Learn French: En hiver, il y a beaucoup de neige. - In winter, there is a lot of snow.

In French, you use en with most seasons: en hiver (in winter), en été (in summer), and en automne (in autumn). The only exception is spring, which takes au (au printemps).

Snow is a feminine noun (la neige), so why does the sentence use de instead of de la?

When you use an expression of quantity like beaucoup (a lot), it is always followed by just de (or d' before a vowel). The usual articles like la are dropped, so it is just beaucoup de neige, never beaucoup de la neige.

Is il y a only used for singular things like snow?

No! Unlike English which changes between 'there is' and 'there are', the French phrase il y a stays exactly the same for both singular and plural things. You can say il y a beaucoup de neige (there is a lot of snow) or il y a deux voitures (there are two cars).