable weather, the temperature can exceed freezing (0 °C or 32 °F) also in winter, so that snow can melt and it can even rain.

Spring, from mid-March to May, is an unstable season, characterized by sudden changes in temperature: initially cold in March, with frequent snowfalls and frosts, it advances gradually, with the first warm days but also the return of cold weather; it is still on average quite cold in April, and becomes mild in May, when the first afternoon thunderstorms begin to occur.

Summer, from June to August, is warm and quite sunny, though there's no shortage of cool and cloudy days, with some chance of rain, while when the weather is fine, thunderstorms can erupt in the afternoon. Sometimes hot periods can occur, with highs around 34/36 °C (93/97 °F).

Autumn, from September to mid-November, is initially pleasant, with many sunny days, in which the temperature exceeds 25 °C (77 °F): these periods, called Indian summer, occur in September, but sometimes also in October. As the weeks pass, however, the temperature decreases rapidly, weather disturbances become more frequent, and there are the first cold days.

The amount of sunshine in the Niagara Falls is overall good in summer, and acceptable in May and September; it is not high in the other months, and it's scarce in winter, especially in November and December, when the sun shows up rarely.

Niagara Falls

The falls are formed by the confluence of Lake Erie and Lake Ontario, at the border between the US state of New York and the Canadian province of Ontario. There are two waterfalls: the Horseshoe Falls, the main ones, at the border between Canada and the United States, and a short distance the smaller American Falls, which are entirely in the U.S. territory, and are separated from the Horseshoe Falls by Goat Island. The American Falls are actually divided into two, because beside them there are the smaller Bridal Veil Falls, separated from them by Luna Island.
The waters flow northwards, because Lake Erie, in the south, is located at about 170 metres (555 feet) above sea level, while Lake Ontario, in the north, is located around 100 metres (325 ft) below. However, the jump itself is about 30 metres (100 ft) in the American Falls (at least compared to the rocks below), and approximately 52 metres (170 ft) in the Horseshoe Falls.
The big cities closest to the falls are Buffalo, NY, Usa, and Toronto, Ontario, Canada.

Lake Ontario and Lake Erie are very cold in winter, when the waters approach the freezing point, while in summer they become almost acceptable for swimming, at least for those who do not suffer from the cold, especially in August, when they reach 21 °C (70 °F). Here are the average water temperatures.
Lake temperature - Lake Erie
Lake ErieJanFebMarAprMayJunJulAugSepOctNovDec
Lake ErieJFMAMJJASOND
Lake (°C)32237132021191495
Lake (°F)373636374555687066574841

When to go

The best time to visit the Niagara Falls runs from mid-May to late September. In midsummer, in July and August, not only the waterfalls are very crowded, but also there may be hot and muggy days, so this period can be excluded if you cannot stand the heat. You can therefore choose May and September; in May and early June it can still be a little cold, and in any case you have to take into account the possibility of rain and temperature swings. In the first half of October it can be cold, but you can admire the trees in the spectacular autumn colours, while in spring you can see flowers blooming in the nearby botanical garden.